The Lone Ranger
01-07-2007, 06:02 AM
An Introduction to Human Anatomy and Physiology
Chapter One: Life, Levels of Organization, and Relevant Terminology
Introduction:
I thought it might be fun to start a series of articles covering the subject of human anatomy and physiology. It’s a topic we can all relate to, after all, so hopefully it’ll be of some interest.
My intention isn’t to go into too much detail, but rather to provide what will hopefully be an accessible overview of the various body systems, what they do, and how they function. It goes without saying (I hope) that most of what applies to human A&P applies just as well to other vertebrates, but I thought it’d be best to concentrate on Homo sapiens, since that’s the one species of vertebrate in which all of us have a vested interest.
Of course, before we can get into the nuts and bolts of how the digestive system functions or what the “sliding filament theory” is about and other such interesting things, we have to first cover the basics. So, I thought I’d start out with a brief overview of some relevant material: what is “biology” and what are “anatomy” and “physiology”? What is “life” and how do we distinguish living things from non-living things? This will doubtless prove pretty basic for most people, but perhaps it will provide a good review. If nothing else, the review will hopefully help once we get to the more complicated stuff.
Biology, Anatomy, and Physiology:
Biology, as I’m sure everyone knows, is the study of life and of living things. You might be surprised at how difficult it is to define “life,” but we’ll get to that momentarily. Within the broader field of biology, anatomy is the study of the structure (the morphology) and the arrangement of body parts. Physiology is the study of the function of body parts – that is, what they do and how they do it.
An old saying amongst anatomists and physiologists is that “form follows function.” What that means is that the function of a body part will to some degree influence its structure and arrangement. For example, an organ adapted for gathering light and thus allowing you to see the world around you could hardly be made of an opaque material, nor could it be located deep within the body cavity. Similarly, a body structure that’s adapted for supporting your weight could scarcely be functional if it were made of a weak and flexible material. What a body structure does will necessarily influence what it’s made of, how it’s shaped, and where it’s located.
Because of the close association between form and function, it’s all but impossible to separate the fields of anatomy and physiology. That’s why it’s typically best to consider both the anatomy and the physiology of any given body structure simultaneously, since each sheds light on the other.
What is “Life” and What are the Characteristics of Living Things?:
Ask 100 biologists to define the word “life” and you’ll surely get at least 100 different answers. A quick consultation of a dictionary reveals that “life” is “the condition of living or the state of being alive.” Well that was helpful! Perhaps an alternate definition will be more illuminating. “Life” is “the period between birth and death.” That was even less helpful! (As an aside, you were alive well before your birth.)
The fact of the matter is that there is no universally agreed-upon definition of what it means to say that something is alive. So, if you consult a biology textbook in hopes of learning what it means to say that something is “alive” or has “life,” you’ll quickly discover that they usually avoid the question and instead give you a list of characteristics that most people agree are shared by living things.
In other words, about the best we can do is say, “things that have these characteristics are agreed to be alive – even though we don’t exactly agree on what ‘alive’ means.” Even then, there are things that we can’t agree upon. Is a virus alive? The answer varies according to whom you ask.
[B]The Characteristics of Living Things:
Living things are generally agreed to have these characteristics in common: complexity and organization, homeostasis, metabolism, irritability and adaptation, evolution, growth, and reproduction. It’s worth keeping in mind that not every living being shares all of these traits, however (indeed, no individual evolves), and there are non-living things that arguably share most of these characteristics. But that just gets us back to arguing about whether or not we should call things like viruses “alive.”
1.) Complexity and Organization:
Living creatures are [B]highly complex and highly organized. The organic and inorganic molecules that make up living creatures can be quite complex and are often made up of tens of thousands or even millions of subunits. Moreover, these molecules are arranged into larger and more complex units, which are arranged into even larger and more complex units . . . and so on. Not only are living creatures quite complex in their makeup, but they are very highly organized. In fact, this is why we refer to living creatures as “organisms.”
Neither complexity nor a high degree of organization means that a thing is alive, of course. Still, living things are both highly complex and very highly organized. In fact, living beings are the most complex chemical structures known. Chemically speaking, a single living cell is a much more complex object than is a star or a planet. On the other hand, while crystals, like living things, are very highly organized, they’re nowhere near as complex as are living things.
The basic unit of life is the cell. That is, it’s generally agreed that every living thing is made up of one or more cells. Most biologists regard things that behave like living things but aren’t cellular as non-living. By this definition, things like viruses and prions – though complex, highly organized, and made of organic molecules – are not alive.
2.) Homeostasis:
What makes living things so special is not just that they’re so complex and highly organized, but that they maintain their complexity and organization. This process by which living things maintain themselves in the face of a Universe which often seems quite hostile to complexity and to organization is known as homeostasis. Homeostasis refers to the fact that living beings actively maintain a more or less stable internal environment. The proper term for an organism that is no longer capable of homeostasis is “dead.”
For the most part, homeostasis occurs through the process of negative feedback. Negative feedback occurs when some portion of the products of a system are fed back into the system as input and cause a decrease in the system’s output of product. In this way, a system can automatically maintain itself at or near some set point.
That’s probably not very clear, so perhaps a couple of examples will help to clarify. An often-used example of a system that uses negative feedback to maintain itself is that of a furnace and a thermostat. The furnace and thermostat can be thought of as a system for producing heat. Once you set your thermostat for a particular temperature, if the room temperature is below the “target” temperature, your furnace produces heat and so the room temperature rises. When the room temperature reaches or exceeds the target temperature, heat production by the furnace decreases or stops entirely. If the room temperature subsequently falls below the target temperature again, the furnace will automatically come on and add more heat to the room. In this way, a very simple system can automatically maintain itself at whatever temperature you choose.
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Negative Feedback
Your body maintains its temperature in very much the same way. The “target” temperature is normally about 37º C, and the hypothalamus of the brain constantly monitors that temperature. If the temperature of blood flowing through the hypothalamus falls below 37º, the hypothalamus triggers physiological changes that cause decreased sweating, increased metabolic rate, constriction of blood vessels, and shivering. These physiological changes cause increased heat production and improved retention of body heat. The further below 37º is the blood flowing through the hypothalamus, the more intense are these physiological changes.
Conversely, if blood temperature rises above 37º, the hypothalamus triggers increased sweating and dilation of blood vessels, which increase heat loss and bring the temperature down.
3.) Metabolism:
All living organisms have some sort of metabolism. Metabolism is the process by which organisms convert nonliving material into cellular components and break down chemicals in order to generate energy which can be used to build and maintain body structures. The process of building cellular structures and other body components (which are not necessarily living matter – for example, your hair and fingernails are not living tissue) is known as synthesis, and the process of breaking down chemicals for energy or so that they can be reassembled into other chemicals is known as catalysis.
Sometimes, you’ll hear metabolism defined as “the sum total of all the chemical reactions occurring within a living organism that support life.” Those chemical reactions which involve breaking larger molecules into smaller molecules are collectively known as catabolism, and those chemical reactions which involve assembling larger molecules from smaller subunits are known as anabolism. (This is why hormones that promote growth of body tissues – say muscle tissue, for example – are known as anabolic hormones.)
4.) Irritability and Adaptation:
All living organisms show irritability and adaptation in response to stimuli. Irritability is the ability of an organism to sense and respond to things in its environment which will affect it – either positively or negatively. A plant might grow toward the sun, for instance, or a human might remove his hand from a hot stove. Closely related to the concept of irritability is that of adaptation, which refers to changes in an organism in response to its environment. For example, a human’s skin might grow darker in response to exposure to intense sunlight.
The term “adaptation” can be confusing, since it’s often used to describe both changes in individuals and to describe long-term changes in populations. Normally, changes to individuals that occur within their lifetimes cannot be passed on to offspring and so are not evolutionarily significant. In addition, these individual changes are typically reversible. For example, a tan will fade if the individual stays out of the sun. Environmental conditions can also cause long-term changes in the genetic makeup of a population that are not reversible in individual organisms. These changes over time in the genetic makeup of populations are certainly evolutionarily significant, since that’s what evolution is.
5.) Evolution:
Changes in populations over time, as noted, is evolution. Because organisms reproduce and pass on their traits to their offspring, populations can and do evolve over time. As a rule, those individuals who happen to possess traits that make them well-suited to their environments are more likely to survive and reproduce than are individuals who happen to possess traits that make them poorly-suited to their environments. Consequently, traits that are beneficial to their bearers tend to become more common over time because their bearers are likely to survive long-enough to pass them on to offspring, while traits that are harmful tend to become less common over time because their bearers tend not to survive long-enough to pass them on to offspring.
In this way, the genetic makeup of populations tends to change over time. This is evolution. It’s important to keep in mind that while both adaptation and evolution are examples of changes in response to organisms’ environments, they are not the same thing. Adaptation refers to non-genetic changes in individuals, while evolution refers to genetic changes in populations. Individuals adapt; populations evolve.
Unfortunately, the word “adaptation” is often used to refer both to physiological changes in individuals and to evolutionary changes in populations, which can be confusing. Some biologists distinguish between individual-level and population-level changes by using the word adaption to refer to any change in an individual that cannot be passed on to offspring, and the word adaptation to refer to a population-level change.
6.) Growth:
All organisms grow. That is, they take in matter and use it to make themselves larger. Of course, lots of things grow. Clouds grow as they accumulate moisture; crystals grow as they accumulate new matrix material; planets grow as their gravity attracts comets and asteroids. What distinguishes the growth of living organisms is that they take non-living matter into themselves and chemically convert it into living material.
7.) Reproduction:
Organisms – at least in principle – are capable of reproduction. All known organisms possess genetic material which determines their characteristics, and the genetic material they pass on to their children in turn determines their characteristics. So, offspring closely resemble their parents. Of course, individual organisms may be incapable of reproduction for one reason or another, but at the cellular level, all organisms can reproduce. Every time a single cell duplicates its genetic material and then divides into two, it reproduces. This is how growth of body tissues occurs in multicellular organisms. So, while a mule (for example) cannot reproduce in the sense that it cannot mate with another animal and produce offspring, the cells that make up its body are certainly capable of growing and reproducing, so no one argues that a mule isn’t alive.
Reproduction can either be asexual, meaning that the offspring in question has only one parent, or it can be sexual, meaning that genetic material from two parents is combined during production of a new individual. Because the genetic makeup of an offspring of sexually-reproducing parents is a random combination of its parents’ genetic makeup, it is genetically distinct from either parent. By contrast, an offspring of an asexually-reproducing parent is typically genetically identical to its parent.
Lots of organisms reproduce asexually, and many can switch from asexual reproduction to sexual reproduction at need. But if the evolutionary “goal” of an organism is to get as many copies of its genes into surviving offspring as possible, then why does sexual reproduction occur at all? After all, you “lose” half your genes every time you reproduce sexually; wouldn’t it be better to reproduce asexually, so that each of your children was a clone of yourself?
The answer seems to have to do with environmental change. Keep in mind that an individual’s evolutionary “goal” isn’t simply to reproduce, but to produce offspring that will survive and successfully reproduce, thus ensuring that the genetic line continues. (This doesn’t imply any sort of consciousness, of course; lineages in which organisms don’t behave as if they have the goal of producing successful offspring soon die out.)
Some animal species have populations that can reproduce either sexually or asexually. Experiments have shown that asexual populations tend to replace sexual populations in stable environments, just as would be expected (since individuals don’t lose half their genes every time they reproduce, good genetic combinations don’t get broken up during sexual recombination). But in unstable environments, the rate of genetic change in asexual populations is too slow to keep up with the changing environment, and asexual populations lose out to sexually-reproducing populations.
So, the advantage of sex appears to be that it allows for relatively rapid evolutionary change in response to a constantly-changing environment.
Energy and Life:
On the night of April 14, 1912, the passenger liner RMS Titanic sank in water that was at about 28° F (-2° C). Of the approximately 1,500 people who went into the water when the ship sank, fewer than a dozen survived. What killed them?
Most people would say that they drowned, but that’s not the case. Few, if any of those unfortunate people drowned. They were killed by a loss of energy.
Energy is defined as “the capacity to do mechanical work.” Mechanical work is the moving of matter from one place to another. In other words, if you want to move matter from one place to another – whether you’re talking about an elephant or a single oxygen molecule – you need a source of energy. Since energy can be stored in the bonds that link together atoms in molecules, breaking those bonds releases energy that can be used to do work. A great many of the metabolic reactions performed by organisms involve breaking larger molecules into smaller ones in order to harvest energy.
If left to itself, any complex and highly-organized system tends to become more disorganized over time. We’re all familiar with this process; clean your bedroom thoroughly and then leave it unattended for a sufficient span of time and it will become rather less organized. Dust will gather, the bedsheets will become rumpled, and so forth. The only way to cope with this inevitable loss of organization is to put energy into the system and move things around to restore them to their “proper” state of organization. In other words, your bed won’t make itself in the morning, no matter how long you wait, nor will the dust sweep itself off the floor.
The same thing applies to living things, which are very highly organized indeed. Deprive an organism of the energy it needs to build and maintain its highly complex structure, and it rapidly begins to lose its highly-organized state. We call this process “death.”
So, if you want your bedroom to remain neat and clean, you must expend energy to maintain it. Similarly, if you want your cat to remain in a highly-organized state (i.e., “alive”), you must supply it with energy in the form of food.
So what killed the Titanic passengers? The water they found themselves in was far colder than were their bodies, and heat is a form of energy. Since energy tends to “flow” from where it’s more concentrated to where it’s less concentrated, heat flowed from the passengers’ bodies into the surrounding water. (Strictly speaking, there’s no such thing as “cold.” A “cold” object is simply one whose molecules have less thermal energy – heat – on average than do those of a warmer object.) The people in the cold water quickly found that their bodies were unable to generate heat as quickly as it flowed out into the water, and so their body temperatures fell. Once their body temperatures fell below a certain point, they were no longer able to maintain the chemical reactions that kept them alive at a sufficient rate (because heat energy is necessary to keep those chemical reactions going at a rate sufficient to maintain life), and they died.
The importance of energy to living organisms cannot be overstated. You need energy to build and maintain body structures which will otherwise fall apart with time. You need energy to move everything from individual ions and molecules to your entire body. And you need energy to keep the very chemical reactions going that provide you with the energy you need for growth, maintenance, and repair of body tissues.
[B]Levels of Organization:
Ultimately, the human body is made up of chemicals. Chemicals, of course, are made up of atoms.
Atoms are made of smaller subunits known as protons, neutrons, and electrons. Protons are positively-charged particles found in the central portion of an atom, the nucleus. The number of protons in an atom’s nucleus determines its identity. For example, a hydrogen atom has one proton in its nucleus, a helium atom has two, and an oxygen atom has eight. Most atoms also contain uncharged neutrons in the nucleus. Outside the nucleus of an atom are found one or more negatively-charged electrons. The number of electrons in an atom is equal to the number of protons (by definition), and it’s the number and arrangement of electrons that determines an atom’s chemical properties.
Atoms can join together chemically to form molecules. Water (H2O), for example, is a common and very simple molecule, consisting of two hydrogen atoms bound to a single oxygen atom. The chemical properties of a molecule are often quite different from those of the atoms that make it up. For example, both hydrogen and oxygen are gases at body temperature (37° C), but water is a liquid. Sodium is a metal that reacts violently with water and chlorine is a poisonous, greenish-yellow gas, but they can combine chemically to produce table salt (NaCl).
Small molecules (called “monomers”) can be joined together in the body to form larger macromolecules or “polymers.” (The process of combining smaller molecules into larger ones is “polymerization.”) A protein, for example, is a macromolecule that can contain hundreds of thousands of atoms, and a DNA molecule may contain millions of atoms.
Molecules and macromolecules form cells, which are the basic units of life. A cell is the smallest thing that carries out all of the basic processes of life – growth, reproduction, metabolism, homeostasis, and response to its environment – which is why we refer to it as the basic unit of life. Humans, like all animals (by definition) are made up of many cells working together. Each of us started out as a single cell (a fertilized ovum) that grew and divided to ultimately produce a human being.
Humans and all other animals (as well as protists, fungi, and plants) are eukaryotes, meaning that their cells contain smaller structures called organelles that are bound within chemical membranes. This is as opposed to bacteria and archaea, whose cells don’t contain organelles. Different organelles within a cell perform different functions. For example, mitochondria are organelles that generate energy, and endoplasmic reticulum is an organelle that transports substances from one part of a cell to another.
Cells can be arranged into masses or layers that perform a common function. These are known as tissues. Tissues that are specialized for contracting are known as muscle tissue, for example.
Groups of different tissues that together form a structure that performs a distinct function make up an organ. The stomach is a good example of an organ, as it contains several different kinds of tissues functioning together.
Several organs working together to perform a common function make up an organ system. The reproductive system, for example, is an organ system, as is the nervous system.
Finally, all of the various organ systems make up an individual organism. In this case, you or me.
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Levels of organization in the human body. Atoms combine to form molecules, which
combine to form macromolecules. Molecules combine to form cells, the simplest
structures that are truly alive. Cells of the same type combine to form tissues,
and different types of tissues combine to form organs. Different organs that perform
a common function make up an organ system, and all of the various organ systems make up an individual organism.
[B]Organization of the Human Body:
At its most basic level, the human body is organized into an axial portion and an appendicular portion. The axial portion of your body consists of your head, neck, and trunk. The appendicular portion consists of your limbs – your arms and legs.
Within the axial portion of the body are two large cavities, the dorsal cavity and the ventral cavity. The organs within these cavities are known as the viscera.
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Body Cavities
The dorsal cavity contains the brain and the spinal cord, and is subdivided into two smaller cavities. The brain is found in the cranial cavity, which is surrounded by the bones that make up the skull. The spinal cord is found within the vertebral cavity (or spinal cavity), which is surrounded by the bony vertebrae (backbone).
The ventral cavity is rather larger than is the dorsal cavity, and it, too, is subdivided into two smaller cavities, the thoracic cavity and the abdominopelvic cavity. The thoracic cavity and abdominopelvic cavity are separated from each other by a broad, thin sheet of muscle known as the diaphragm, which is important in breathing.
The thoracic cavity contains the heart, lungs, esophagus, trachea, and thymus gland. It is divided into right and left halves by a band of fat and other connective tissues called the mediastinum. Embedded within the mediastinum is the pericardial cavity, containing the heart. The thymus gland, esophagus, and trachea are also embedded within the mediastinum. Each lung is embedded within a pleural cavity. (The pressure within each pleural cavity is actually slightly less than air pressure. Because of this, the greater pressure from the air in the lungs keeps the lungs inflated. If a pleural cavity is punctured and air gets in, causing a rise in pressure, this can cause the lung to collapse.)
The abdominopelvic cavity is subdivided into the abdominal cavity and the pelvic cavity. The viscera found within the abdominal cavity include the stomach, liver, spleen, gallbladder, and kidneys. The pelvic cavity is that portion of the abdominopelvic cavity enclosed by the hip bones, and it contains portions of the large and small intestines, the urinary bladder, and the internal reproductive organs.
In addition to these large cavities, there are several smaller body cavities. The kidneys, for example, are sometimes said to lie within the retroabdominal cavity, since they lie somewhat behind the main portion of the abdominal cavity and are separated from the rest of the abdominal organs by layers of connective tissue. In the head, the oral cavity contains the teeth and the tongue. The nasal cavity is located within the nose and contains several air-filled sinuses. The orbital cavities contain the eyes. The middle ear cavities contain the bones of the middle portion of the ears.
Within a given body cavity, the organs are surrounded and protected by thin membranes. Membranes typically secrete fluids that help keep organs moist and that also help protect against abrasion or shock.
Most body cavities contain two membranes with a layer of secreted fluid between them. Parietal membranes line the cavity itself, while visceral membranes surround the organs within the cavity. For example, each pleural cavity (which, as you recall, contains a lung) is lined by a parietal pleural membrane, and the lung within the cavity is surrounded by a visceral pleural membrane. The fluid secreted by the two membranes helps keep the lung moist and also lubricates it against damage that might occur from abrading the ribs.
Similarly, the pericardial cavity is lined by parietal pericardium and the heart is surrounded by visceral pericardium. Parietal peritoneum lines the abdominopelvic cavity and visceral peritoneum surrounds the organs within the addominopelvic cavity.
[B]Organ Systems:
The various organ systems, as noted above, consist of sets of interrelated organs that work together to perform specific functions. The functions of the organ systems include covering the body, providing support and movement, integrating and coordinating body functions, transportation of substances, absorption and excretion of materials, and reproduction. Hopefully, we’ll be able to discuss the functions of each body system in some detail by the time we’re finished with this series of articles.
[b]Body Covering:The organs of the integumentary system cover and protect the body. These organs include the skin (your largest and most massive organ) and various accessory organs and structures, including sweat glands, sebaceous glands, hair, and nails.
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The integumentary system forms the outer covering of the body, protects
deeper tissues, and synthesizes Vitamin D. It also contains receptors that
allow you to sense changes in your environment, plus sweat and oil glands.
Support and Movement:The organs of the skeletal system and of the muscular system support and move body parts. The skeletal system consists of bone and cartilage, as well as the ligaments that bind bones together. The skeletal system provides the framework for the body and protects underlying soft tissues. It also serves as an attachment point for muscles, allowing movement of the body.
Muscles are the main organs of the muscular system. When they contract, they grow shorter and pull on the body parts to which they’re connected. Muscles are also important in maintaining body posture. Contraction of muscles is an important source of body heat.
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The skeletal system forms the framework of the body. The muscular system
moves the body and also moves substances within the body.
Integration and Coordination:For the body to function properly, the different organs must perform in an integrated and coordinated manner. The nervous system and the endocrine system are primarily responsible for this integration and coordination.
The nervous system consists of the brain, spinal cord, nerves, and sense organs. Nerve cells (neurons) within these organs transmit electrochemical nerve impulses that allow rapid communication within the body and rapid response to external conditions.
The organs of the endocrine system secrete chemicals called hormones that travel through the blood to affect other organs. Hormones take much longer to function than do nerve impulses, but their effects are much longer-lasting.
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The nervous system and the endocrine system integrate and coordinate bodily functions.
Transportation:The two systems that are primarily responsible for transporting substances from one part of the body to another are the cardiovascular system (or circulatory system) and the lymphatic system. The cardiovascular system consists of the heart, arteries, veins, capillaries, and blood. It transports oxygen and nutrients to body tissues, and transports metabolic wastes such as carbon dioxide to the lungs, kidneys, or other organs for disposal. Blood also transports hormones from the endocrine organs that produce them to the “target organs” that are affected by them.
The lymphatic system consists of the lymphatic vessels, lymph fluid, lymph nodes, the thymus gland, and the spleen. The lymph transports fatty substances that cannot be transported effectively through the blood, returns lost fluid to the cardiovascular system, and helps transport disease-fighting “white blood cells” (especially lymphocytes) to regions of injury or infection.
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The cardiovascular system and the lymphatic system transport substances within the body.
Absorption and Excretion:Food is taken into the body to provide the raw materials for building body tissues. In addition food molecules are broken down for energy. Oxygen is also taken in, because it is a necessary component in the chemical reactions wherein larger molecules are broken into smaller molecules for energy.
The organs of the digestive system take in food and break it down into small molecules that can be transported through the blood and lymph and then absorbed into body cells where they will be used for energy production and for raw materials. Ingested matter that cannot be digested is eliminated from the body by the digestive system.
The respiratory system absorbs oxygen from the air and delivers it to the blood, which can then transport it to body tissues. The respiratory system also removes the metabolic waste product carbon dioxide from the blood and expels it from the body.
The urinary system removes metabolic waste products and other potentially harmful substances from the blood and dilutes them in water. The result is known as urine, which is excreted from the body.
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The digestive system takes in and processes food, then eliminates undigestible matter.
The respiratory system takes in oxygen and excretes harmful carbon dioxide.
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The urinary system removes metabolic wastes and other
potentially harmful substances from the body.
Reproduction:The reproductive system consists of the various organs associated with producing new offspring. The organs of the male reproductive system produce gametes known as sperm and are adapted for transferring the sperm into the female reproductive tract. The female reproductive system produces gametes known as ova (eggs) and is adapted for supporting a developing embryo.
One thing that it’s important to keep in mind is that because a given organ can have more than one function, some organs are parts of more than one organ system. For example, the urethra is part of both the urinary system and the reproductive system (in males, anyway). Similarly, the pancreas is part of both the endocrine system and the digestive system.
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The reproductive system produces and supports gametes.
Anatomical Terminology:
[B]The Anatomical Position:
You may have noticed that anatomical illustrations are almost always very similar in appearance. Specifically, when you see a person illustrated in an anatomy textbook, (s)he is almost always oriented such that the body is erect, the face is forward, the arms are at the sides of the body, and the palms of the hands are facing forward.
It’s no coincidence that subjects are oriented like this for purposes of illustration. This posture is known as the anatomical position, and the location of body structures is always assumed to refer to a person who is in the anatomical position. Also, we talk about “right” and “left,” we’re always referring to the subject’s right and left, not the viewer’s.
A person who is lying in the anatomical position and facing downward (that is, the person is lying on his/her belly) is said to be prone. A person who’s lying in the anatomical position but facing upward (that is, lying on his/her back) is said to be supine.
I remember reading a movie critic’s rather brutal assessment of a certain actress’ talents some time ago. Evidently, the reviewer thought that the actress’ fame had more to do with her looks and her willingness to disrobe on camera than with her acting ability. He claimed than in all of her movies she quickly went “from the ridiculous to the supine.”
Relative Positions of Body Structures:
Various terms are used to indicate the positions of body structures relative to one another. It’s important to keep in mind that each of these terms is relative, and means nothing by itself. It makes no sense to say that the elbow is proximal, for instance. Proximal to what?
Superior/Inferior: A body part is superior to another if it is above that part or is closer to the head. Similarly, a body part is inferior to another if it is below that part or is farther from the head. For example, the neck is superior to the chest, but inferior to the head.
Anterior/Posterior: Anterior means toward the front of the body, while posterior means toward the back of the body. For example, the eyes are anterior to the ears and the ears are posterior to the eyes.
Dorsal/Ventral: Dorsal means toward the back, while ventral means toward the belly. For example, the nose is ventral to the ears and the ears are dorsal to the eyes.
You may have noticed that the terms anterior and ventral are essentially identical in humans, as are the terms posterior and dorsal. That’s because humans stand erect. In non-human animals that typically walk on all fours, that is not the case. “Anterior” always means “toward the front of the body” and “posterior” always means “toward the back of the body.” Similarly, “ventral” always means “toward the belly” and “dorsal” always means “toward the back.” It’s just that “front” and “back” mean different things in animals that walk on all fours, compared to animals that walk upright.
Medial/Lateral: Medial refers to an imaginary line down the middle of the body that divides it into right and left halves. A structure is medial to another structure if it’s closer to the body midline. A structure is lateral to another structure if it’s farther away from the body midline. For example, the eyes are lateral to the nose and medial to the ears.
Proximal/Distal: A body part is proximal to another if it is closer to the point of attachment or to the trunk of the body. A body part is distal to another if it is farther from the point of attachment or to the trunk of the body. For example, the wrist is proximal to the fingers and distal to the elbow.
Superficial/Deep A body part is superficial (or external) to another if it is closer to the surface of the body. A body part is deep (or internal) to another if it is farther from the surface of the body. For example, the ribcage is superficial to the lungs and deep to the pectoral muscles.
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Our subjects have graciously agreed to demonstrate the anatomical position.
Some of the more common directional terms used in anatomy are also shown.
[B]Movement of Body Parts:
In addition to relative terms used to locate body parts in reference to each other, we also have specific terms used to indicate movement of body parts.
Supination/Pronation: Supination means to rotate the forearm so that it’s facing upward. That is, so that it’s in the anatomical position. Pronation, of course, means to rotate the forearm so that it’s facing downward. (The supinators are the muscles in the arm that rotate your arm upward, and the pronators are those which rotate it downward.)
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Elevation/Depression: Elevation means to move a body part upward, and depression means to lower a body part. For instance, someone might elevate his eyebrows in surprise and depress them when angered.
http://www.freethought-forum.com/forum/gallery/files/5/0/elevation.jpg
Protraction/Retraction: Protraction means to move a body part anteriorally (that is, toward the front of the body). Retraction means to move a body part posteriorally (that is, toward the back of the body).
http://www.freethought-forum.com/forum/gallery/files/5/0/protraction.jpg
Abduction/Adduction: Abduction means to move a body part away from the body, while adduction means to move a body part toward the body. (To help them remember the difference, I always tell my students that an “abductor” carries something away, while an “adductor” adds to something.)
http://www.freethought-forum.com/forum/gallery/files/5/0/adduct.jpg
Flexion/Extension: Flexion is bending a joint so that the angle of the joint grows smaller; usually, this means bending the joint so that the body part comes closer to the trunk of the body. Think of weight-lifters flexing their arms to make their biceps muscles bulge. Extension refers to changing the angle of a joint so that the angle of the joint grows larger; usually, this means moving the body part farther away from the trunk of the body. Think of someone extending his or her arm to wave to a departing loved-one. (If you extend a body part beyond where it’s located when you’re in the anatomical position, that is hyperextension.)
http://www.freethought-forum.com/forum/gallery/files/5/0/flexion.jpg
The arm on the left is flexed; the arm on the right is extended.
http://www.freethought-forum.com/forum/gallery/files/5/0/hyper.jpg
Circumduction: Circumduction is moving a body part so that it makes a circle or a cone. Circumduction does not mean rotating the body part; that is something different.
http://www.freethought-forum.com/forum/gallery/files/5/0/circumduction.jpg
Rotation: Rotation is turning a body part around its long axis. Rotating a body part away from the midline of the body is lateral rotation. Rotating a body part toward the midline of the body is medial rotation.
http://www.freethought-forum.com/forum/gallery/files/5/0/rotation.jpg
[B]Body Sections:
Any three-dimensional object can be divided into sections along three planes, and the human body is no different. When we refer to cutting or sectioning of the body, we refer to the three planes.
A sagittal section is a cut (real or imaginary) along the sagittal plane that divides the body into right and left halves. A sagittal section is also known as a median section. (Because it runs along the body midline and divides the head into right and left halves, the crest that runs along the top of the skull of a gorilla or a Klingon is known as a sagittal crest.)
A transverse section is a cut along the transverse plane that divides the body into superior and inferior portions. A transverse section is also known as a horizontal section.
A coronal section is a cut that divides the body into anterior and posterior portions. It is also known as a frontal section.
A diagonal cut is known as an oblique section.
http://www.freethought-forum.com/images/anatomy/bodyplanes.jpg
Body Planes and Sectioning the Body
[B]Body Regions:There are quite a lot of terms used to refer to different regions of the body. They can sound rather intimidating, but use of these terms allows a physician or anatomist to precisely indicate an area of interest.
The simplest way to indicate regions of the body is to divide the trunk of the body into four quadrants. To do this, draw an imaginary line down the midline of the body and a second imaginary line at a right angle to it that goes through the umbilicus (“navel” or “belly button”). This is a simple and easily-understood way to refer to different regions of the body, but it isn’t very precise.
http://www.freethought-forum.com/forum/gallery/files/5/0/quadrants.jpg
The four quadrants of the body.
A more precise way to indicate different areas of the body is to divide it into nine different regions. In the diagram below, the nine abdominopelvic regions are indicated.
http://www.freethought-forum.com/forum/gallery/files/5/0/regions_original.jpg
The nine abdominopelvic regions and some of the organs that can be found in each of them.
In the upper portion of the abdomen, the right hypochondriac and left hypochondriac regions flank the epigastric region. “Hypo” means “below,” and “chondriac” means “cartilage.” So the hypochondriac regions of the body are named for the fact that they lie more or less just below the costal cartilage that attaches the ribs to the sternum or breastbone.
The epigastric region lies between the hypochondriac regions. “Epi” means “above” and “gastric” refers to the “stomach.” So the epigastric region is named for its location just above the stomach. (A lot of people seem to be under the mistaken impression that the stomach lies in the lower portion of the abdomen. Actually, it’s right up under the tip of your breastbone, almost in your chest.)
In the middle portion of the abdomen, the right lumbar and left lumbar regions flank the umbilical region. “Lumbar” refers to the “lower back,” which is why this is referred to as the lumbar region. (The large vertebrae in the lower portion of your spine that support most of your weight are the lumbar vertebrae.)
Between the right and left lumbar regions is the umbilical region, where your umbilicus or “navel” is located. (As I’m sure most people know, the navel is where the umbilical cord was attached before your birth.)
In the lower portion of the abdomen, the right iliac (or right inguinal) and left iliac (left inguinal) regions flank the hypogastric (or pubic) region. The ilium is the portion of the pelvis that flares outward and makes up your hips, which is why these are known as the iliac regions. “Inguinal” refers to the “groin,” so the inguinal (iliac) regions make up the lateral boundaries of the groin.
Between the right and left iliac regions is the hypogastric region. “Hypo” means “below,” so this region is named for the fact that it lies below the stomach. It is also known as the pubic region because the pubic bones lie beneath it.
Here’s a more complete list of terms you might encounter that refer to specific regions of the body, and what those terms mean.
[B]Abdominal: The region between the thorax and the pelvis.
Acromial: The top of the shoulder.
Antebrachial: The forearm.
Antecubital: The space in front of the elbow.
Axillary: The armpit.
Brachial: The arm.
Buccal: The cheek.
Calcaneal: The heel.
Carpal: The wrist.
Celiac: The abdomen.
Cephalic: The head.
Cervical: The neck.
Costal: The ribs.
Coxal: The hips.
Cranial: The skull.
Crural: The (lower) leg.
Cubital: The elbow.
Digital: The fingers (and less commonly, the toes).
Dorsal: The back.
Facial: The face.
Femoral: The thighs.
Frontal: The forehead.
Genital: The external reproductive organs.
Gluteal: The buttocks.
Inguinal: The depressed area in the region of the groin.
Lumbar: The lower back.
Mammary: The breast.
Manual: The hand.
Mental: The chin.
Nasal: The nose.
Occipital: The back of the head.
Ocular: The eye.
Olecranal: The back of the elbow.
Oral: The mouth.
Orbital: The eye cavity.
Otic: The ear.
Palmar: The palm of the hand.
Patellar: The front of the knee.
Pectoral: The chest.
Pedal: The foot.
Pelvic: The pelvis.
Perineal: The region between the anus and the external genitalia.
Plantar: The sole of the foot.
Popliteal: The back of the knee.
Pubic: The region anterior to the pubis bones in the pelvis.
Sacral: The posterior region between the hips.
Sternal: The region above the breastbone.
Sural: The calf.
Tarsal: The ankles.
Thoracic: The chest.
Umbilical: The navel.
Vertebral: The spinal column.
http://www.freethought-forum.com/images/anatomy/ventrum.jpg
Regions of the Body (Ventral Surface)
http://www.freethought-forum.com/images/anatomy/dorsum.jpg
Regions of the Body (Dorsal Surface)
[B]A Final Note on Terminology:
Anatomy and physiology uses lots of scary-looking words, to be sure. This isn’t done to impress people, but because it’s important that the language used be both precise and standardized. After all, it’s rather important that a surgeon knows exactly where to cut as she begins an operation. “Somewhere in the region of the chest” just won’t suffice as instructions on where to make the first incision.
As intimidating as the terminology may appear at first, it’s actually not that difficult to learn. Almost all anatomical and physiological terms have Greek or Latin roots, and when you get used to what the prefixes and suffixes mean, you’ll discover that the term itself almost always tells you what it means or refers to. With that in mind, I’ll provide a list of some of the more common word roots below. You might find it helpful to refer to these terms from time to time in order to more easily understand the terms we’ll encounter from time to time.
[B]a– “absence” [Arrhythmia refers to absence of rhythm in the heartbeat.]
–able “capable” [A fetus is viable if it is capable of surviving outside of its mother’s body.]
acetabul– “vinegar cup” [The acetabulum is a cup-shaped depression in the hip into which the head of the femur bone fits.]
adip– “fat” [Adipose tissue stores fat.]
agglutin– “to glue together” [Agglutination is the clumping-together of blood cells.]
alb– “white” [An albino has white skin and hair.]
–algia “pain” [Neuralgia is a painful disorder of the cranial nerves.]
aliment– “food” [The alimentary canal contains and transports food.]
allant– “sausage-shaped” [The allantois is a membrane that surrounds a developing fetus.]
alveoli– “small cavity” [The alveoli are small sacs within the lungs.]
ambi– “both” [One who is ambidextrous can use both hands equally well.]
an– “without” [Anaerobic respiration takes place in the absence of oxygen.]
ana– “up” [Anabolic steroids cause buildup of muscle tissues.]
andr– “man” [Androgens are steroids that promote development of male characteristics.
ante– “in front of” [The antecubital region is the front of the elbow.]
anti– “against” [An anticoagulant prevents blood from coagulating.]
append– “to hang” [The appendix hangs off the intestinal wall.]
arthr– “joint” [Arthritis is inflammation of a joint.]
auto– “self” [An autoimmune disease occurs when the immune system attacks its own tissues.]
ax– “axis” [The axial skeleton makes up the framework of the midline of the body.]
bil– “bile” [Bilirubin is a chemical that is used to produce bile.
–blast “to create or grow” [Osteoblasts are cells that create new bone matrix.]
brachi– “arm” [The brachialis is a muscle in the arm.]
brady– “slow” [Bradycardia is an abnormally slow heart rate.]
bronch– “the windpipe” [The bronchial tubes connect the windpipe and the lungs.
calat– “something inserted”
calc– “stone” [A kidney stone is properly known as a calculus.]
calyc– “small cup” [Urine passes from the kidney into the ureter through small, cuplike structures called calyces.]
carcin– “cancer” [A carcinogen can cause cancer to occur.]
cardi– “heart” [The heart lies within the pericardial cavity.]
carp– “wrist” [The carpal bones are found in the wrist.]
cata– “down” [A catatonic person has much lower life functions than normal.]
cerebro– “brain” [The brain is bathed in cerebrospinal fluid.]
chondr– “cartilage” [The living cells within cartilage are known as chondrocytes.]
–cide “destroy” [A germicide kills germs.]
–clast “break” [Osteoclasts are cells that break down old or damaged bone.]
condyl– “knob” [Condyles are enlarged, knob-like portions at the ends of certain bones.]
corac– “beaklike” [The coracoid process is a beaklike projection from the scapula bone.]
cort– “covering” [The cortex of the kidney is the outer portion.]
cric– “ring” [Cricoid cartilage forms ring-like structures surrounding the windpipe and holding it open.]
crin– “to secrete” [Endocrine glands secrete hormones.]
crypt– “hidden” [Cryptorchidism occurs when a male’s testes fail to descend into the scrotum and are “hidden” inside the body cavity.]
cyan– “blue” [Cyanosis occurs when lack of oxygen causes the skin to turn blue.]
cyt– “cell” [Osteocytes are bone cells.]
de– “to undo” [Deamination is the removal of an amine group from a molecule.]
decidu– “falling off” [The deciduous teeth are shed during childhood.]
dendr– “tree” [The dendrites of a nerve cell look like the branches of a tree.]
derm– “skin” [A hypodermic needle passes through the skin to deliver drugs directly into the blood.]
di– “two” [A disaccharide is a sugar molecule made of two smaller sugar molecules.]
dis– “apart” [A joint is disarticulated when the bones are pulled away from each other.]
–ectomy “surgical removal” [Surgical removal of the appendix is an appendectomy.]
ede– “swelling” [Edema is swelling of body tissues because of excess fluid volume.]
ejacul– “to shoot forth” [Ejaculation occurs when semen is ejected from the male’s reproductive system.]
embol– “stopper or plug” [An embolism occurs when a blood clot prevents blood flow through a vessel.]
–emia “relating to the blood” [Anemia occurs when there are insufficient numbers of red blood cells in circulation.]
endo– “within” [The endometrium is the inner lining of the uterus.]
enter– “the intestine” [Enteritis is inflammation of the intestine.]
epi– “above or upon” [The epidermis is the outermost layer of skin.]
erythro– “red” [“Red blood cells” are properly known as erythrocytes.]
follic– “small bag” [Hairs grow out of structures known as follicles.]
fenestr– “window” [A fenestra is a relatively large opening in a bone, like a window.]
–ferent “to carry” [Afferent nerve fibers carry impulses toward the central nervous system.]
for– “opening” [A foramen is an opening in a bone through which blood vessels or nerves pass.]
–form “shaped” [The vermiform appendix is worm-shaped.]
foss– “to dig” [A fossa is a depression in a bone that looks as if it has been excavated.]
gangli– “swelling” [A ganglion is a cluster of nerve cells.]
gastr– “stomach” [Gastritis is inflammation of the stomach lining.]
–gen “to produce” [An allergen causes an allergic reaction.]
genesis– “origin” [Spermatogenesis is the process of sperm production.]
glom– “little ball” [Glomeruli are tangled masses of blood vessels in the kidneys that look like little balls.]
gloss– “tongue” [The glossopharyngeal nerve services the tongue.]
glyco– “sweet or sugar” [Glycolysis is the chemical breakdown of the sugar glucose for energy.]
gyn– “female” [Gynecology literally means “the study of women.”]
hem(at) – “blood” [Hemoglobin is a blood protein that transports oxygen, and haematopoiesis is the process by which blood cells are produced.]
hepat– “liver” [Hepatitis is inflammation of the liver.]
homeo– “the same” [Homeostasis is the process in which living organisms maintain near-constant internal environments.
hyper– “above” [Hypertension is abnormally high blood pressure.]
hypo– “below” [Hypotension is abnormally low blood pressure.]
ia– “abnormal state” [Hypoglycemia refers to abnormally low blood sugar levels.]
im– “not” [An imbalance means that things are not in balance.]
infra– “below” [The infraorbital region is that region of the face just beneath the eye sockets.]
inter– “between” [The interclavicular notch is the depression in your throat between the two clavicle bones.]
intra– “inside” [Intramembranous bones form inside membranes.]
iso– “equal” [An isotonic saline solution has the same salt concentration as your body tissues.]
–itis “inflammation” [Meningitis is inflammation of the tissues surrounding the brain.]
kerat– “horn” [Keratin is the protein that makes up the horns of goats – and your hair and fingernails.]
labi– “lip” [The labia are lip-like structures covering the external opening into the female reproductive tract.]
lacri– “tears” [Tears are produced by the lacrimal glands.]
lacun– “pool” [The cells in cartilage and bone are found in fluid-filled spaces called lacunae.]
leuko– “white” [“White blood cells” are properly known as leukocytes.]
lingu– “tongue” [The lingual tonsils are located at the root of the tongue.]
lip– “fat” [Lipases are enzymes that break down fat molecules.]
–logy “study of”
[B]–lysis “to dissolve or break down” [The venom of some snakes can cause hemolysis the breakdown of blood cells.]
–lyte “dissolvable” [Electrolytes dissolve in water and form electrically-charged subunits.]
macro– “large” [Macrophages are large white blood cells.]
mal– “bad” [A malignant tumor is rather bad for you.]
meat– “passage” [The auditory meatus is an passage in the skull which contains the internal portions of the ear.]
melan– “black” [Melanin is a dark pigment produced in the skin.]
mening– “membrane” [The meninges are membranes surrounding the brain and spinal cord.]
mens– “month” [The menstrual cycle takes roughly one month.]
meta– “change” [Metabolism involves lots of chemical changes.]
meta– “beyond” (Yeah, sometimes the same prefix has more than one meaning.) [The metacarpal bones are distal to the carpal bones.]
micro– “small” [A microscope can help you see very small things.]
mono– “one” [A monomer is a small molecule from which larger molecules can be constructed.]
mons– “mountain” [Mons pubis literally means “mountain of the groin.”]
morph– “shape or form” [An organism’s morphology is its body form.]
multi– “many” [A multinucleated cell has many nuclei.]
mut– “change” [A mutagen can cause genetic changes to occur.]
myo– “muscle” [Myosin is a protein found in muscle fibers.]
narc– “numbness or stupor” [Narcotics cause numbness.]
nat– “to be born” [Your natal day is the day you were born.]
necro– “dead or death” [Necrosis is the death of body tissue from injury or infection.]
neo– “new” [A neonate is a newborn.]
nephr– “kidney” [Nephritis is inflammation of the kidney.]
neuro– “nerve” [Neurons are nerve cells.]
nutri– “nourish” [Nutrients are quite nourishing.]
odont– “tooth” [The odontoid process is a tooth-shaped projection on the second cervical vertebra.]
oid– “shaped like” [The odontoid process is shaped like a tooth.]
olfact– “to smell” [Your olfactory nerves allow you to smell.]
–oma “tumor” [Lymphoma is cancer of lymphatic tissues.]
oo– “egg” [Oocytes are cells that develop into eggs.]
orchi– “testis” [Polyorchidism occurs when a man has more than two testes.]
oss– “bone” osseous tissue.]
[B]osteo– “bone” [Loss of bone mass can cause osteoporosis.]
oto– “ear” [An otolaryngologist specializes in the diagnosis and treatment of ailments of the ear, nose, and throat.]
ovo– “egg” [Ovolactovegetarians eat eggs and dairy products, but not meat.]
papill– “nipple” [Whoever decided that the small bumps on the tongue looked like nipples and decided to name them papillae had an interesting way of thinking.]
para– “beside” [The parathyroid glands are beside the thyroid glands.]
pariet– “wall” [Parietal membranes wall off body cavities.]
patho– “disease” [Pathogens are disease-causing agents.]
pelv– “basin” [The pelvis forms a basin of sorts that supports some of the internal organs.]
peri– “around” [Pericardial membranes surround the heart.]
phag– “to eat” [Phagocytes are cells that engulf and destroy bacteria and other invaders.]
pleur– “rib” [The pleural cavities lie within the ribcage.]
poly– “many” [A polysaccharide is a large carbohydrate molecule made by linking together many smaller carbohydrate molecules.]
post– “after” [Postnatal care occurs after birth.]
pre– “before” [Prenatal care occurs before birth.]
pseudo– “false” [Pseudostratified tissues look as if they’re laid down in layers at first glance, but they aren’t.]
puber– “adult” [Our bodies begin to acquire adult characteristics at puberty.]
pylor– “gatekeeper” [The pyloric sphincter prevents food from leaving the stomach too soon.]
rect– “straight” [The rectus abdominis is a straight, strap-like muscle in the abdomen.]
ren- “kidney” [Renal failure is when the kidneys stop functioning.]
retro– “behind or backward” [The kidneys are located in the retroabdominal cavity, behind the main portion of the abdominal cavity.]
rhino– “nose” [Rhinoviruses infect tissues in the nose and throat.]
–rrhea “flowing” [Rhinorrhea is a runny nose.]
sacchar– “sugar” [Sucrose is a well-known disaccharide.]
seb– “grease” [The sebaceous glands produce skin oil.]
squam– “flat or scale-like” [Squamous epithelium is made of flattened cells.]
–stasis “standing still” [Homeostasis involves keeping more or less constant body conditions.]
strat– “layer” [Stratified epithelium is layered.]
sub– “below” [Subcutaneous tissues lie beneath the outer layer of skin.]
super– supra– “above” [Superficial tissues lie above deep tissues.]
tachy– “fast” [Tachycardia is an abnormally fast heartbeat.]
tetan– “stiff” [Tetanus refers to stiffening of muscle fibers.]
thorac– “chest” [The thoracic cavity contains the heart and lungs.]
thrombo– “lump or clot” [Thrombocytes are cells that initiate clotting of the blood.]
–tropic “influencing” [Adrenocorticotropic hormone influences the adrenal cortex.]
tympan– “drum” [The tympanic membrane is also known as the eardrum.]
umbil– “navel” [The navel is where the umbilical cord joined the body.]
ur(o) – “urine” [The ureters transport urine from the kidneys to the urinary bladder.]
vas– “vessel” [The cardiovascular system consists of the heart and various blood vessels.]
viscer– “organ” [Visceral peritoneum surrounds the organs of the abdominopelvic cavity.]
vit– “life” [Vitamins are essential for life.]
zyg– “joining” [A zygote is formed when a sperm and an ovum join at conception.]
Chapter One: Life, Levels of Organization, and Relevant Terminology
Introduction:
I thought it might be fun to start a series of articles covering the subject of human anatomy and physiology. It’s a topic we can all relate to, after all, so hopefully it’ll be of some interest.
My intention isn’t to go into too much detail, but rather to provide what will hopefully be an accessible overview of the various body systems, what they do, and how they function. It goes without saying (I hope) that most of what applies to human A&P applies just as well to other vertebrates, but I thought it’d be best to concentrate on Homo sapiens, since that’s the one species of vertebrate in which all of us have a vested interest.
Of course, before we can get into the nuts and bolts of how the digestive system functions or what the “sliding filament theory” is about and other such interesting things, we have to first cover the basics. So, I thought I’d start out with a brief overview of some relevant material: what is “biology” and what are “anatomy” and “physiology”? What is “life” and how do we distinguish living things from non-living things? This will doubtless prove pretty basic for most people, but perhaps it will provide a good review. If nothing else, the review will hopefully help once we get to the more complicated stuff.
Biology, Anatomy, and Physiology:
Biology, as I’m sure everyone knows, is the study of life and of living things. You might be surprised at how difficult it is to define “life,” but we’ll get to that momentarily. Within the broader field of biology, anatomy is the study of the structure (the morphology) and the arrangement of body parts. Physiology is the study of the function of body parts – that is, what they do and how they do it.
An old saying amongst anatomists and physiologists is that “form follows function.” What that means is that the function of a body part will to some degree influence its structure and arrangement. For example, an organ adapted for gathering light and thus allowing you to see the world around you could hardly be made of an opaque material, nor could it be located deep within the body cavity. Similarly, a body structure that’s adapted for supporting your weight could scarcely be functional if it were made of a weak and flexible material. What a body structure does will necessarily influence what it’s made of, how it’s shaped, and where it’s located.
Because of the close association between form and function, it’s all but impossible to separate the fields of anatomy and physiology. That’s why it’s typically best to consider both the anatomy and the physiology of any given body structure simultaneously, since each sheds light on the other.
What is “Life” and What are the Characteristics of Living Things?:
Ask 100 biologists to define the word “life” and you’ll surely get at least 100 different answers. A quick consultation of a dictionary reveals that “life” is “the condition of living or the state of being alive.” Well that was helpful! Perhaps an alternate definition will be more illuminating. “Life” is “the period between birth and death.” That was even less helpful! (As an aside, you were alive well before your birth.)
The fact of the matter is that there is no universally agreed-upon definition of what it means to say that something is alive. So, if you consult a biology textbook in hopes of learning what it means to say that something is “alive” or has “life,” you’ll quickly discover that they usually avoid the question and instead give you a list of characteristics that most people agree are shared by living things.
In other words, about the best we can do is say, “things that have these characteristics are agreed to be alive – even though we don’t exactly agree on what ‘alive’ means.” Even then, there are things that we can’t agree upon. Is a virus alive? The answer varies according to whom you ask.
[B]The Characteristics of Living Things:
Living things are generally agreed to have these characteristics in common: complexity and organization, homeostasis, metabolism, irritability and adaptation, evolution, growth, and reproduction. It’s worth keeping in mind that not every living being shares all of these traits, however (indeed, no individual evolves), and there are non-living things that arguably share most of these characteristics. But that just gets us back to arguing about whether or not we should call things like viruses “alive.”
1.) Complexity and Organization:
Living creatures are [B]highly complex and highly organized. The organic and inorganic molecules that make up living creatures can be quite complex and are often made up of tens of thousands or even millions of subunits. Moreover, these molecules are arranged into larger and more complex units, which are arranged into even larger and more complex units . . . and so on. Not only are living creatures quite complex in their makeup, but they are very highly organized. In fact, this is why we refer to living creatures as “organisms.”
Neither complexity nor a high degree of organization means that a thing is alive, of course. Still, living things are both highly complex and very highly organized. In fact, living beings are the most complex chemical structures known. Chemically speaking, a single living cell is a much more complex object than is a star or a planet. On the other hand, while crystals, like living things, are very highly organized, they’re nowhere near as complex as are living things.
The basic unit of life is the cell. That is, it’s generally agreed that every living thing is made up of one or more cells. Most biologists regard things that behave like living things but aren’t cellular as non-living. By this definition, things like viruses and prions – though complex, highly organized, and made of organic molecules – are not alive.
2.) Homeostasis:
What makes living things so special is not just that they’re so complex and highly organized, but that they maintain their complexity and organization. This process by which living things maintain themselves in the face of a Universe which often seems quite hostile to complexity and to organization is known as homeostasis. Homeostasis refers to the fact that living beings actively maintain a more or less stable internal environment. The proper term for an organism that is no longer capable of homeostasis is “dead.”
For the most part, homeostasis occurs through the process of negative feedback. Negative feedback occurs when some portion of the products of a system are fed back into the system as input and cause a decrease in the system’s output of product. In this way, a system can automatically maintain itself at or near some set point.
That’s probably not very clear, so perhaps a couple of examples will help to clarify. An often-used example of a system that uses negative feedback to maintain itself is that of a furnace and a thermostat. The furnace and thermostat can be thought of as a system for producing heat. Once you set your thermostat for a particular temperature, if the room temperature is below the “target” temperature, your furnace produces heat and so the room temperature rises. When the room temperature reaches or exceeds the target temperature, heat production by the furnace decreases or stops entirely. If the room temperature subsequently falls below the target temperature again, the furnace will automatically come on and add more heat to the room. In this way, a very simple system can automatically maintain itself at whatever temperature you choose.
http://www.freethought-forum.com/images/anatomy/nfeedback.gif
Negative Feedback
Your body maintains its temperature in very much the same way. The “target” temperature is normally about 37º C, and the hypothalamus of the brain constantly monitors that temperature. If the temperature of blood flowing through the hypothalamus falls below 37º, the hypothalamus triggers physiological changes that cause decreased sweating, increased metabolic rate, constriction of blood vessels, and shivering. These physiological changes cause increased heat production and improved retention of body heat. The further below 37º is the blood flowing through the hypothalamus, the more intense are these physiological changes.
Conversely, if blood temperature rises above 37º, the hypothalamus triggers increased sweating and dilation of blood vessels, which increase heat loss and bring the temperature down.
3.) Metabolism:
All living organisms have some sort of metabolism. Metabolism is the process by which organisms convert nonliving material into cellular components and break down chemicals in order to generate energy which can be used to build and maintain body structures. The process of building cellular structures and other body components (which are not necessarily living matter – for example, your hair and fingernails are not living tissue) is known as synthesis, and the process of breaking down chemicals for energy or so that they can be reassembled into other chemicals is known as catalysis.
Sometimes, you’ll hear metabolism defined as “the sum total of all the chemical reactions occurring within a living organism that support life.” Those chemical reactions which involve breaking larger molecules into smaller molecules are collectively known as catabolism, and those chemical reactions which involve assembling larger molecules from smaller subunits are known as anabolism. (This is why hormones that promote growth of body tissues – say muscle tissue, for example – are known as anabolic hormones.)
4.) Irritability and Adaptation:
All living organisms show irritability and adaptation in response to stimuli. Irritability is the ability of an organism to sense and respond to things in its environment which will affect it – either positively or negatively. A plant might grow toward the sun, for instance, or a human might remove his hand from a hot stove. Closely related to the concept of irritability is that of adaptation, which refers to changes in an organism in response to its environment. For example, a human’s skin might grow darker in response to exposure to intense sunlight.
The term “adaptation” can be confusing, since it’s often used to describe both changes in individuals and to describe long-term changes in populations. Normally, changes to individuals that occur within their lifetimes cannot be passed on to offspring and so are not evolutionarily significant. In addition, these individual changes are typically reversible. For example, a tan will fade if the individual stays out of the sun. Environmental conditions can also cause long-term changes in the genetic makeup of a population that are not reversible in individual organisms. These changes over time in the genetic makeup of populations are certainly evolutionarily significant, since that’s what evolution is.
5.) Evolution:
Changes in populations over time, as noted, is evolution. Because organisms reproduce and pass on their traits to their offspring, populations can and do evolve over time. As a rule, those individuals who happen to possess traits that make them well-suited to their environments are more likely to survive and reproduce than are individuals who happen to possess traits that make them poorly-suited to their environments. Consequently, traits that are beneficial to their bearers tend to become more common over time because their bearers are likely to survive long-enough to pass them on to offspring, while traits that are harmful tend to become less common over time because their bearers tend not to survive long-enough to pass them on to offspring.
In this way, the genetic makeup of populations tends to change over time. This is evolution. It’s important to keep in mind that while both adaptation and evolution are examples of changes in response to organisms’ environments, they are not the same thing. Adaptation refers to non-genetic changes in individuals, while evolution refers to genetic changes in populations. Individuals adapt; populations evolve.
Unfortunately, the word “adaptation” is often used to refer both to physiological changes in individuals and to evolutionary changes in populations, which can be confusing. Some biologists distinguish between individual-level and population-level changes by using the word adaption to refer to any change in an individual that cannot be passed on to offspring, and the word adaptation to refer to a population-level change.
6.) Growth:
All organisms grow. That is, they take in matter and use it to make themselves larger. Of course, lots of things grow. Clouds grow as they accumulate moisture; crystals grow as they accumulate new matrix material; planets grow as their gravity attracts comets and asteroids. What distinguishes the growth of living organisms is that they take non-living matter into themselves and chemically convert it into living material.
7.) Reproduction:
Organisms – at least in principle – are capable of reproduction. All known organisms possess genetic material which determines their characteristics, and the genetic material they pass on to their children in turn determines their characteristics. So, offspring closely resemble their parents. Of course, individual organisms may be incapable of reproduction for one reason or another, but at the cellular level, all organisms can reproduce. Every time a single cell duplicates its genetic material and then divides into two, it reproduces. This is how growth of body tissues occurs in multicellular organisms. So, while a mule (for example) cannot reproduce in the sense that it cannot mate with another animal and produce offspring, the cells that make up its body are certainly capable of growing and reproducing, so no one argues that a mule isn’t alive.
Reproduction can either be asexual, meaning that the offspring in question has only one parent, or it can be sexual, meaning that genetic material from two parents is combined during production of a new individual. Because the genetic makeup of an offspring of sexually-reproducing parents is a random combination of its parents’ genetic makeup, it is genetically distinct from either parent. By contrast, an offspring of an asexually-reproducing parent is typically genetically identical to its parent.
Lots of organisms reproduce asexually, and many can switch from asexual reproduction to sexual reproduction at need. But if the evolutionary “goal” of an organism is to get as many copies of its genes into surviving offspring as possible, then why does sexual reproduction occur at all? After all, you “lose” half your genes every time you reproduce sexually; wouldn’t it be better to reproduce asexually, so that each of your children was a clone of yourself?
The answer seems to have to do with environmental change. Keep in mind that an individual’s evolutionary “goal” isn’t simply to reproduce, but to produce offspring that will survive and successfully reproduce, thus ensuring that the genetic line continues. (This doesn’t imply any sort of consciousness, of course; lineages in which organisms don’t behave as if they have the goal of producing successful offspring soon die out.)
Some animal species have populations that can reproduce either sexually or asexually. Experiments have shown that asexual populations tend to replace sexual populations in stable environments, just as would be expected (since individuals don’t lose half their genes every time they reproduce, good genetic combinations don’t get broken up during sexual recombination). But in unstable environments, the rate of genetic change in asexual populations is too slow to keep up with the changing environment, and asexual populations lose out to sexually-reproducing populations.
So, the advantage of sex appears to be that it allows for relatively rapid evolutionary change in response to a constantly-changing environment.
Energy and Life:
On the night of April 14, 1912, the passenger liner RMS Titanic sank in water that was at about 28° F (-2° C). Of the approximately 1,500 people who went into the water when the ship sank, fewer than a dozen survived. What killed them?
Most people would say that they drowned, but that’s not the case. Few, if any of those unfortunate people drowned. They were killed by a loss of energy.
Energy is defined as “the capacity to do mechanical work.” Mechanical work is the moving of matter from one place to another. In other words, if you want to move matter from one place to another – whether you’re talking about an elephant or a single oxygen molecule – you need a source of energy. Since energy can be stored in the bonds that link together atoms in molecules, breaking those bonds releases energy that can be used to do work. A great many of the metabolic reactions performed by organisms involve breaking larger molecules into smaller ones in order to harvest energy.
If left to itself, any complex and highly-organized system tends to become more disorganized over time. We’re all familiar with this process; clean your bedroom thoroughly and then leave it unattended for a sufficient span of time and it will become rather less organized. Dust will gather, the bedsheets will become rumpled, and so forth. The only way to cope with this inevitable loss of organization is to put energy into the system and move things around to restore them to their “proper” state of organization. In other words, your bed won’t make itself in the morning, no matter how long you wait, nor will the dust sweep itself off the floor.
The same thing applies to living things, which are very highly organized indeed. Deprive an organism of the energy it needs to build and maintain its highly complex structure, and it rapidly begins to lose its highly-organized state. We call this process “death.”
So, if you want your bedroom to remain neat and clean, you must expend energy to maintain it. Similarly, if you want your cat to remain in a highly-organized state (i.e., “alive”), you must supply it with energy in the form of food.
So what killed the Titanic passengers? The water they found themselves in was far colder than were their bodies, and heat is a form of energy. Since energy tends to “flow” from where it’s more concentrated to where it’s less concentrated, heat flowed from the passengers’ bodies into the surrounding water. (Strictly speaking, there’s no such thing as “cold.” A “cold” object is simply one whose molecules have less thermal energy – heat – on average than do those of a warmer object.) The people in the cold water quickly found that their bodies were unable to generate heat as quickly as it flowed out into the water, and so their body temperatures fell. Once their body temperatures fell below a certain point, they were no longer able to maintain the chemical reactions that kept them alive at a sufficient rate (because heat energy is necessary to keep those chemical reactions going at a rate sufficient to maintain life), and they died.
The importance of energy to living organisms cannot be overstated. You need energy to build and maintain body structures which will otherwise fall apart with time. You need energy to move everything from individual ions and molecules to your entire body. And you need energy to keep the very chemical reactions going that provide you with the energy you need for growth, maintenance, and repair of body tissues.
[B]Levels of Organization:
Ultimately, the human body is made up of chemicals. Chemicals, of course, are made up of atoms.
Atoms are made of smaller subunits known as protons, neutrons, and electrons. Protons are positively-charged particles found in the central portion of an atom, the nucleus. The number of protons in an atom’s nucleus determines its identity. For example, a hydrogen atom has one proton in its nucleus, a helium atom has two, and an oxygen atom has eight. Most atoms also contain uncharged neutrons in the nucleus. Outside the nucleus of an atom are found one or more negatively-charged electrons. The number of electrons in an atom is equal to the number of protons (by definition), and it’s the number and arrangement of electrons that determines an atom’s chemical properties.
Atoms can join together chemically to form molecules. Water (H2O), for example, is a common and very simple molecule, consisting of two hydrogen atoms bound to a single oxygen atom. The chemical properties of a molecule are often quite different from those of the atoms that make it up. For example, both hydrogen and oxygen are gases at body temperature (37° C), but water is a liquid. Sodium is a metal that reacts violently with water and chlorine is a poisonous, greenish-yellow gas, but they can combine chemically to produce table salt (NaCl).
Small molecules (called “monomers”) can be joined together in the body to form larger macromolecules or “polymers.” (The process of combining smaller molecules into larger ones is “polymerization.”) A protein, for example, is a macromolecule that can contain hundreds of thousands of atoms, and a DNA molecule may contain millions of atoms.
Molecules and macromolecules form cells, which are the basic units of life. A cell is the smallest thing that carries out all of the basic processes of life – growth, reproduction, metabolism, homeostasis, and response to its environment – which is why we refer to it as the basic unit of life. Humans, like all animals (by definition) are made up of many cells working together. Each of us started out as a single cell (a fertilized ovum) that grew and divided to ultimately produce a human being.
Humans and all other animals (as well as protists, fungi, and plants) are eukaryotes, meaning that their cells contain smaller structures called organelles that are bound within chemical membranes. This is as opposed to bacteria and archaea, whose cells don’t contain organelles. Different organelles within a cell perform different functions. For example, mitochondria are organelles that generate energy, and endoplasmic reticulum is an organelle that transports substances from one part of a cell to another.
Cells can be arranged into masses or layers that perform a common function. These are known as tissues. Tissues that are specialized for contracting are known as muscle tissue, for example.
Groups of different tissues that together form a structure that performs a distinct function make up an organ. The stomach is a good example of an organ, as it contains several different kinds of tissues functioning together.
Several organs working together to perform a common function make up an organ system. The reproductive system, for example, is an organ system, as is the nervous system.
Finally, all of the various organ systems make up an individual organism. In this case, you or me.
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Levels of organization in the human body. Atoms combine to form molecules, which
combine to form macromolecules. Molecules combine to form cells, the simplest
structures that are truly alive. Cells of the same type combine to form tissues,
and different types of tissues combine to form organs. Different organs that perform
a common function make up an organ system, and all of the various organ systems make up an individual organism.
[B]Organization of the Human Body:
At its most basic level, the human body is organized into an axial portion and an appendicular portion. The axial portion of your body consists of your head, neck, and trunk. The appendicular portion consists of your limbs – your arms and legs.
Within the axial portion of the body are two large cavities, the dorsal cavity and the ventral cavity. The organs within these cavities are known as the viscera.
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Body Cavities
The dorsal cavity contains the brain and the spinal cord, and is subdivided into two smaller cavities. The brain is found in the cranial cavity, which is surrounded by the bones that make up the skull. The spinal cord is found within the vertebral cavity (or spinal cavity), which is surrounded by the bony vertebrae (backbone).
The ventral cavity is rather larger than is the dorsal cavity, and it, too, is subdivided into two smaller cavities, the thoracic cavity and the abdominopelvic cavity. The thoracic cavity and abdominopelvic cavity are separated from each other by a broad, thin sheet of muscle known as the diaphragm, which is important in breathing.
The thoracic cavity contains the heart, lungs, esophagus, trachea, and thymus gland. It is divided into right and left halves by a band of fat and other connective tissues called the mediastinum. Embedded within the mediastinum is the pericardial cavity, containing the heart. The thymus gland, esophagus, and trachea are also embedded within the mediastinum. Each lung is embedded within a pleural cavity. (The pressure within each pleural cavity is actually slightly less than air pressure. Because of this, the greater pressure from the air in the lungs keeps the lungs inflated. If a pleural cavity is punctured and air gets in, causing a rise in pressure, this can cause the lung to collapse.)
The abdominopelvic cavity is subdivided into the abdominal cavity and the pelvic cavity. The viscera found within the abdominal cavity include the stomach, liver, spleen, gallbladder, and kidneys. The pelvic cavity is that portion of the abdominopelvic cavity enclosed by the hip bones, and it contains portions of the large and small intestines, the urinary bladder, and the internal reproductive organs.
In addition to these large cavities, there are several smaller body cavities. The kidneys, for example, are sometimes said to lie within the retroabdominal cavity, since they lie somewhat behind the main portion of the abdominal cavity and are separated from the rest of the abdominal organs by layers of connective tissue. In the head, the oral cavity contains the teeth and the tongue. The nasal cavity is located within the nose and contains several air-filled sinuses. The orbital cavities contain the eyes. The middle ear cavities contain the bones of the middle portion of the ears.
Within a given body cavity, the organs are surrounded and protected by thin membranes. Membranes typically secrete fluids that help keep organs moist and that also help protect against abrasion or shock.
Most body cavities contain two membranes with a layer of secreted fluid between them. Parietal membranes line the cavity itself, while visceral membranes surround the organs within the cavity. For example, each pleural cavity (which, as you recall, contains a lung) is lined by a parietal pleural membrane, and the lung within the cavity is surrounded by a visceral pleural membrane. The fluid secreted by the two membranes helps keep the lung moist and also lubricates it against damage that might occur from abrading the ribs.
Similarly, the pericardial cavity is lined by parietal pericardium and the heart is surrounded by visceral pericardium. Parietal peritoneum lines the abdominopelvic cavity and visceral peritoneum surrounds the organs within the addominopelvic cavity.
[B]Organ Systems:
The various organ systems, as noted above, consist of sets of interrelated organs that work together to perform specific functions. The functions of the organ systems include covering the body, providing support and movement, integrating and coordinating body functions, transportation of substances, absorption and excretion of materials, and reproduction. Hopefully, we’ll be able to discuss the functions of each body system in some detail by the time we’re finished with this series of articles.
[b]Body Covering:The organs of the integumentary system cover and protect the body. These organs include the skin (your largest and most massive organ) and various accessory organs and structures, including sweat glands, sebaceous glands, hair, and nails.
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The integumentary system forms the outer covering of the body, protects
deeper tissues, and synthesizes Vitamin D. It also contains receptors that
allow you to sense changes in your environment, plus sweat and oil glands.
Support and Movement:The organs of the skeletal system and of the muscular system support and move body parts. The skeletal system consists of bone and cartilage, as well as the ligaments that bind bones together. The skeletal system provides the framework for the body and protects underlying soft tissues. It also serves as an attachment point for muscles, allowing movement of the body.
Muscles are the main organs of the muscular system. When they contract, they grow shorter and pull on the body parts to which they’re connected. Muscles are also important in maintaining body posture. Contraction of muscles is an important source of body heat.
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The skeletal system forms the framework of the body. The muscular system
moves the body and also moves substances within the body.
Integration and Coordination:For the body to function properly, the different organs must perform in an integrated and coordinated manner. The nervous system and the endocrine system are primarily responsible for this integration and coordination.
The nervous system consists of the brain, spinal cord, nerves, and sense organs. Nerve cells (neurons) within these organs transmit electrochemical nerve impulses that allow rapid communication within the body and rapid response to external conditions.
The organs of the endocrine system secrete chemicals called hormones that travel through the blood to affect other organs. Hormones take much longer to function than do nerve impulses, but their effects are much longer-lasting.
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The nervous system and the endocrine system integrate and coordinate bodily functions.
Transportation:The two systems that are primarily responsible for transporting substances from one part of the body to another are the cardiovascular system (or circulatory system) and the lymphatic system. The cardiovascular system consists of the heart, arteries, veins, capillaries, and blood. It transports oxygen and nutrients to body tissues, and transports metabolic wastes such as carbon dioxide to the lungs, kidneys, or other organs for disposal. Blood also transports hormones from the endocrine organs that produce them to the “target organs” that are affected by them.
The lymphatic system consists of the lymphatic vessels, lymph fluid, lymph nodes, the thymus gland, and the spleen. The lymph transports fatty substances that cannot be transported effectively through the blood, returns lost fluid to the cardiovascular system, and helps transport disease-fighting “white blood cells” (especially lymphocytes) to regions of injury or infection.
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The cardiovascular system and the lymphatic system transport substances within the body.
Absorption and Excretion:Food is taken into the body to provide the raw materials for building body tissues. In addition food molecules are broken down for energy. Oxygen is also taken in, because it is a necessary component in the chemical reactions wherein larger molecules are broken into smaller molecules for energy.
The organs of the digestive system take in food and break it down into small molecules that can be transported through the blood and lymph and then absorbed into body cells where they will be used for energy production and for raw materials. Ingested matter that cannot be digested is eliminated from the body by the digestive system.
The respiratory system absorbs oxygen from the air and delivers it to the blood, which can then transport it to body tissues. The respiratory system also removes the metabolic waste product carbon dioxide from the blood and expels it from the body.
The urinary system removes metabolic waste products and other potentially harmful substances from the blood and dilutes them in water. The result is known as urine, which is excreted from the body.
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The digestive system takes in and processes food, then eliminates undigestible matter.
The respiratory system takes in oxygen and excretes harmful carbon dioxide.
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The urinary system removes metabolic wastes and other
potentially harmful substances from the body.
Reproduction:The reproductive system consists of the various organs associated with producing new offspring. The organs of the male reproductive system produce gametes known as sperm and are adapted for transferring the sperm into the female reproductive tract. The female reproductive system produces gametes known as ova (eggs) and is adapted for supporting a developing embryo.
One thing that it’s important to keep in mind is that because a given organ can have more than one function, some organs are parts of more than one organ system. For example, the urethra is part of both the urinary system and the reproductive system (in males, anyway). Similarly, the pancreas is part of both the endocrine system and the digestive system.
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The reproductive system produces and supports gametes.
Anatomical Terminology:
[B]The Anatomical Position:
You may have noticed that anatomical illustrations are almost always very similar in appearance. Specifically, when you see a person illustrated in an anatomy textbook, (s)he is almost always oriented such that the body is erect, the face is forward, the arms are at the sides of the body, and the palms of the hands are facing forward.
It’s no coincidence that subjects are oriented like this for purposes of illustration. This posture is known as the anatomical position, and the location of body structures is always assumed to refer to a person who is in the anatomical position. Also, we talk about “right” and “left,” we’re always referring to the subject’s right and left, not the viewer’s.
A person who is lying in the anatomical position and facing downward (that is, the person is lying on his/her belly) is said to be prone. A person who’s lying in the anatomical position but facing upward (that is, lying on his/her back) is said to be supine.
I remember reading a movie critic’s rather brutal assessment of a certain actress’ talents some time ago. Evidently, the reviewer thought that the actress’ fame had more to do with her looks and her willingness to disrobe on camera than with her acting ability. He claimed than in all of her movies she quickly went “from the ridiculous to the supine.”
Relative Positions of Body Structures:
Various terms are used to indicate the positions of body structures relative to one another. It’s important to keep in mind that each of these terms is relative, and means nothing by itself. It makes no sense to say that the elbow is proximal, for instance. Proximal to what?
Superior/Inferior: A body part is superior to another if it is above that part or is closer to the head. Similarly, a body part is inferior to another if it is below that part or is farther from the head. For example, the neck is superior to the chest, but inferior to the head.
Anterior/Posterior: Anterior means toward the front of the body, while posterior means toward the back of the body. For example, the eyes are anterior to the ears and the ears are posterior to the eyes.
Dorsal/Ventral: Dorsal means toward the back, while ventral means toward the belly. For example, the nose is ventral to the ears and the ears are dorsal to the eyes.
You may have noticed that the terms anterior and ventral are essentially identical in humans, as are the terms posterior and dorsal. That’s because humans stand erect. In non-human animals that typically walk on all fours, that is not the case. “Anterior” always means “toward the front of the body” and “posterior” always means “toward the back of the body.” Similarly, “ventral” always means “toward the belly” and “dorsal” always means “toward the back.” It’s just that “front” and “back” mean different things in animals that walk on all fours, compared to animals that walk upright.
Medial/Lateral: Medial refers to an imaginary line down the middle of the body that divides it into right and left halves. A structure is medial to another structure if it’s closer to the body midline. A structure is lateral to another structure if it’s farther away from the body midline. For example, the eyes are lateral to the nose and medial to the ears.
Proximal/Distal: A body part is proximal to another if it is closer to the point of attachment or to the trunk of the body. A body part is distal to another if it is farther from the point of attachment or to the trunk of the body. For example, the wrist is proximal to the fingers and distal to the elbow.
Superficial/Deep A body part is superficial (or external) to another if it is closer to the surface of the body. A body part is deep (or internal) to another if it is farther from the surface of the body. For example, the ribcage is superficial to the lungs and deep to the pectoral muscles.
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Our subjects have graciously agreed to demonstrate the anatomical position.
Some of the more common directional terms used in anatomy are also shown.
[B]Movement of Body Parts:
In addition to relative terms used to locate body parts in reference to each other, we also have specific terms used to indicate movement of body parts.
Supination/Pronation: Supination means to rotate the forearm so that it’s facing upward. That is, so that it’s in the anatomical position. Pronation, of course, means to rotate the forearm so that it’s facing downward. (The supinators are the muscles in the arm that rotate your arm upward, and the pronators are those which rotate it downward.)
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Elevation/Depression: Elevation means to move a body part upward, and depression means to lower a body part. For instance, someone might elevate his eyebrows in surprise and depress them when angered.
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Protraction/Retraction: Protraction means to move a body part anteriorally (that is, toward the front of the body). Retraction means to move a body part posteriorally (that is, toward the back of the body).
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Abduction/Adduction: Abduction means to move a body part away from the body, while adduction means to move a body part toward the body. (To help them remember the difference, I always tell my students that an “abductor” carries something away, while an “adductor” adds to something.)
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Flexion/Extension: Flexion is bending a joint so that the angle of the joint grows smaller; usually, this means bending the joint so that the body part comes closer to the trunk of the body. Think of weight-lifters flexing their arms to make their biceps muscles bulge. Extension refers to changing the angle of a joint so that the angle of the joint grows larger; usually, this means moving the body part farther away from the trunk of the body. Think of someone extending his or her arm to wave to a departing loved-one. (If you extend a body part beyond where it’s located when you’re in the anatomical position, that is hyperextension.)
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The arm on the left is flexed; the arm on the right is extended.
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Circumduction: Circumduction is moving a body part so that it makes a circle or a cone. Circumduction does not mean rotating the body part; that is something different.
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Rotation: Rotation is turning a body part around its long axis. Rotating a body part away from the midline of the body is lateral rotation. Rotating a body part toward the midline of the body is medial rotation.
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[B]Body Sections:
Any three-dimensional object can be divided into sections along three planes, and the human body is no different. When we refer to cutting or sectioning of the body, we refer to the three planes.
A sagittal section is a cut (real or imaginary) along the sagittal plane that divides the body into right and left halves. A sagittal section is also known as a median section. (Because it runs along the body midline and divides the head into right and left halves, the crest that runs along the top of the skull of a gorilla or a Klingon is known as a sagittal crest.)
A transverse section is a cut along the transverse plane that divides the body into superior and inferior portions. A transverse section is also known as a horizontal section.
A coronal section is a cut that divides the body into anterior and posterior portions. It is also known as a frontal section.
A diagonal cut is known as an oblique section.
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Body Planes and Sectioning the Body
[B]Body Regions:There are quite a lot of terms used to refer to different regions of the body. They can sound rather intimidating, but use of these terms allows a physician or anatomist to precisely indicate an area of interest.
The simplest way to indicate regions of the body is to divide the trunk of the body into four quadrants. To do this, draw an imaginary line down the midline of the body and a second imaginary line at a right angle to it that goes through the umbilicus (“navel” or “belly button”). This is a simple and easily-understood way to refer to different regions of the body, but it isn’t very precise.
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The four quadrants of the body.
A more precise way to indicate different areas of the body is to divide it into nine different regions. In the diagram below, the nine abdominopelvic regions are indicated.
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The nine abdominopelvic regions and some of the organs that can be found in each of them.
In the upper portion of the abdomen, the right hypochondriac and left hypochondriac regions flank the epigastric region. “Hypo” means “below,” and “chondriac” means “cartilage.” So the hypochondriac regions of the body are named for the fact that they lie more or less just below the costal cartilage that attaches the ribs to the sternum or breastbone.
The epigastric region lies between the hypochondriac regions. “Epi” means “above” and “gastric” refers to the “stomach.” So the epigastric region is named for its location just above the stomach. (A lot of people seem to be under the mistaken impression that the stomach lies in the lower portion of the abdomen. Actually, it’s right up under the tip of your breastbone, almost in your chest.)
In the middle portion of the abdomen, the right lumbar and left lumbar regions flank the umbilical region. “Lumbar” refers to the “lower back,” which is why this is referred to as the lumbar region. (The large vertebrae in the lower portion of your spine that support most of your weight are the lumbar vertebrae.)
Between the right and left lumbar regions is the umbilical region, where your umbilicus or “navel” is located. (As I’m sure most people know, the navel is where the umbilical cord was attached before your birth.)
In the lower portion of the abdomen, the right iliac (or right inguinal) and left iliac (left inguinal) regions flank the hypogastric (or pubic) region. The ilium is the portion of the pelvis that flares outward and makes up your hips, which is why these are known as the iliac regions. “Inguinal” refers to the “groin,” so the inguinal (iliac) regions make up the lateral boundaries of the groin.
Between the right and left iliac regions is the hypogastric region. “Hypo” means “below,” so this region is named for the fact that it lies below the stomach. It is also known as the pubic region because the pubic bones lie beneath it.
Here’s a more complete list of terms you might encounter that refer to specific regions of the body, and what those terms mean.
[B]Abdominal: The region between the thorax and the pelvis.
Acromial: The top of the shoulder.
Antebrachial: The forearm.
Antecubital: The space in front of the elbow.
Axillary: The armpit.
Brachial: The arm.
Buccal: The cheek.
Calcaneal: The heel.
Carpal: The wrist.
Celiac: The abdomen.
Cephalic: The head.
Cervical: The neck.
Costal: The ribs.
Coxal: The hips.
Cranial: The skull.
Crural: The (lower) leg.
Cubital: The elbow.
Digital: The fingers (and less commonly, the toes).
Dorsal: The back.
Facial: The face.
Femoral: The thighs.
Frontal: The forehead.
Genital: The external reproductive organs.
Gluteal: The buttocks.
Inguinal: The depressed area in the region of the groin.
Lumbar: The lower back.
Mammary: The breast.
Manual: The hand.
Mental: The chin.
Nasal: The nose.
Occipital: The back of the head.
Ocular: The eye.
Olecranal: The back of the elbow.
Oral: The mouth.
Orbital: The eye cavity.
Otic: The ear.
Palmar: The palm of the hand.
Patellar: The front of the knee.
Pectoral: The chest.
Pedal: The foot.
Pelvic: The pelvis.
Perineal: The region between the anus and the external genitalia.
Plantar: The sole of the foot.
Popliteal: The back of the knee.
Pubic: The region anterior to the pubis bones in the pelvis.
Sacral: The posterior region between the hips.
Sternal: The region above the breastbone.
Sural: The calf.
Tarsal: The ankles.
Thoracic: The chest.
Umbilical: The navel.
Vertebral: The spinal column.
http://www.freethought-forum.com/images/anatomy/ventrum.jpg
Regions of the Body (Ventral Surface)
http://www.freethought-forum.com/images/anatomy/dorsum.jpg
Regions of the Body (Dorsal Surface)
[B]A Final Note on Terminology:
Anatomy and physiology uses lots of scary-looking words, to be sure. This isn’t done to impress people, but because it’s important that the language used be both precise and standardized. After all, it’s rather important that a surgeon knows exactly where to cut as she begins an operation. “Somewhere in the region of the chest” just won’t suffice as instructions on where to make the first incision.
As intimidating as the terminology may appear at first, it’s actually not that difficult to learn. Almost all anatomical and physiological terms have Greek or Latin roots, and when you get used to what the prefixes and suffixes mean, you’ll discover that the term itself almost always tells you what it means or refers to. With that in mind, I’ll provide a list of some of the more common word roots below. You might find it helpful to refer to these terms from time to time in order to more easily understand the terms we’ll encounter from time to time.
[B]a– “absence” [Arrhythmia refers to absence of rhythm in the heartbeat.]
–able “capable” [A fetus is viable if it is capable of surviving outside of its mother’s body.]
acetabul– “vinegar cup” [The acetabulum is a cup-shaped depression in the hip into which the head of the femur bone fits.]
adip– “fat” [Adipose tissue stores fat.]
agglutin– “to glue together” [Agglutination is the clumping-together of blood cells.]
alb– “white” [An albino has white skin and hair.]
–algia “pain” [Neuralgia is a painful disorder of the cranial nerves.]
aliment– “food” [The alimentary canal contains and transports food.]
allant– “sausage-shaped” [The allantois is a membrane that surrounds a developing fetus.]
alveoli– “small cavity” [The alveoli are small sacs within the lungs.]
ambi– “both” [One who is ambidextrous can use both hands equally well.]
an– “without” [Anaerobic respiration takes place in the absence of oxygen.]
ana– “up” [Anabolic steroids cause buildup of muscle tissues.]
andr– “man” [Androgens are steroids that promote development of male characteristics.
ante– “in front of” [The antecubital region is the front of the elbow.]
anti– “against” [An anticoagulant prevents blood from coagulating.]
append– “to hang” [The appendix hangs off the intestinal wall.]
arthr– “joint” [Arthritis is inflammation of a joint.]
auto– “self” [An autoimmune disease occurs when the immune system attacks its own tissues.]
ax– “axis” [The axial skeleton makes up the framework of the midline of the body.]
bil– “bile” [Bilirubin is a chemical that is used to produce bile.
–blast “to create or grow” [Osteoblasts are cells that create new bone matrix.]
brachi– “arm” [The brachialis is a muscle in the arm.]
brady– “slow” [Bradycardia is an abnormally slow heart rate.]
bronch– “the windpipe” [The bronchial tubes connect the windpipe and the lungs.
calat– “something inserted”
calc– “stone” [A kidney stone is properly known as a calculus.]
calyc– “small cup” [Urine passes from the kidney into the ureter through small, cuplike structures called calyces.]
carcin– “cancer” [A carcinogen can cause cancer to occur.]
cardi– “heart” [The heart lies within the pericardial cavity.]
carp– “wrist” [The carpal bones are found in the wrist.]
cata– “down” [A catatonic person has much lower life functions than normal.]
cerebro– “brain” [The brain is bathed in cerebrospinal fluid.]
chondr– “cartilage” [The living cells within cartilage are known as chondrocytes.]
–cide “destroy” [A germicide kills germs.]
–clast “break” [Osteoclasts are cells that break down old or damaged bone.]
condyl– “knob” [Condyles are enlarged, knob-like portions at the ends of certain bones.]
corac– “beaklike” [The coracoid process is a beaklike projection from the scapula bone.]
cort– “covering” [The cortex of the kidney is the outer portion.]
cric– “ring” [Cricoid cartilage forms ring-like structures surrounding the windpipe and holding it open.]
crin– “to secrete” [Endocrine glands secrete hormones.]
crypt– “hidden” [Cryptorchidism occurs when a male’s testes fail to descend into the scrotum and are “hidden” inside the body cavity.]
cyan– “blue” [Cyanosis occurs when lack of oxygen causes the skin to turn blue.]
cyt– “cell” [Osteocytes are bone cells.]
de– “to undo” [Deamination is the removal of an amine group from a molecule.]
decidu– “falling off” [The deciduous teeth are shed during childhood.]
dendr– “tree” [The dendrites of a nerve cell look like the branches of a tree.]
derm– “skin” [A hypodermic needle passes through the skin to deliver drugs directly into the blood.]
di– “two” [A disaccharide is a sugar molecule made of two smaller sugar molecules.]
dis– “apart” [A joint is disarticulated when the bones are pulled away from each other.]
–ectomy “surgical removal” [Surgical removal of the appendix is an appendectomy.]
ede– “swelling” [Edema is swelling of body tissues because of excess fluid volume.]
ejacul– “to shoot forth” [Ejaculation occurs when semen is ejected from the male’s reproductive system.]
embol– “stopper or plug” [An embolism occurs when a blood clot prevents blood flow through a vessel.]
–emia “relating to the blood” [Anemia occurs when there are insufficient numbers of red blood cells in circulation.]
endo– “within” [The endometrium is the inner lining of the uterus.]
enter– “the intestine” [Enteritis is inflammation of the intestine.]
epi– “above or upon” [The epidermis is the outermost layer of skin.]
erythro– “red” [“Red blood cells” are properly known as erythrocytes.]
follic– “small bag” [Hairs grow out of structures known as follicles.]
fenestr– “window” [A fenestra is a relatively large opening in a bone, like a window.]
–ferent “to carry” [Afferent nerve fibers carry impulses toward the central nervous system.]
for– “opening” [A foramen is an opening in a bone through which blood vessels or nerves pass.]
–form “shaped” [The vermiform appendix is worm-shaped.]
foss– “to dig” [A fossa is a depression in a bone that looks as if it has been excavated.]
gangli– “swelling” [A ganglion is a cluster of nerve cells.]
gastr– “stomach” [Gastritis is inflammation of the stomach lining.]
–gen “to produce” [An allergen causes an allergic reaction.]
genesis– “origin” [Spermatogenesis is the process of sperm production.]
glom– “little ball” [Glomeruli are tangled masses of blood vessels in the kidneys that look like little balls.]
gloss– “tongue” [The glossopharyngeal nerve services the tongue.]
glyco– “sweet or sugar” [Glycolysis is the chemical breakdown of the sugar glucose for energy.]
gyn– “female” [Gynecology literally means “the study of women.”]
hem(at) – “blood” [Hemoglobin is a blood protein that transports oxygen, and haematopoiesis is the process by which blood cells are produced.]
hepat– “liver” [Hepatitis is inflammation of the liver.]
homeo– “the same” [Homeostasis is the process in which living organisms maintain near-constant internal environments.
hyper– “above” [Hypertension is abnormally high blood pressure.]
hypo– “below” [Hypotension is abnormally low blood pressure.]
ia– “abnormal state” [Hypoglycemia refers to abnormally low blood sugar levels.]
im– “not” [An imbalance means that things are not in balance.]
infra– “below” [The infraorbital region is that region of the face just beneath the eye sockets.]
inter– “between” [The interclavicular notch is the depression in your throat between the two clavicle bones.]
intra– “inside” [Intramembranous bones form inside membranes.]
iso– “equal” [An isotonic saline solution has the same salt concentration as your body tissues.]
–itis “inflammation” [Meningitis is inflammation of the tissues surrounding the brain.]
kerat– “horn” [Keratin is the protein that makes up the horns of goats – and your hair and fingernails.]
labi– “lip” [The labia are lip-like structures covering the external opening into the female reproductive tract.]
lacri– “tears” [Tears are produced by the lacrimal glands.]
lacun– “pool” [The cells in cartilage and bone are found in fluid-filled spaces called lacunae.]
leuko– “white” [“White blood cells” are properly known as leukocytes.]
lingu– “tongue” [The lingual tonsils are located at the root of the tongue.]
lip– “fat” [Lipases are enzymes that break down fat molecules.]
–logy “study of”
[B]–lysis “to dissolve or break down” [The venom of some snakes can cause hemolysis the breakdown of blood cells.]
–lyte “dissolvable” [Electrolytes dissolve in water and form electrically-charged subunits.]
macro– “large” [Macrophages are large white blood cells.]
mal– “bad” [A malignant tumor is rather bad for you.]
meat– “passage” [The auditory meatus is an passage in the skull which contains the internal portions of the ear.]
melan– “black” [Melanin is a dark pigment produced in the skin.]
mening– “membrane” [The meninges are membranes surrounding the brain and spinal cord.]
mens– “month” [The menstrual cycle takes roughly one month.]
meta– “change” [Metabolism involves lots of chemical changes.]
meta– “beyond” (Yeah, sometimes the same prefix has more than one meaning.) [The metacarpal bones are distal to the carpal bones.]
micro– “small” [A microscope can help you see very small things.]
mono– “one” [A monomer is a small molecule from which larger molecules can be constructed.]
mons– “mountain” [Mons pubis literally means “mountain of the groin.”]
morph– “shape or form” [An organism’s morphology is its body form.]
multi– “many” [A multinucleated cell has many nuclei.]
mut– “change” [A mutagen can cause genetic changes to occur.]
myo– “muscle” [Myosin is a protein found in muscle fibers.]
narc– “numbness or stupor” [Narcotics cause numbness.]
nat– “to be born” [Your natal day is the day you were born.]
necro– “dead or death” [Necrosis is the death of body tissue from injury or infection.]
neo– “new” [A neonate is a newborn.]
nephr– “kidney” [Nephritis is inflammation of the kidney.]
neuro– “nerve” [Neurons are nerve cells.]
nutri– “nourish” [Nutrients are quite nourishing.]
odont– “tooth” [The odontoid process is a tooth-shaped projection on the second cervical vertebra.]
oid– “shaped like” [The odontoid process is shaped like a tooth.]
olfact– “to smell” [Your olfactory nerves allow you to smell.]
–oma “tumor” [Lymphoma is cancer of lymphatic tissues.]
oo– “egg” [Oocytes are cells that develop into eggs.]
orchi– “testis” [Polyorchidism occurs when a man has more than two testes.]
oss– “bone” osseous tissue.]
[B]osteo– “bone” [Loss of bone mass can cause osteoporosis.]
oto– “ear” [An otolaryngologist specializes in the diagnosis and treatment of ailments of the ear, nose, and throat.]
ovo– “egg” [Ovolactovegetarians eat eggs and dairy products, but not meat.]
papill– “nipple” [Whoever decided that the small bumps on the tongue looked like nipples and decided to name them papillae had an interesting way of thinking.]
para– “beside” [The parathyroid glands are beside the thyroid glands.]
pariet– “wall” [Parietal membranes wall off body cavities.]
patho– “disease” [Pathogens are disease-causing agents.]
pelv– “basin” [The pelvis forms a basin of sorts that supports some of the internal organs.]
peri– “around” [Pericardial membranes surround the heart.]
phag– “to eat” [Phagocytes are cells that engulf and destroy bacteria and other invaders.]
pleur– “rib” [The pleural cavities lie within the ribcage.]
poly– “many” [A polysaccharide is a large carbohydrate molecule made by linking together many smaller carbohydrate molecules.]
post– “after” [Postnatal care occurs after birth.]
pre– “before” [Prenatal care occurs before birth.]
pseudo– “false” [Pseudostratified tissues look as if they’re laid down in layers at first glance, but they aren’t.]
puber– “adult” [Our bodies begin to acquire adult characteristics at puberty.]
pylor– “gatekeeper” [The pyloric sphincter prevents food from leaving the stomach too soon.]
rect– “straight” [The rectus abdominis is a straight, strap-like muscle in the abdomen.]
ren- “kidney” [Renal failure is when the kidneys stop functioning.]
retro– “behind or backward” [The kidneys are located in the retroabdominal cavity, behind the main portion of the abdominal cavity.]
rhino– “nose” [Rhinoviruses infect tissues in the nose and throat.]
–rrhea “flowing” [Rhinorrhea is a runny nose.]
sacchar– “sugar” [Sucrose is a well-known disaccharide.]
seb– “grease” [The sebaceous glands produce skin oil.]
squam– “flat or scale-like” [Squamous epithelium is made of flattened cells.]
–stasis “standing still” [Homeostasis involves keeping more or less constant body conditions.]
strat– “layer” [Stratified epithelium is layered.]
sub– “below” [Subcutaneous tissues lie beneath the outer layer of skin.]
super– supra– “above” [Superficial tissues lie above deep tissues.]
tachy– “fast” [Tachycardia is an abnormally fast heartbeat.]
tetan– “stiff” [Tetanus refers to stiffening of muscle fibers.]
thorac– “chest” [The thoracic cavity contains the heart and lungs.]
thrombo– “lump or clot” [Thrombocytes are cells that initiate clotting of the blood.]
–tropic “influencing” [Adrenocorticotropic hormone influences the adrenal cortex.]
tympan– “drum” [The tympanic membrane is also known as the eardrum.]
umbil– “navel” [The navel is where the umbilical cord joined the body.]
ur(o) – “urine” [The ureters transport urine from the kidneys to the urinary bladder.]
vas– “vessel” [The cardiovascular system consists of the heart and various blood vessels.]
viscer– “organ” [Visceral peritoneum surrounds the organs of the abdominopelvic cavity.]
vit– “life” [Vitamins are essential for life.]
zyg– “joining” [A zygote is formed when a sperm and an ovum join at conception.]