An Introduction to Human Anatomy and Physiology
Chapter Six: Tissues, Membranes, Organs, and Organ Systems
So, the fundamental unit of life is the cell. For many organisms, that’s as complex as it gets. In fact, the great majority of living organisms are single-celled. However, most of the organisms we’re familiar with are comprised of millions, billions, or even trillions of cells. In multicellular animals such as ourselves, the cells are organized into tissues
, and organ systems
. All of these tissues, etc. work together to make up a living organism. It’s to these structures that we’ll now turn our attention. You were probably wondering when we’d start talking about some actual anatomy
, weren’t you?
is a structure made up of cells of the same general type, in which the cells perform a common function. In animals such as ourselves, there are four basic types of tissues. Epithelial tissue
covers body surfaces and lines body cavities. Connective tissue
binds body structures together, provides structural support and protection, stores substances, and fills body spaces. Muscle tissue
is specialized to contract and to cause movement. Nervous tissue
is specialized for transmitting information throughout the body and for coordinating body movements.
Epithelial Tissue (Epithelium):Epithelial tissue
(also called epithelium
) covers body surfaces. It also lines body cavities and covers many of the internal organs. It forms a protective layer around body organs (and the epidermis of the skin protects the entire body against water loss and against invasion by pathogenic organisms), and is often specialized for secreting substances.
Epithelium lines various body tubes, including blood vessels, the passageways in the lungs, the kidney tubules, and the digestive tract. The epithelium lining the inside of body tubes such as blood vessels is sometimes referred to as endothelium
. (The hollow space within a body tube is known as a lumen
Epithelium almost always forms relatively thin sheets of tissue, and the underside of epithelium is typically attached to an underlying layer of connective tissue by a non-cellular structure (largely made up of protein fibers) called the basal lamina
or basement membrane
. (The term “basement membrane” isn’t used too often nowadays, since the basal lamina isn’t a true membrane.) You can think of the basal lamina as consisting of protein fibers (especially collagen
) that penetrate into both the epithelium above and the connective tissue below and so knit the tissues together.
The cells of epithelium are typically packed together very tightly, and blood vessels don’t normally penetrate into it. On the other hand, since epithelium tends to be rather thin and because the connective tissue that underlies it is well-supplied with blood vessels, the cells that make up epithelium are never far from blood vessels and so are well-supplied with oxygen and nutrients. This is important because epithelial tissue is often damaged. Fortunately, the cells that make up epithelium generally grow and reproduce quite rapidly, and so epithelial tissues are quick to heal when injured.
Epithelial tissues are generally classified according to two factors: the arrangement
of the cells that make them up, and the shape
of the cells. Simple
epithelium consists of a single layer of cells. Stratified
epithelium consists of layers of cells. Then there’s pseudostratified epithelium
; pseudostratified epithelium looks
like it’s layered at first glance, but if you look carefully, you can see that it consists of only a single layer of cells.
epithelium is made up of cells that are flattened in shape. Flattened cells allow for the quick movement of dissolved substances across the tissue. Cuboidal
epithelium, as you might imagine, is made up of cube-shaped cells. Finally, there’s columnar
epithelium, in which the cells are elongated and much “taller” than they are wide.
Types of Epithelial Tissues:
Simple Squamous Epithelium:
Simple squamous epithelium, of course, consists of a single layer of flattened cells. Because the cells are flattened (and therefore very thin) and because there’s only a single layer of them, dissolved substances can quickly and easily cross simple squamous epithelium. So, you probably wouldn’t be surprised to learn that this type of epithelium is found lining the insides of your lungs. Oxygen can rapidly pass from the air in the lungs into your blood through the epithelium, and CO2
can rapidly pass from the blood and into your lungs to be exhaled. Simple squamous epithelium also lines the smaller blood vessels, allowing rapid exchange of gases and nutrients between the blood and surrounding tissues.
Simple Squamous Epithelium in a Chicken Embryo
The arrows indicate a layer of simple squamous epithelium.
Simple squamous epithelium surrounds most of the internal organs, such as the intestines, for instance. These tissues don’t have much of a protective function, of course, but they may help to hold the organs in position, and they sometimes secrete fluids that lubricate and cushion the organs.
The skin of most frogs and other amphibians consists of simple squamous epithelium. This is important to them, because most amphibians get much (in some species, all
) of their oxygen not from their lungs, but by absorbing it across the skin surface. The downside to this, of course, is that they rapidly lose water across their very thin skins, and so must live in moist environments.
Simple Cuboidal Epithelium:
Simple Cuboidal Epithelium Lining the Kidney Tubules
Simple cuboidal epithelium consists of a single layer of cube-shaped cells – that is, cells that are as wide as they are tall. Since it is much thicker than simple squamous epithelium, substances don’t cross simple cuboidal epithelium so easily. Instead, simple cuboidal epithelium often consists of cells that are specialized for actively transporting substances, and so they either secrete
substances into body cavities or absorb
substances from body cavities. For example, the tubules of the kidney are lined with simple cuboidal epithelium. The cells of this tissue actively transport urea and other substances out of the blood and then secrete it into the lumen of the kidney tubules, where it can be excreted out of the body in the form of urine.
Because of their shape, cuboidal cells can be packed together very tightly, and that’s just what happens in the simple cuboidal epithelium lining the kidney tubules. The cells are bound together by tight junctions
(remember those?), so that even water molecules cannot easily pass between them and escape from the lumen of the kidney tubules.
Simple Columnar Epithelium:
Simple columnar epithelium consists of a single layer of cells that are much taller than they are wide, and so appear column-shaped under a microscope. These cells, though they form a relatively thick layer, tend to be fairly fragile, and so they typically line body cavities where there’s a minimum of wear and tear. They are found lining the uterus and most of the digestive tract, for instance.
Simple Columnar Epithelium
The scale bar represents 20 micrometers, giving you an idea of the size of a typical cell.
(A micrometer is one-millionth of a meter.)
Simple columnar epithelium is often specialized for secretion
. For example, the epithelium lining both the uterus and the digestive tract secretes large amounts of mucus. The mucous layer lining the digestive tract helps protect the tissues of the digestive tract from digestive enzymes and acids, and the mucous layer lining the uterus helps protect against invasion by bacteria and viruses.
[By the way, just in case you’re wondering why “muc(o)us” seems to be spelled inconsistently, the word “mucus” is a noun; “mucous” (as in mucous membrane) is an adjective.]
The cells of the simple columnar epithelium that lines the lumen of the intestine often have finger-like projections that extend into the lumen. These microvilli
greatly increase the cells’ surface areas, making them very efficient at absorbing amino acids, simple sugars, and other small molecules produced by digestion of larger molecules. The molecules can then by secreted by the intestinal epithelium into the blood, which carries them to wherever they’re needed in the body.