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Intro to Anatomy 6: Tissues, Membranes, Organs
Intro to Anatomy 6: Tissues, Membranes, Organs
The Lone Ranger
Published by The Lone Ranger
Default Bone

Bone (osseous tissue) is a connective tissue in which the matrix is a rigid solid made up mostly of calcium and phosphorous salts. (The mineral that makes up the matrix is known as calcium hydroxyapatite.) Of course, the matrix of bone also contains collagen fibers, so bones will bend – slightly – when stressed. Even so, bone is far less flexible than is cartilage; on the other hand, it’s much stronger.

Bones make up most of the framework of the body and provide structural support. Bones surround and protect many of the vital organs, including the brain. Blood cells are manufactured inside bone tissue. Bones provide attachment points for muscles, allowing for body movement. Finally, bones can store certain substances until they are needed by the body, including the minerals calcium and phosphorous, and fat.

The living cells within bone are known as osteocytes, and they are located within lacunae, just as are the chondrocytes in cartilage. Unlike chondrocytes, however, osteocytes typically have direct access to blood vessels. This is possible because the matrix of bone is rigid, unlike that of cartilage.

Bone matrix is laid down in concentric layers (called lamellae) around central canals (or Haversian canals). Each central canal contains blood vessels and nerves. From the osteocytes in the lamellae around each central canal extend tiny canals (called canaliculi). These fluid-filled canaliculi penetrate the matrix and connect osteocytes to the central canal, ensuring that the osteocytes are in contact with blood vessels.

Because osteocytes are in contact with blood vessels, they receive oxygen and nutrients from the blood at a much greater rate than do the chondrocytes in cartilaginous tissues. This means that bone cells grow and reproduce much faster than do cartilage cells. That is why bone, unlike cartilage, heals quickly and – ideally – completely after it is broken.

We’ll discuss the internal structure of osseous tissues in considerable detail when we get to the skeletal system.

Compact Bone
The relatively large black structures are the central canals, containing blood vessels and nerves. Note how the osteocytes (small black structures) are arranged in concentric rings around the central canals. You can just make out canaliculi linking the osteocytes to the central canals and to each other.

Most people don’t think of blood as a tissue, but it is. Blood is a connective tissue with a liquid matrix. Like the matrix of all connective tissues, the matrix of blood contains protein fibers (indeed, they’re vital components of the clotting process, which prevents you from bleeding to death from minor wounds), but they aren’t sufficiently numerous as to prevent the blood from flowing freely.

The primary function of blood is to transport substances through the body. Blood transports oxygen from the lungs to body tissues for aerobic respiration, and transports carbon dioxide from body tissues to the lungs for excretion. Blood transports urea and other metabolic wastes to the kidneys for excretion. Blood transports potentially toxic substances to the liver for detoxification. Blood transports digested molecules (including amino acids, simple sugars, and nucleotides – but not large amounts of water-insoluble lipids) from the intestines to body tissues that need them. Some lipids can be transported in the blood if they’re first bound to proteins; this makes them (more or less) water-soluble, so that they can be transported in the blood.

Blood also contains specialized cells known as leukocytes (white blood cells) that are important in defending the body against invasion by pathogens. Your blood is therefore your second line of defense (after the skin) against infection.

Blood consists of a liquid matrix (known as blood plasma) and various solids that are collectively known as the formed elements. Blood plasma consists mostly of water, with various proteins and electrolytes dissolved in it. Glucose, some lipids, hormones, amino acids, carbon dioxide, (small amounts of) oxygen, metabolic wastes such as urea, and other substances can be found dissolved into the blood plasma as well.

Blood plasma is transparent and slightly yellowish in color. When the clotting factors are separated out, blood plasma is known as serum. Removal of the clotting factors allows blood plasma to be stored without concern that it will clot and thus be rendered useless.

The formed elements in blood consist of three different types of cells: erythrocytes, leukocytes, and throbocytes. By far the most common are the erythrocytes, or red blood cells. The erythrocytes, like all blood cells, are manufactured inside of bones. In mammals such as ourselves, erythrocytes have no nuclei or other organelles, which means that they don’t live for very long – about 120 days at most. (Most other vertebrates have nucleated erythrocytes.)

The primary function of eythrocytes is to transport oxygen. Erythrocytes contain large amounts of the protein hemoglobin, which binds to oxygen in the lungs and then releases it to body tissues. The hemoglobin in erythrocytes allows blood to transport far more oxygen than could be dissolved into the blood plasma. It's the presence of large numbers of red blood cells that makes blood look red.

A person has the condition known as anemia if (s)he has reduced amounts of erythrocytes and/or hemoglobin for some reason. (“Anemia” literally means “without blood.”) Anemia can be life-threatening, since severe anemia means that the blood cannot deliver sufficient oxygen to body tissues.

Anemia can be caused by severe blood loss, or by insufficient amounts of iron in the diet. (Iron, you recall, is a key component of hemoglobin; iron deficiency is particularly common in menstruating women.) Some diseases (particularly those that affect the bone marrow, where all blood cells originate) can cause anemia, as can certain inherited conditions, such as sickle-cell disease. (Sufferers of sickle-cell disease don't lack erythrocytes; it's that their erythrocytes don't function normally.)

Leukocytes are also known as white blood cells, because they’re more or less white in color, when seen under a microscope. They are primarily important in defending you against infectious agents. There are several different kinds of leukocytes, and we’ll discuss their specific functions when we cover the immune system.

Leukemia is a cancer of the blood or of the bone marrow that causes abnormal production of leukocytes. Because the leukocytes produced by leukemia sufferers typically function poorly or not at all, victims of this condition are often much more vulnerable to infections than are most of us.

Thrombocytes (platelets) are responsible for coagulation (clotting) of the blood. Most forms of hemophilia are the result of a genetically-caused inability to manufacture one or more proteins that are important in clotting, but if your body doesn’t manufacture enough platelets for one reason or another, even small wounds may bleed so much that they can be life-threatening.

The condition known as thrombocytopenia occurs when a person has an abnormally low thrombocyte concentration. TCP sufferers tend to bruise very easily, and often suffer from nosebleeds or bleeding of the gums. Even small cuts bleed for a long time.

TCP can be caused by some forms of leukemia as well as infection of bone marrow. Since Vitamin B12 is important in platelet formation, too little of it in the diet can cause TCP. Ironically, some anti-cancer medications can induce TCP. TCP can be a life-threatening condition, because if bleeding occurs inside the skull, increased pressure on the brain can cause severe brain damage or even death.


Most of the cells you see are erythrocytes. Two leukocytes (neutrophils, to be precise) can also be seen.
The small purple-stained structures are thrombocytes.


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