BLOOD

Red blood cells (also called erythrocytes) are the most numerous, making up 40-45 percent of

one's blood, and they give blood its characteristic color. Red blood cells are shaped like tiny

doughnuts, with an indentation in the center instead of a hole. They contain a special molecule

called hemoglobin, which carries the oxygen. In the lungs, where there is a lot of oxygen, the

hemoglobin molecules loosely bind with oxygen. Each molecule of hemoglobin contains four iron

atoms, and each iron atom can bind with one molecule of oxygen, allowing each hemoglobin

molecule to carry four molecules of oxygen. In the capillaries, where there is little oxygen, the

hemoglobin readily sheds the oxygen it is carrying and allows it to be absorbed by the body's

cells. The iron in hemoglobin is what makes blood red.

White blood cells (leukocytes) are the body's mobile warriors in the battle against infection and

invasion. There are three types of white blood cell: granulocytes, lymphocytes, and monocytes.

There are, in turn, three kinds of granulocyte: neutrophils, eosinophils, and basophils.

(Granulocytes are called that because they contain granules that hold digestive enzymes.)

Neutrophils kill invading bacteria by ingesting and then digesting them. Eosinophils kill
parasites,

and are involved in allergic reactions. Basophils also function in allergic reactions, but are not

well understood.

Lymphocytes are key parts of the body's immune system. There are two kinds of lymphocyte: T

cells and B lymphocytes. T cells direct the activity of the immune system. B lymphocytes

produce antibodies, which destroy foreign bodies. Monocytes, the largest kind of white blood

cells, enter the tissues of the body and turn into even larger cells called macrophages. These eat

foreign bacteria and destroy damaged, old, and dead cells of the body itself.

The blood cells called platelets (thrombocytes) help blood to clot, in several different ways. When

bleeding occurs, platelets clump together to help form a clot. Also, when they are exposed to air

(as they would be by a wound), platelets start breaking down and release a substance into the

bloodstream. This substance starts a chain of chemical events that eventually causes a protein in

the blood, fibrinogen, to turn into a different substance, fibrin, which forms long threads. These

threads tangle up red blood cells to help form a clot, or scab, over the wound.

In their "resting" state, platelets look like two plates stuck together (hence the name). When

"activated" and helping to form a clot, they change shape and look like tiny roundish blobs with

tentacles. At only two to three microns, they are the smallest kind of blood cell.

Plasma is a clear, straw-colored liquid that carries the blood cells and various hormones,

nutrients, and so on through the body. It makes up a little more than half of the total blood

volume. Plasma is about 90 percent water. Much of the other ten percent comprises various

kinds of protein molecules, including enzymes, clotting agents, immunoglobulins (part of the

immune system), and proteins that carry hormones, vitamins, cholesterol, and other things the

body needs. Plasma also contains sugar (glucose) and electrolytes like sodium, potassium, and

calcium, as well as other things like hormones, vitamins, and cholesterol.