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Blood is a mixture of cells and a watery liquid, called plasma, that
the cells float in. It also contains other things like nutrients (such
as sugar), hormones, clotting agents, and waste products to be flushed
out of the body.
 There
are three kinds of cells in the blood: red blood cells, white blood
cells, and platelets. Red blood cells carry oxygen from the lungs
throughout the body, white blood cells help fight infection, and platelets
help in clotting.
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 the aforementioned hormones, vitamins, and cholesterol.
-- Sue Wilson
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