(1) Red blood cells (erythrocytes). Red blood cells (RBCs) are biconcave
discs. That is, they are shaped something like an inner tube from an automobile tire,
but with a thin middle portion instead of a hole. There are approximately 5,000,000
RBCs in a cubic millimeter of normal adult blood. Red blood cells contain hemoglobin,
a protein that carries most of the oxygen transported by the blood.
(2) White blood cells (leukocytes). There are various types of WBCs, but
the most common are neutrophils and lymphocytes. Neutrophils phagocytize (swallow
up) foreign particles and organisms, and digest them. Lymphocytes produce antibodies
and serve other functions in immunity. In normal adults, there are about 5,000 to
11,000 WBCs per cubic millimeter of blood.
(3) Platelets. Platelets are about half the size of erythrocytes. They are
fragments of cells. Since they are fragile, they last only about 3-5 days. Their main
function is to aid in clotting by clumping together and by releasing chemical factors
relating to clotting. There are 150,000-350,000 platelets in a cubic millimeter of normal
Some General Functions of the Blood.
(1) Blood serves as a vehicle for oxygen nutrients, carbon dioxide and other
wastes, hormones, antibodies, heat, and so forth.
(2) Blood aids in temperature control. Beneath the skin, there is a network
of vessels that functions much like a radiator. To avoid accumulation of excess heat in
the body, the flow of blood to these vessels can be increased greatly. Here, aided by
the evaporative cooling provided by the sweat glands, large amounts of heat can be
rapidly given off. The flow of blood also keeps the outer parts of the body from
becoming too cold.
(3) The blood aids in protecting our bodies by providing immunity. Some
WBCs phagocytize (swallow up) foreign particles and microorganisms. Other WBCs
produce antibodies. The blood transports antibodies throughout the body.
(4) Blood clotting is another function of blood. Not only does this prevent
continued blood loss; it also helps prevent invasion of the body by microorganisms and
viruses by sealing the wound opening.
2-6. BLOOD VESSELS
The blood is conducted or carried through the body by tubular structures known
as blood vessels. Since at no time does the whole blood ever leave a blood vessel of
some sort, we refer to this system as a closed system.