(1) Red blood cells (erythrocytes). 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. RBCs contain hemoglobin, a protein which 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 frag-
ments of cells. Since they are fragile, they last only about three to five days. Their main
function is to aid in clotting by clumping together and by releasing chemical factors
related to clotting. There are 150,000 - 350,000 platelets in a cubic millimeter of normal
c. Some General Functions of the Blood.
(1) Blood serves as a vehicle for oxygen, nutrients, carbon dioxide and other
wastes, hormones, antibodies, heat, etc.
(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 helps keep the outer parts of the body from becoming
(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.
9-6. 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.
a. General Construction. The blood vessels in general are tubular and have a