Blood is the vehicle for the human cardiovascular system. Its major
subdivisions are the plasma, a fluid containing proteins, and the formed elements,
including red blood cells, white blood cells, and platelets.
a. Plasma.
(1) Plasma makes up about 55 percent of the total blood volume. It is
mainly composed of water. A variety of materials are dissolved in plasma. Among the most
important of these are proteins.
(2) After the blood clots, the clear fluid remaining is called serum.
Serum does not contain the proteins used for clotting. Otherwise, it is very similar to
plasma.
b. Formed Elements. The formed elements make up about 45 percent of
the total blood volume. The formed elements are cellular in nature. While the red blood
cells (RBCs) and white blood cells (WBCs) are cells, the platelets are only fragments of
cells.
(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
fragments 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
blood.
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 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.
Primary Content Providers: The U. S. Army, The
U.S. Navy
Ancillary Content and Online Version: David L. Heiserman
Publisher: SweetHaven Publishing Services
