a. Red Blood Cells (RBCs; Erythrocytes). The primary function of RBCs is to
contain the protein called hemoglobin, which in turn carries oxygen. Thus, RBCs carry
the majority of the oxygen to the individual cells of the body.
(1) Structure. The normal, mature red blood cell is a biconcave disc. The
biconcave shape results from the loss of the nucleus just before the final maturation of
the RBC. Since this shape increases the surface area of the disc, there is an increase
in the capacity for the flow of substances into and out of the RBC.
(2) Hemoglobin. Within the cytoplasm of the RBC is a special protein called
hemoglobin. Because of its iron atoms, hemoglobin has a great affinity for oxygen. It
will readily pick up oxygen until it is saturated. At the same time, however, hemoglobin
will readily give up oxygen in areas of low concentration.
(3) Life cycle of the RBC. Because of the loss of its nucleus, the RBC has a
limited life period (about 120 days). At the end of this period, the spleen removes the
"worn out" RBC, and the liver salvages the "pieces," particularly the iron.
b. White Blood Cells (WBCs; Leukocytes). The white blood cells are also
formed elements of the blood. There are several types.
(1) Neutrophils and other phagocytic WBCs. The phagocytic WBCs can
move independently out of the capillaries and penetrate into the tissues of the body.
There, they actively attack foreign substances and engulf them in a process called
phagocytosis. When these WBCs are overcome by foreign substances and die, their
bodies accumulate to form a substance called pus.
(2) Lymphocytes. The lymphocytes are involved with the immune system of
the body, including the production of antibodies.
c. Platelets. The platelets are the third type of formed element in the blood.
Platelets are fragments of former cells. They are very important in the clotting process.
After blood has been treated to remove the formed elements and the protein
fibrinogen, there is a clear light-straw-colored fluid remaining. This fluid is called serum.
10-13. TRANSPORT OF GASES
One very important transport function of the blood is to carry gases back and
forth between the lungs and the individual cells of the body. The alveoli and the
individual body cells are the sites of exchange of gases to and from the blood. At these
sites, the gases move according to the directions of pressure of concentration gradients.
That is, each gas moves from an area where it is in higher concentration to an area of