The cells contained in the blood are of two main types: red blood cells
(erythrocytes) and white blood cells (leukocytes). The erythrocytes are produced in the
red bone marrow that is primarily present (in the adult body) in the sternum, ribs,
hipbones, and cranial bones. Most of the granular leukocytes are also formed in the red
bone marrow. Most of the non-granular leukocytes are formed in the lymph nodes,
thymus gland, and the spleen.
a. Erythrocytes. The erythrocyte is the red blood cell (RBC) of the blood. The
mature RBC contains about 34 percent hemoglobin (a complex iron-bearing protein
which transports gases). Erythrocytes are produced primarily in the red marrow of the
spongy bones. Hemoglobin is the main functioning component of the cell. The RBCs
transport oxygen from the lungs to the tissues and carbon dioxide from the tissues to
the lungs. The hemoglobin protein is composed of carbon, nitrogen, and iron.
Hemoglobin carries 98 percent of the oxygen. Less than 2 percent is carried in simple
solution in the plasma. These cells have an average life span of 80 to 120 days. When
a red blood cell becomes too old, it is broken down into its elements (especially in the
spleen). A portion is carried to the liver and excreted as "bilirubin" with the bile. This
gives the bile its golden-yellow color. The iron is retained and either reused or stored
for future use. (See figure 1-1.)
Figure 1-1. Red blood cells.
b. Leukocytes. There are two main groups: granular and nongranular
leukocytes. These are further subdivided into neutrophils, eosinophils, and basophils,
which are granular. Nongranular are classed as lymphocytes and monocytes.
(1) General functions. The leukocytes remove invading antigens (that is,
bacteria) and, to some extent, transport and distribute antibodies. In antigen removal,
some nongranular and granular leukocytes show directional movement, subject to
chemical stimulus (chemotaxis). In this way, they are attracted to substances, which
they must either transport or engulf. The process of engulfing and destroying bacteria
(phagocytosis) is the prime function of leukocytes. (See figure 1-2.)