1-3

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1-3. BLOOD CELLS

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.)

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.)

Figure 1-2. Leukocytes-granular.

(2) Types and their specific functions. The three types of polymorphonuclear (called polys or granular) are neutrophils, esoinophils, and basophils. The neutrophils increase in number when fighting pyrogenic infections (fever producing). They decrease in number when reacting to toxic substances. This can occur with certain drugs. The esoinophils will increase in reaction to allergy or parasitic infections, and they will decrease in reaction to adrenocortical hormones. The basophils will increase in the presence of leukemia (but their exact function is not known). Lymphocytes and monocytes are nongranular leukocytes. The lymphocytes are believed to be directly involved with the antibody production. They increase in number in the presence of certain infections such as mononucleosis. Monocytes will increase during tuberculosis and protozoal infections. All types of leukocytes will increase during leukemia and will decrease because of acquired immunodeficiency syndrome (AIDS). (See figure 1-3.)

Figure 1-3. Leukocytes-nongranular.

c. Thrombocytes. Thrombocytes are commonly called "platelets." They are detached irregular shaped fragments of their precursor cells (which are found in the bone marrow). These precursor cells are giants in size compared to the other blood cells. The platelets live for only 3 to 5 days. The platelets contain different clotting factors and other components that are either known or presumed to participate in the clotting process. They clump together to form a plug in the initial phase of controlling bleeding. This process is speeded up by thrombin, an enzyme involved in blood clotting. A deficiency in platelets causes a tendency to bleed. The platelets possess ameboid movements. This property is very necessary for the clotting process. (See figure 1-4.)

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