red blood cell
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blood
Erythrocytes (Red Blood Cells)
The erythrocytes, or red blood cells, make up the largest population of blood cells, numbering from 4.5 million to 6 million per cubic millimeter of blood. They carry out the exchange of oxygen and carbon dioxide between the lungs and the body tissues. To effectively combine with oxygen, the erythrocytes must contain a normal amount of the red protein pigment hemoglobin, the amount of which in turn depends on the iron level in the body. A deficiency of iron and therefore of hemoglobin leads to anemia and poor oxygenation of the body tissues.
Erythrocytes are constantly developing from stem cells, the undifferentiated, self-regenerating cells that give rise to both erythrocytes and leukocytes in the bone marrow. In the fetus, red blood cells are produced in the spleen. As they mature, the erythrocytes lose their nuclei, become disk-shaped, and begin to produce hemoglobin. After circulating for about 120 days, the erythrocytes wear out and undergo destruction by the spleen. Although all red blood cells are essentially similar, certain structures on their surfaces vary from person to person. These serve as the basis for the classification into blood groups. There are four major blood groups, whose compatibility or incompatibility is an important consideration in successful blood transfusion.
Leukocytes (White Blood Cells)
Granulocytes
Agranulocytes
The agranulocytes include the monocytes and the lymphocytes. Monocytes are derived from the phagocytic cells that line many vascular and lymph channels, called the reticuloendothelial system. Monocytes ordinarily number 4% to 8% of the white cells. They move to areas of infection, where they are transformed into macrophages, large phagocytic cells that trap and destroy organisms left behind by the granulocytes and lymphocytes. In certain diseases of long duration (tuberculosis, malaria, and typhoid) the monocytes act as the main instrument of defense.
Lymphocytes, under normal conditions, make up about 20% to 35% of all white cells, but proliferate rapidly in the face of infection. There are two basic types of lymphocytes: the B lymphocytes and the T lymphocytes. B lymphocytes tend to migrate into the connective tissue, where they develop into plasma cells that produce highly specific antibodies against foreign antigens. Other B lymphocytes act as memory cells, ready for subsequent infection by the same organism. Some T lymphocytes kill invading cells directly; others interact with other immune system cells, regulating the immune response.
Other Constituents of Blood
The blood also contains platelets, or thrombocytes, and at least 15 other factors active in blood clotting. Platelets are tiny plate-shaped cytoplasmic bags of blood-clotting chemicals produced by megakaryocytes; if their production is hindered, as by AIDS or chemotherapy, there is an increased risk of bleeding. Also circulating in the plasma are the hormones that the endocrine glands secrete directly into the bloodstream. In addition, essential salts (such as those of sodium and potassium), essential plasma proteins (albumin, globulins, and fibrinogen), and metabolic wastes (such as urea) circulate in the plasma.
Serum, a straw-colored liquid, essentially composed of plasma without fibrinogen, makes up the liquid component of blood that separates from the clot. Serum is separated from whole blood by centrifuging and can serve various medical uses. Normal human serum is sometimes used to treat shock and the loss of fluid resulting from severe burns.
Bibliography
See D. Starr, Blood (1998).