erythrocyte(redirected from achromatic erythrocyte)
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a red blood corpuscle, or cell, in man, the vertebrates, and some invertebrates (echinoderms). Erythrocytes transport oxygen from the lungs to the tissues, and carbon dioxide from the tissues to the lungs. They regulate the acid-base balance and maintain osmotic balance in the blood and tissues. In addition, amino acids and lipids are absorbed from the blood plasma, and transported to the tissues, by erythrocytes.
Mature mammalian and human erythrocytes lack the nucleus that is present in the early stages of their development—that is, in the erythroblasts. They have the shape of a biconcave disk. Erythrocytes consist mainly of the respiratory pigment hemoglobin, which is responsible for the red color of blood. The erythrocytes of birds, reptiles, amphibians, and fishes are nucleated. The erythroblasts’ actively functioning nuclei gradually become inactive in the course of the erythrocytes’ development; they can, however, be reactivated. At the same time, ribosomes and other constituents that participate in protein synthesis disappear from the cytoplasm. The cell (or plasma) membrane plays an important role by permitting the passage of gases, ions, and water in the erythrocytes. On the surface of the lipoprotein membrane are specific glycoprotein antigens, or agglutinogens—the blood group factors responsible for the agglutination of erythrocytes.
The efficient functioning of hemoglobin depends on the erythrocytes’ area of surface contact with the environment. The smaller the erythrocytes, the greater their total surface area. The lower invertebrates have large erythrocytes (measuring, for example, 70 micrometers in diameter in the caudate amphibian Amphiuma), and the higher vertebrates have smaller ones (4 micrometers in diameter in goats). Human erythrocytes vary in diameter from 7.2 to 7.5 micrometers.
The number of erythrocytes in the blood normally remains constant, ranging from 4.5 to 5 million in 1 mm3 of human blood. The life-span of a human erythrocyte averages 125 days; approximately 2.5 million erythrocytes are formed and an equal number are destroyed every second. The total number decreases in anemia and increases in polycythemia. In the anemias, erythrocytes are found to change shape and size; they may be large (such as the megalocytes in Addison-Biermer anemia) or small, and they may, for example, be oval in shape (as in hemolytic anemia).
K. G. GAZARIAN and A. N. SMIRNOV