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[erythro- + -cyte]
A circulating cell that contains hemoglobin and carries oxygen to tissue; a red blood cell. Each erythrocyte is a nonnucleated, biconcave disk averaging 7.7 μm in diameter. An erythrocyte has a typical cell membrane and an internal stroma of lipids and proteins to which more than 200 million molecules of hemoglobin are attached. The total surface area of the erythrocytes of an average adult is 3820 sq m, or about 2000 times more than the external total body surface area.
SYN: SEE: red blood cell; SEE: red blood corpuscle; SEE: red cell; SEE: red corpuscle
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ERYTHROCYTES Note the different size and shape of the red blood cells
erythrocytic (ĕ-rith″rŏ-sit′ik), adj.

In a normal person, the number of erythrocytes averages about 5,000,000/μL (5,500,000 for men and 4,500,000 for women). The total number in an average-sized person is about 35 trillion. The number per μL varies with age (higher in infants), time of day (lower during sleep), activity and environmental temperature (increasing with both), and altitude. People living at altitudes of 10,000 ft (3048 m) or more may have an erythrocyte count of 8,000,000/μL or more.

If a person has a normal blood volume of 5 L (70 mL/ kg of body weight) and 5,000,000 erythrocytes per μL of blood, and the erythrocytes live 120 days, the red bone marrow must produce 2,400,000 erythrocytes per second to maintain this erythrocyte count.

The primary function of erythrocytes is to carry oxygen. The hemoglobin also contributes to the acid-base balance of the blood by acting as a buffer for the transport of carbon dioxide in the plasma as bicarbonate ions.

Erythrocyte formation (erythropoiesis) in adults takes place in the bone marrow, principally in the vertebrae, ribs, sternum, hip bone, diploë of cranial bones, and proximal ends of the humerus and femur. Erythrocytes arise from large nucleated stem cells (promegaloblasts), which give rise to pronormoblasts, in which hemoglobin appears. These become normoblasts, which extrude their nuclei. Erythrocytes at this stage possess a fine reticular network and are known as reticulocytes. This reticular structure is usually lost before the cells enter circulation as mature erythrocytes. The proper formation of erythrocytes depends primarily on nutrition, with protein, iron, and copper essential for the formation of hemoglobin, and vitamin B12 and folic acid necessary for DNA synthesis in stem cells of the red bone marrow.

As erythrocytes age and become fragile, they are removed from circulation by macrophages in the liver, spleen, and red bone marrow. The protein and iron of hemoglobin are reused; iron may be stored in the liver until needed for the production of new erythrocytes in the bone marrow. The heme portion of the hemoglobin is converted to bilirubin, which is excreted in bile as one of the bile pigments.

On microscopic examination, erythrocytes may reveal variations in the following respects: size (anisocytosis), shape (poikilocytosis), staining reaction (achromia, hypochromia, hyperchromia, polychromatophilia), structure (possession of bodies such as Cabot's rings, Howell-Jolly bodies, Heinz bodies; parasites such as malaria; a reticular network; or nuclei), and number (anemia, polycythemia).

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