An anemia is a decrease in the hemoglobin of the blood below the limit of normal. Since hemoglobin is contained only in the red blood cell, an anemia must result from quantitative or qualitative changes in the erythrocytes. The erythrocyte count may be low or the amount of hemoglobin per cell may be decreased. Red blood cells are always being formed and destroyed rapidly, so that the entire mass of cells is replaced every two to six weeks. An anemia results from a disturbance in the normal balance between destruction and regeneration of erythrocytes and hemoglobin. The red blood cells are formed primarily from the endothelial cells lining the capillary spaces in the bone marrow, especially of the flat bones. Successive stages in the growth of an erythrocyte from the endothelial cell include the megaloblast, erythroblast, normoblast, reticulocyte and mature erythrocyte. Red blood cells in the circulation do not all
HADEN RL. CLASSIFICATION AND DIFFERENTIAL DIAGNOSIS OF THE ANEMIAS. JAMA. 1935;104(9):706–710. doi:10.1001/jama.1935.02760090010003
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