Etiology of Anemia

ByGloria F. Gerber, MD, Johns Hopkins School of Medicine, Division of Hematology
Reviewed/Revised May 2024
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    Anemia is a decrease in the number of red blood cells (RBCs), which leads to a decrease in the hematocrit and the red cell hemoglobin content. (See also Red Blood Cell Production.)

    The RBC mass represents the balance between the production and destruction or loss of RBCs. Thus, anemia can result from one or more of 3 basic mechanisms (see table Classification of Anemia by Cause):

    Table
    Table

    Blood loss can be

    • Acute

    • Chronic

    Anemia may not develop until several hours after acute blood loss when interstitial fluid diffuses into the intravascular space and dilutes the remaining RBC mass. During the first few hours, however, levels of polymorphonuclear granulocytes, platelets, and, in severe hemorrhage, immature white blood cells and normoblasts may rise. Chronic blood loss results in anemia if loss is more rapid than can be replaced or, more commonly, if accelerated erythropoiesis depletes body iron stores (see Iron Deficiency Anemia).

    Deficient or inefficient erythropoiesis has myriad causes. Complete cessation of erythropoiesis results in a decline in RBCs of about 7 to 10%/week (1%/day). Impaired erythropoiesis, even if not sufficient to decrease the numbers of RBCs, often causes abnormal RBC size and shape.

    Excessive hemolysis can be caused by intrinsic abnormalities of RBCs or by extrinsic factors, such as the presence of antibodies or complement on their surface, that lead to their early destruction. An enlarged spleen sequesters and destroys RBCs more rapidly than normal. Some causes of hemolysis deform as well as destroy RBCs. Hemolysis normally causes increased reticulocyte production unless iron or other essential nutrients are depleted or there is erythropoietin deficiency.

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