Haut, Arthur Division of Hematology and Oncology, University of Arkansas for Medical Sciences, Little Rock, Arkansas.
Last reviewed:May 2019
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A medical condition resulting from a reduction in the total quantity of hemoglobin or of red blood cells in the circulation. Anemia is a disorder marked by significant decreases in hemoglobin concentration and in the number of circulating red blood cells (erythrocytes). Because it is generally impractical to measure the total quantity of hemoglobin or red blood cells, measures of concentration are used instead. Hemoglobin (Fig. 1) is contained in red blood cells, which are suspended in plasma (the liquid component of blood). Therefore, concentration is affected not only by quantities of hemoglobin and red blood cells, but also by plasma volume. Thus, the apparent anemia found in many women in the third trimester of pregnancy is not really anemia at all; in fact, the red cell mass is actually increased, but the plasma volume is expanded even more. In other words, hemodilution is present. Conversely, in dehydration and other circumstances of hemoconcentration, the plasma volume is reduced, thereby tending to mask anemia. See also: Blood; Hematologic disorders; Hemoglobin
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