Anemia is a decrease in the number of red blood cells (RBCs), which leads to a decrease in hematocrit and hemoglobin content. (See also Red Blood Cell Production Red Blood Cell Production Red blood cell (RBC) production (erythropoiesis) takes place in the bone marrow under the control of the hormone erythropoietin (EPO). Juxtaglomerular cells in the kidney produce erythropoietin... read more .)
The RBC mass represents the balance between 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 Classification of Anemia by Cause 
):
Blood loss
Blood loss can be
Acute
Chronic
Anemia does 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 Iron Deficiency Anemia Iron deficiency is the most common cause of anemia and usually results from blood loss; malabsorption, such as with celiac disease, is a much less common cause. Symptoms are usually nonspecific... read more 
).
Deficient 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 Splenomegaly Splenomegaly is abnormal enlargement of the spleen. (See also Overview of the Spleen.) Splenomegaly is almost always secondary to other disorders. Causes of splenomegaly are myriad, as are the... read more 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.
