Anemia

Anemia is defined as an insufficient quantity of normal red blood cells (RBCs) or decreased levels of hemoglobin. Hemoglobin is the iron-containing protein present in RBCs. It is synthesized in the mitochondria of the RBC as the cell develops within the bone marrow. Subsequent to maturation, the RBC enters the vasculature where it functions to transport oxygen throughout the body. Anemia is quite variable in the sense that it can result from impaired red cell production, increased red cell destruction, and/or significant blood loss. There are many serious health consequences associated with anemia, and it can act to make individuals more susceptible to other adverse health events. Often, it is difficult to determine if health complications are the result of anemia or serve to produce the anemia conditions. The variation among the etiologies of anemia has thus fostered health professionals to categorize the disorder into more specific terms.

The class of anemia that receives the most attention is nutritional iron deficiency anemia (IDA). Inadequate intake of iron, poor iron availability, or increased utilization of iron can lead to iron deficiencies within the body, which can evolve in stages beginning with depleted iron stores. This stage is indicated when hemoglobin levels are at an adequate level but iron is no longer stored in tissues, as indicated by low serum ferritin levels. As iron availability begins to depreciate, normal erythropoeisis, the development of RBCs that requires the greatest amount of iron, becomes interrupted and leads to the stage known as iron-de-ficient erythropoeisis. However, in this stage, hemoglobin levels continue to remain at normal values but are accompanied by an increase in transferrin receptor (TfR). The deteriorating iron supply eventually inhibits the production of hemoglobin, decreasing the levels to below normal cutoff values, and culminates in the final stage known as IDA.

Macrocytic anemias are often the result of inadequate intake or absorption of vitamin B12 or folic acid caused by diet, disease, or medication use. Pernicious anemia is a type of megaloblastic macrocytic anemia, a chronic illness that occurs when the stomach fails to produce intrinsic factor to assist in the absorption of vitamin B12. Both vitamin B and folic acid are required for proper DNA production and thus successful erythropoeisis. Deficiencies in one of these micronutrients can also stimulate deficiencies in the other. When erythropoeisis is interrupted, macrocytes, abnormally large RBCs with irregularly shaped nuclei, and megaloblasts, the precursor to RBCs, are present in the blood vessels and bone marrow, respectively. Macrocytic anemias can also be initiated by causes not associated with vitamin B12 or folic acid deficiencies, such as viral illnesses (e.g., HIV), bone marrow diseases, and other infections that inhibit DNA synthesis. These macrocytic anemias may or may not be accompanied by the presence of megaloblasts.

Hemolytic anemia, a general category of nonnutritional forms of anemia, includes hemogloblinopathies that are genetic blood diseases such as sickle-cell anemia and thalassemia; autoimmune diseases; and [Page 134]infections such as malaria, toxins/drugs, and trauma. Hemolytic anemia occurs when RBCs are prematurely destroyed and bone marrow cannot produce enough RBCs to compensate for the decline, resulting in decreased levels of RBCs. In this case, low levels of hemoglobin are typical and accompanied by elevated levels of bilirubin and reticulocytes, the precursors to RBCs.

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