Anemia is a condition in which the number of red blood cells or the amount of hemoglobin (the protein that carries oxygen in them) is low.
Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts of the body. When the number of red blood cells is reduced or the amount of hemoglobin in them is low, the blood cannot carry an adequate supply of oxygen. An inadequate supply of oxygen in the tissues produces the symptoms of anemia.
The causes of anemia are numerous, but most can be grouped within three major mechanisms that produce anemia:
Anemia may be caused by excessive bleeding. Bleeding may be sudden, as may occur in an injury or during surgery. Often, bleeding is gradual and repetitive, typically due to abnormalities in the digestive or urinary tract or heavy menstrual periods. Chronic bleeding typically leads to low levels of iron, which leads to worsening anemia.
Anemia may also result when the body does not produce enough red blood cells. Many nutrients are needed for red blood cell production. The most critical are iron, vitamin B12, and folate (folic acid), but the body also needs trace amounts of vitamin C, riboflavin, and copper, as well as a proper balance of hormones, especially erythropoietin (a hormone that stimulates red blood cell production). Without these nutrients and hormones, production of red blood cells is slow and inadequate, or the red blood cells may be deformed and unable to carry oxygen adequately. Chronic disease also may affect red blood cell production. In some circumstances, the bone marrow space may be invaded and replaced (for example, by leukemia, lymphoma, or metastatic cancer), resulting in decreased production of red blood cells.
Anemia may also result when too many red blood cells are destroyed. Normally, red blood cells live about 120 days. Scavenger cells in the bone marrow, spleen, and liver detect and destroy red blood cells that are near or beyond their usual life span. If red blood cells are destroyed prematurely (hemolysis), the bone marrow tries to compensate by producing new cells faster. When destruction of red blood cells exceeds their production, hemolytic anemia results. Hemolytic anemia is relatively uncommon compared with the anemia caused by excessive bleeding and decreased red blood cell production. Hemolytic anemia may result from disorders of the red blood cells themselves, but more often it results from other disorders that cause red blood cells to be destroyed.