Introduction
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.
Causes
The individual causes of anemia are numerous, but most can be grouped within three major mechanisms that produce anemia: blood loss (excessive bleeding), inadequate production of red blood cells, or excessive destruction of red blood cells.
Anemia may be caused by excessive bleeding. Bleeding may be sudden, as may occur in an accident or during surgery. Often, bleeding is gradual and repetitive, typically from abnormalities in the digestive or urinary tract. 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 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), and this results 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.
 See the table The Major Causes of Anemia.
See the table Other Causes of Anemia.
Symptoms and Diagnosis
Symptoms vary depending on the severity of the anemia and how rapidly it develops. Some people with mild anemia, particularly when it develops slowly, have no symptoms at all; others may experience symptoms only with physical exertion. More severe anemia may produce symptoms even when a person is resting. Symptoms are more severe when mild or severe anemia develops rapidly, such as with bleeding that occurs when a blood vessel ruptures.
Mild anemia often causes fatigue, weakness, and paleness. In addition to these symptoms, more severe anemia may produce faintness, dizziness, increased thirst, sweating, a weak and rapid pulse, and rapid breathing. Severe anemia may produce painful lower leg cramps during exercise, shortness of breath, and chest pain, especially if a person already has impaired blood circulation in the legs or certain types of lung or heart disease.
Sometimes anemia is detected before a person notices symptoms, when routine blood tests are performed.
Low levels of hemoglobin and a low hematocrit (the percentage of red blood cells in the total blood volume) confirm the anemia. Other tests help determine the cause of the anemia.
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