Anemia (a-ne'me-ah) is a deficiency of hemoglobin in the blood. It can result from a decrease in the number of erythrocytes, a decreased in the amount of hemoglobin in each erythrocyte, or both. The decreased hemoglobin reduces the ability of the blood to transport oxygen. Anemic patients suffer from a lack of energy and feel excessively tired and listless. They can appear pale and quickly become short of breath with only slight exertion.

One general cause of anemia is insufficient production of erythrocytes. Aplastic anemia is caused by an inability of the red bone marrow to produce erythrocytes. It is usually acquired as a result of damage to the red marrow by chemicals (for example, benzene), drugs (for example, certain antibiotics and sedatives), or radiation.

Erythrocyte production can also be reduced because of nutritional deficiencies. Iron deficiency anemia results from a deficient intake or absorption of iron or from excessive iron loss. Consequently, not enough hemoglobin is produced, the number of erythrocytes decreases, and the erythrocytes that are manufactured are smaller than normal. Another type of nutritional anemia is pernicious (per-nish'us) anemia, which is caused by inadequate vitamin B12. Because vitamin B12 is necessary for the cell divisions that result in erythrocyte formation, a shortage of vitamin B12 reduces erythrocyte production. Although inadequate levels of vitamin B12 in the diet can cause pernicious anemia, the usual cause is insufficient absorption of the vitamin. Normally the stomach produces intrinsic factor, a protein that binds to vitamin B12. The combined molecules pass into the lower intestine, where intrinsic factor facilitates the absorption of the vitamin. Without adequate levels of intrinsic factor, insufficient vitamin B12 is absorbed, and pernicious anemia develops.

Present evidence suggests that the inability to produce intrinsic factor is an autoimmune disease in which the body's immune system damages the cells in the stomach that produce intrinsic factor. Folic acid deficiency can also hinder cell divisions and cause anemia. Inadequate amounts of folic acid in the diet are the usual cause, with the disorder developing most often in the poor, in pregnant women, and in chronic alcoholics.

Another general cause of anemia is loss or destruction of erythrocytes. Hemorrhagic (hem-o-raj'ik) anemia results from a loss of blood such as can result from trauma, ulcers, or excessive menstrual bleeding. Chronic blood loss, in which small amounts of blood are lost over a period of time, can result in iron deficiency anemia. Hemolytic (he-mo-lit'ik) anemia is a disorder in which erythrocytes rupture or are destroyed at an excessive rate. It can be caused by inherited defects within the erythrocytes. For example, one kind of inherited hemolytic anemia results from a defect in the cell membrane that causes erythrocytes to rupture easily. Many kinds of hemolytic anemia result from unusual damage to the erythrocytes by drugs, snake venom, artificial heart valves, autoimmune disease, or hemolytic disease of the new born.

Some anemias result from inadequate or defective hemoglobin production. Thalassemia (thal-a-se'me-ah) is a hereditary disease found in people of Mediterranean, Asian, and African ancestry. It is caused by insufficient production of the globin part of the hemoglobin molecule. The major form of the disease results in death by age 20, and the minor form in a mild anemia. Sickle cell anemia is a hereditary disease found mostly in blacks that results in the formation of an abnormal hemoglobin. The erythrocytes assume a rigid sickle shape and plug up small blood vessels. They are also more fragile than normal. In its severe form, sickle cell anemia is usually fatal before the person is 30 years of age, whereas in its minor form, sickle cell trait, there are usually no symptoms.