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Anemia (Am.) or anaemia (Brit.) is the state of having low hemoglobin levels. It is the most common disorder of the blood. It occurs when the blood cannot transport sufficent amounts of oxygen, because the number of red blood cells is too low or something is wrong with the hemoglobin (the oxygen-carrying protein in the red blood cells). The tissues of an anemic person become, in a sense, "starved" of oxygen. Deprived of oxygen, the tissues cannot produce enough energy to function properly.

There are several kinds of anemia, produced by a variety of underlying causes. There are two broad classifications of types of anaemia.

Classification based on size of the red blood cells

One form of anaemia classification is made on the basis of the appearance of the red blood cells on microscopic examination of a peripheral blood smear. If the cells are smaller than normal, the anemia is said to be microcytic; if they are normal size, normocytic; and if they are larger than normal, the anemia is classified as macrocytic. Other characteristics visible on the peripheral smear may provide valuable clues about a more specific diagnosis.

Macrocytic anaemia

The most common causes of macrocytic anemia is a deficiency of either vitamin B12 or folic acid (or both) due either to inadequate intake or insufficient absorption. Pernicious anemia is an autoimmune condition where the body lacks intrinsic factor, required to absorb vitamin B12 from food. Alcoholism can cause macrocytic anemia. The treatment was first devised by William P. Murphy who bled dogs to make them anemic and then fed them various substances to see what worked. He discovered that ingesting large amounts of liver seemed to cure the disease. George R. Minot and George H. Whipple then set about to chemically isolate the curative substance and ultimately were able to isolate the vitamin B12 from the liver. For this, all three shared the 1934 Nobel prize in medicine.

Normocytic anaemia

Normocytic anemia can be caused by acute blood loss, chronic disease or failure to produce enough red blood cells.
Renal failure or liver failure causes normocytic anemia. Certain hormonal deficiencies, like testosterone, can cause normocytic anemia. Lastly, sideroblastic anemia is caused by abnormal production of red blood cells and can lead to hematologic malignancies.

Aplastic anemia (bone marrow failure) is anemia caused by the inability of the bone marrow to produce blood cells. Aplastic anemias are much rarer than dietary deficiency or genetic defect anemias.

Microcytic anaemia

The most common type of microcytic anemia, and the most common cause of anemia overall, is iron deficiency anemia. Other causes of microcytic anaemia include hemoglobinopathies such as sickle cell anaemia and thalassemia.

Iron deficiency anemia is caused when the dietary intake or absorption of iron is insufficient (Hemoglobin contains iron). In the United States, 20% of all women of childbearing age have iron deficiency anemia, compared with only 2% of adult men. The principal cause of iron deficiency anemia in premenopausal women is blood lost during menses.

Iron deficiency anemia is the final stage of iron deficiency. When the body has sufficient iron to meet its needs (functional iron), the remainder is stored for later use in the bone marrow, liver, and spleen. Iron deficiency ranges from iron depletion, which yields little physiological damage, to iron deficiency anemia, which can affect the function of numerous organ systems. Iron depletion causes the amount of stored iron to be reduced, but has no affect on the functional iron. However, a person with no stored iron has no reserves to use if the body requires more iron. In essence, the amount of iron absorbed by the body is not adequate for growth and development or to replace the amount lost.

Studies have shown that iron deficiency without anemia causes poor school performance and lower IQ in teenage girls.

Classification based on the cause of the anemia

The other major classification type for anaemia is based on the cause of the anaemia. There are three broad subcategories in this class which are:

  1. problems with production of red blood cells (hematopoiesis)
  2. problems with red blood cell function
  3. excessive destruction of existing red blood cells (hemolysis)\n