Anemia is an inadequate amount of red blood cells. These blood cells are made by the body in the bone marrow. Red blood cells contain hemoglobin, a substance that picks up oxygen from your lungs, carries it throughout your body, and gives it to your cells. Your cells need this oxygen to perform the basic functions that generate energy and keep you alive. In addition, hemoglobin picks up some of the carbon dioxide given off by your cells and returns it to the lungs, where it is exhaled when you breathe out. Without enough red blood cells to transport oxygen and carbon dioxide, your body functions at a less than optimal level.

Hemoglobin

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There are many causes of anemia, which can be broadly grouped into 3 categories:

Blood Loss

Any loss of blood automatically decrease the amount of red blood cells. If you are bleeding heavily, you will rapidly become anemic and may develop severe symptoms including shock.

Slow leaks that you are unaware of, such as bleeding from a stomach ulcer or from colon cancer, can also lead to anemia if the blood loss exceeds your body's ability to create new blood cells.

Failure to Make Enough Normal Red Blood Cells or Hemoglobin

Dietary intake of iron, folic acid, and vitamin B 12 are necessary to make red blood cells. Low levels of these nutrients can effect how the bone marrow functions, and fewer red blood cells will be made. In addition, cancers, certain drugs and toxins, allergic reactions to medicines, and chronic illness can cripple the bone marrow so that it makes damaged or low numbers of red blood cells. Hereditary defects, such as sickle cell disease, also may lead to anemia. When the bone marrow fails completely the condition is known as aplastic anemia.

Rapid Destruction of Red Blood Cells

Red blood cells normally last for 3-4 months before they are destroyed and their contents recycled. If they are defective, or if the recycling process is sped up, the marrow may not be able to keep up with the demand for new red cells. Defective red blood cells are also more fragile and therefore do not last as long. Normal red blood cells can also be destroyed rapidly by diseases, such as malaria and Rh incompatibility between a mother and her unborn child.