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Hemoglobin  

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Anemia is a condition characterized by an inadequate amount of red blood cells, which are produced in your 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 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 to your cells and carbon dioxide away from your cells, your body functions at a less than optimal level.

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

Blood Loss  

If you are bleeding heavily, you will rapidly become anemic and may develop severe symptoms including shock. Slower leakage of blood that you are unaware of, such as bleeding from a stomach ulcer or from colon cancer, can also exceed your bone marrow’s ability to replace blood supplies, eventually resulting in anemia.

Failure to Make Enough Normal Red Blood Cells or Hemoglobin  

Dietary intake of iron, folic acid, and vitamin B 12 are necessary for red blood cell formation. Deficiencies of these nutrients can impair bone marrow function, thus reducing production of adequate numbers of red blood cells. In addition, cancers, certain drugs and toxins, allergic reactions to medicines, and chronic illness can cripple the bone marrow so that it makes defective or insufficient 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 three to four months before they are destroyed and their contents recycled. If they are defective, or if the recycling process is accelerated, the marrow may not be able to keep up with the demand for new red cells. Defective red blood cells are more fragile and therefore do not last as long. Normal red blood cells may be destroyed rapidly by diseases, such as malaria and Rh incompatibility between a mother and her unborn child.