Causes of Anemia

Any process that can disrupt the normal life span of a red blood cell may cause anemia. Normal life span of a red blood cell is typically around 120 days. Red blood cells are made in the bone marrow.

Anemia is caused essentially through two basic pathways. Anemia is either caused:

1. by a decrease in production of red blood cell or hemoglobin, or
   
   
2. by a loss or destruction of blood.

As more common classifications of anemia (low hemoglobin) is based on the MCV, or the volume of individual red blood cells.

1. If the MCV is low (less than 80), the anemia is categorized as microcytic anemia (low cell volume).
   
   
2. If the MCV is in the normal range (80-100), it is called a normocytic anemia (normal cell volume).
   
   
3. If the MCV is high, then it is called a macrocytic anemia (large cell volume).

Looking at each of the components of a complete blood count (CBC), especially the MCV, a physician can gather clues as what may be the most common reason for anemia.

Source : http://www.medicinenet.com/anemia/article.htm