Red blood cells normally live for 110 - 120 days. After that, they naturally break down and are usually removed from the circulation by the spleen.
Some diseases and processes cause red blood cells to break down too soon. This requires the bone marrow to make more red blood cells than normal. The balance between red blood cell breakdown and production determines how low the red blood cell count becomes.
Conditions that can cause hemolysis include:
Scwartz RS. Autoimmune and intravascular hemolytic anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 164.