A normal red blood cell lives for about 120 days. It takes about two days for the body to manufacture each red blood cell, and about two million are turned out every second. Production of new red blood cells occurs in the bone marrow.
The main job of red blood cells is to carry oxygen from the lungs to cells throughout the body. Red blood cells aid in the removal of carbon dioxide, which is a waste product of several of the body's chemical processes. The body must produce additional red blood cells because they eventually break down and cannot perform their functions. Cells in the spleen, liver and bone marrow eliminate useless red blood cells.