Different components of human blood are replaced at different rates. According to the AABB, a non-profit organization that represents organizations that conduct blood transfusions, plasma is replaced within a few hours of a blood donation while the red blood cells take a few weeks to replace. Doctors usually only remove about one pint of blood during a transfusion, which represents about 10 percent of the total blood volume.
According to Palomar Community College, human blood contains approximately 4,000 different components. These are replaced on a regular basis by different processes in the body. For example, bone marrow produces stem cells which in turn produce red blood cells. Red blood cells live for about 4 months before they are removed and recycled by the spleen. White blood cells, by contrast, only last an average of 18 to 36 hours before they are removed. Occasional white blood cells last for up to 1 year before they are removed. Platelets, which help to form clots, only survive about 10 days before they are removed from the blood stream.
People are usually able to donate blood every 56 days. Blood donations are an important component of health care systems because to date, no synthetic substitute exists, according to the AABB.