Two body organs the liver and the spleen remove worn-out red blood cells from the bloodstream and break them down. The liver uses coloring matter from the old cells in producing a digestive liquid called bile. The iron from hemoglobin is reused by the body to make new red blood cells. Worn-out white blood cells migrate to body tissues, where they die. Platelets probably wear out plugging tiny leaks in blood vessels.
As red blood cells develop in the marrow, they make hemoglobin. They also shrink and lose their nuclei. At maturity, they enter the bloodstream through tiny blood-filled cavities in the marrow.
Although all white blood cells originate in the red bone marrow, lymphocytes the T cells and B cells mature elsewhere in the body. T cells enter the bloodstream through the sinuses and move to the thymus, a gland near the base of the neck, where they complete their development. The mature T cells then travel to structures called lymph nodes, which occur in many areas of the body. B cells complete their maturation in the lymph nodes and spleen.