Blood cells are produced in the bone marrow, a jellylike substance inside the bones that is composed of, among other things, fat, blood, and special cells that turn into the various kinds of blood cells. In children, the marrow of most of the bones produces blood. But in adults, only the marrow of certain bones -- the spine, ribs, pelvis, and some others -- continues to make blood. Bone marrow that actively produces blood cells is called red marrow, and bone marrow that no longer produces blood cells is called yellow marrow.
All blood cells come from the same kind of stem cell, which has the potential to turn into any kind of blood cell. These stem cells are called pluripotential hematopoietic stem cells.
As the blood cells develop from the stem cells in the marrow, they seep into the blood that passes through the bones and on into the bloodstream. The different kinds of blood cells have different "life spans" -- red blood cells last about 120 days in the bloodstream; platelets about 10 days; and the various kinds of white blood cells can last anywhere from days to years.
The body has a feedback system that tells it when to make new red blood cells. If bodily oxygen levels are low (as they would be if there are too few red blood cells circulating), the kidneys produce a hormone called erythropoietin, which stimulates the stem cells in the marrow to produce more red blood cells.
-- Sue Wilson