词条 | blood cell formation |
释义 | blood cell formation biochemistry also called Hematopoiesis, or Hemopoiesis, ![]() ![]() In the human embryo, the first site of blood formation is the yolk sac. Later in embryonic life, the liver becomes the most important red blood cell-forming organ, but it is soon succeeded by the bone marrow, which in adult life is the only source of both red blood cells and the granulocytes. Both the red and white blood cells arise through a series of complex, gradual, and successive transformations from primitive stem cells (stem cell), which have the ability to form any of the precursors of a blood cell. Precursor cells are stem cells that have developed to the stage where they are committed to forming a particular kind of new blood cell. In a normal adult the red cells of about half a litre (almost one pint) of blood are produced by the bone marrow every week. Almost 1 percent of the body's red cells are generated each day, and the balance between red cell production and the removal of aging red cells from the circulation is precisely maintained. The rate of blood cell formation varies depending on the individual, but a typical production might average 200,000,000,000 red cells per day, 10,000,000,000 white cells per day, and 400,000,000,000 platelets per day. |
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