The various cellular elements of blood are produced by the hematopoietic system (see Table 15-10). The main components of the hematopoietic system are bone marrow and blood; in addition, the liver and spleen are important accessory organs. The spleen is a blood reservoir, and it can also degrade time-expired red blood cells. The liver stores vitamin BI2, which is essential for erythrocyte production as well as the hemoglobin biosynthesis.

The term "erythron" describes the circulating cells and their precursors present in the blood compartment. The erythron may be thought of as a disperse organ whose prime function is the transport of oxygen and carbon dioxide, as well as the maintenance of blood pH. In adults, the erythron is in a steady state where the loss of cells is precisely balanced by the new production of cells. Every minute a human must biosyn-thesize approximately 150 million erythrocytes and 120 million granulocytes, as well as numerous mononuclear cells and platelets. There are numerous hormones and regulatory processes that control crucial aspects of the various cell populations of the erythron.

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