Hematopoiesis: The production of all types of blood cells generated by a remarkable self-regulated system that is responsive to the demands put upon it.
The levels of the different types of the white blood cells in the granulocyte series (the neutrophils, eosinophils, and basophils) are normally maintained within preset normal ranges and prompt adjustments are made in response to demands such as infections, allergic reactions, etc.
In much the same way, the levels of the other types of white cells (the monocytes and lymphocytes), the platelet, and red blood cell are regulated within normal ranges and respond rapidly to demands. The monocytes respond to infections, the lymphocytes respond to immune challenges, the platelets respond to hemorrhage or inflammation, and the red blood cells (erythrocytes) respond to hypoxia (low oxygen levels).
When hematopoiesis is disturbed, the first cells to drop are usually the neutrophils (which have a life span in the blood of only 6 to 8 hours) followed by platelets (with a 10-day life span). Anemia develops more slowly, over a much longer span of time (since the red blood cells have a 120-day lifespan).
All types of blood cells are derived from primitive cells (stem cells) in the bone marrow. The production of blood cells is largely controlled by feedback. When the demand for production of cells of a particular type of cells increases or the levels of the cells fall in blood, stimulatory substances called cytokines are released. And the cytokines stimulate the stem cells to generate new mature blood cells. This occurs in the few days required for blood cell maturation.
The production of lymphocytes is an exception to this neat scenario. Many more lymphocytes are generated daily than are needed in the blood. Most lymphocytes are destroyed during development. Thus, lymphopoiesis (the production of lymphocytes) is inefficient compared to all other hematopoiesis.