Hemopoiesis

The life span of the formed elements is very short. Most erythrocytes, leukocytes, and platelets normally live only a few hours to a few weeks, so the body must form new blood cells and platelets quickly and continuously. The process by which this replacement occurs is called hemopoiesis, or hematopoiesis (from the Greek root haima- = “blood”; -poiesis = “production”). For example, when you donate a unit of blood during a blood drive (approximately 475 mL, or about 1 pint), your body typically replaces the donated plasma within 24 hours, but it takes about four to six weeks to replace the blood cells.

Most hemopoiesis occurs in the red bone marrow, a connective tissue within the spaces of spongy bone. In children, hemopoiesis can occur in the medullary cavity of long bones, but in adults, the process is largely restricted to the cranial and pelvic bones, the vertebrae, the sternum, and the proximal epiphyses of the femur and humerus.

All formed elements arise from stem cells of the red bone marrow. Hemopoiesis begins when the hematopoietic stem cell is exposed to chemical stimuli that prompt it to divide and differentiate. Development from stem cells to mature cells is initiated by hemopoietic growth factors that include the following:

• Erythropoietin (EPO) is a hormone made by the kidneys in response to low oxygen levels. It prompts the production of erythrocytes. Some athletes use synthetic EPO as a performance-enhancing drug (called blood doping) to increase RBC counts and subsequently increase oxygen delivery to tissues throughout the body. EPO is a banned substance in most organized sports, but it is also used medically in the treatment of certain anemias, specifically those triggered by renal failure and certain types of cancer, and other disorders in which increased erythrocyte counts and oxygen levels are desirable.
• Colony-stimulating factors (CSF) are glycoproteins needed to stimulate leukopoiesis, the process of white blood cell formation and differentiation. CSFs are often given to patients undergoing chemotherapy and bone marrow transplants to prevent low white blood cell counts.
• Thrombopoietin, another hormone, is produced by the liver and kidneys. It triggers the development of megakaryocytes into platelets (thrombocytes).