Erythropoietin, also known as EPO, is a hormone that is produced by the kidneys to stimulate the production of red blood cells. The kidneys monitor the condition of the blood that passes through their capillaries, including the oxygen-carrying capacity of the blood. When the blood becomes hypoxic, meaning that it is carrying deficient levels of oxygen, cells lining the capillaries begin producing EPO and release it into the bloodstream. EPO travels through the blood to the r ed bone marrow , where it stimulates hematopoietic cells to increase their rate of red blood cell production. Red blood cells contain hemoglobin, which greatly increases the blood’s oxygen-carrying capacity and effectively ends the hypoxic conditions.
In normal circumstances glomerular pressure is believed to be about 45 millimetres of mercury (mmHg), which is a higher pressure than that found in capillaries elsewhere in the body. As is the case in renal blood flow, the glomerular filtration rate is also kept within the limits between which autoregulation of blood flow operates. Outside these limits, however, major changes in blood flow occur. Thus, severe constriction of the afferent vessels reduces blood flow, glomerular pressure, and filtration rate, while efferent constriction causes reduced blood flow but increases glomerular pressure and filtration.