Erythropoietin also known as EPO, is a glycoprotein that acts specifically on the bone marrow to cause an increased secretion of erythrocytes (red blood cells). The stimulus for the production of EPO is reduced red blood cell count (hematocrit). Accidents which result in bleeding or staying in high altitude areas can result in reduced hematocrit and once this occurs, EPO is prodced by the extraglomerular mesangial cells in order to restore the hematocrit levels.