Erythropoietin is a glycoprotein hormone which induces the bone marrow stem cells to produce red blood cells, which is called erythropoiesis. Erythropoietin acts on the bone marrow to increase the production of red blood cells. Stimuli such as bleeding or moving to high altitudes, where oxygen is scarcer, trigger the release of erythropoietin (EPO). This hormone is produced by the peritubular capillary endothelial cells of the kidney. Erythropoietin is a cytokine for erythrocyte (red blood cell) precursors in the bone marrow.
Erythropoietin is also called hematopoietin and has other known biological functons; it is involved in the brain's response to neuronal injury, taking part in the wound healing process. When exogenous erythropoietin is used as a performance-enhancing drug, it is classified as an erythropoiesis-stimulating agent (ESA).
Since Erythropoietin increases the hematocrit, which is the proportion of blood volume that is occupied by red blood cells, it enables more oxygen to flow to the skeletal muscles. Some cyclists have used recombinant hematopoietin to enhance their performance. Although recombinant EPO has exactly the same sequence of amino acids as the natural hormone, the sugars attached by the cells used in the pharmaceutical industry differ from those attached by the cells of the human kidney. This difference can be detected by a test of the athlete's urine.