Erythropoietin is a hormone in the bloodstream that stimulates the the production of red blood cells. Red blood cells are also called erythrocytes and poiesis is from the Greek word for "make." Erythropoietin travels to the red blood marrow in the bones to signal the start of production of red blood cells. Erythropoietin is synthesized in the kidneys. Oxygen is a feedback inhibitor for its synthesis. In other words, low levels of oxygen in the blood are detected by the kidneys and stimulate the production of erythropoietin. When oxygen levels are higher, the kidneys sense this and shut down the production of the hormone.