erythropoietin

Erythropoietin (EPO) is the kidney’s “make more red blood cells” order. The kidneys constantly check how much oxygen the blood is carrying; when oxygen runs low, they release EPO to ramp up production of oxygen-carrying red cells.

EPO is a glycoprotein hormone made mostly by specialized cells in the kidney (with a small contribution from the liver). It travels to the bone marrow and stimulates progenitor cells to mature into red blood cells. Its release is governed by oxygen sensing through a system involving hypoxia-inducible factor (HIF): low oxygen ramps EPO up, restoring oxygen delivery — a clean example of a feedback loop tuned to a physiological need rather than a single hormone.

Clinically, EPO explains both a disease and a therapy. Chronic kidney disease causes anemia because failing kidneys make too little EPO, which is treated with synthetic EPO. Living at high altitude raises EPO naturally to compensate for thin air. EPO is also infamously abused as a performance-enhancing drug in endurance sports, where boosting red cells raises oxygen delivery but also dangerously thickens the blood.