Which hormone produced by the kidneys stimulates erythropoiesis?

• A. Erythropoietin
• B. Renin
• C. Antidiuretic hormone
• D. Calcitonin

Answer: (A)

Erythropoiesis, the production of red blood cells, is driven by erythropoietin, a hormone released by the kidneys in response to low oxygen levels. When kidney cells sense hypoxia, they emit erythropoietin into the blood. This hormone travels to the bone marrow and binds to receptors on early red blood cell precursors, promoting their survival, proliferation, and maturation into mature erythrocytes. As a result, the body ramps up red blood cell production to improve oxygen delivery. While the liver contributes some erythropoietin during fetal life, the kidneys are the primary source in adults. The other substances listed have different roles: renin regulates blood pressure through the RAAS, antidiuretic hormone controls water reabsorption, and calcitonin lowers blood calcium. Thus, erythropoietin is the hormone that stimulates erythropoiesis.