Erythropoietin (EPO) is released in response to hypoxia from which organ?

• A. Liver
• B. Kidney
• C. Spleen
• D. Bone marrow

Answer: (B)

The kidneys are the main source of erythropoietin when oxygen delivery is low. Special cells in the kidney (peritubular interstitial cells near the juxtaglomerular apparatus) sense hypoxia and release erythropoietin into the bloodstream. Erythropoietin then travels to the bone marrow and stimulates erythroid precursor cells to proliferate and mature into red blood cells, increasing the blood’s oxygen-carrying capacity. The liver can produce some erythropoietin, especially during fetal life, but in adults the kidney is the primary source. The spleen and bone marrow are not producers of erythropoietin; the bone marrow is where red cells are produced in response to EPO, and the spleen mainly handles storage and destruction of RBCs.