Which organ detects low oxygen levels and releases erythropoietin?

• A. Lungs
• B. Liver
• C. Spleen
• D. Kidneys

Answer: (D)

Low oxygen in the blood triggers the kidneys to release erythropoietin. Specialized cells in the renal peritubular interstitium sense the drop in PO2 and, via hypoxia‑responsive pathways, increase EPO production and release into the circulation. EPO then stimulates the bone marrow to produce more red blood cells, raising the oxygen‑carrying capacity of the blood and helping to correct the hypoxia. In adults, the kidneys are the primary source of erythropoietin; the liver contributes mainly during fetal life and to a lesser extent afterward under certain conditions. The lungs are the site of gas exchange, not hormone signaling for red blood cell production, and the spleen is involved in blood storage rather than sensing oxygen levels for EPO release.