Which cells secrete erythropoietin, and what stimulus prompts them?

1. Erythropoietin is a glycoprotein hormone that prompts the marrow to make erythrocytes. 2. It is secreted by the interstitial fibroblast cells of the kidneys. 3. The trigger is low oxygen, not low iron, which is why anemia of chronic kidney disease is a production problem rather than a substrate problem. 4. The kidneys are well placed for this job because they filter about 20 percent of the resting blood volume each day. 5. Rising oxygen saturation switches erythropoietin secretion back down in a negative feedback loop. 6. The liver option is a trap because the liver does make thrombopoietin and most plasma proteins, but not erythropoietin. 7. The splenic macrophage option pairs the correct stimulus with the wrong cell, so reading only the second half of each option is fatal here. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.2 Production of the Formed Elements_