Final answer:
Erythropoietin (EPO) is the hormone secreted by the kidneys that stimulates red blood cell production, in a process called erythropoiesis, in response to low oxygen levels in the blood.
Step-by-step explanation:
The hormone secreted by the kidney to increase red blood cells is erythropoietin (EPO). Erythropoietin is a glycoprotein hormone produced by the interstitial fibroblast cells in the kidneys in response to low levels of oxygen in the blood. It stimulates the process of erythropoiesis, which is the production of red blood cells (RBCs) in the bone marrow. The increase in RBC count enhances the oxygen-carrying capacity of the blood. Although EPO has legitimate medical uses, such as treatment for certain types of anemia, it is also known to be misused as a performance-enhancing drug in sports, which can lead to health risks like increased blood viscosity and the risk of blood clots