Final answer:
Erythropoietin is the hormone secreted by the kidneys to increase red blood cell synthesis.
Step-by-step explanation:
Erythropoietin is the hormone secreted by the kidneys to increase red blood cell synthesis. When the kidneys detect low levels of oxygen in the blood, they release erythropoietin, which stimulates the production of red blood cells in the bone marrow. This increase in red blood cell production helps to deliver more oxygen throughout the body.