Which hormone is primarily responsible for stimulating the production of red blood cells?

Erythropoietin is the hormone primarily responsible for stimulating the production of red blood cells, a process known as erythropoiesis. This hormone is produced primarily by the kidneys in response to low oxygen levels in the blood. When oxygen levels decrease, such as in cases of anemia or at high altitudes, erythropoietin is secreted into the bloodstream, where it targets the bone marrow. In the bone marrow, erythropoietin encourages the differentiation and proliferation of erythroid progenitor cells into mature red blood cells, thereby increasing the oxygen-carrying capacity of the blood.

This function is crucial for maintaining adequate oxygen levels in the body's tissues, especially during times of physical demand or in pathological conditions where oxygen supply is compromised. The other hormones listed do not have a direct role in red blood cell production; insulin primarily influences glucose metabolism, cortisol is involved in metabolism and stress responses, and adrenaline primarily prepares the body for 'fight or flight' responses. Thus, erythropoietin stands out as the key hormone linked to the production of red blood cells.