Final answer:
Erythropoietin (EPO) is the key cytokine hormone that accelerates the recovery of red blood cell mass post hemorrhage, by stimulating the production of red blood cells in the bone marrow.
Step-by-step explanation:
The principal factor for accelerating recovery of red blood cell mass following hemorrhage is erythropoietin (EPO). EPO is a glycoprotein hormone secreted by the kidneys in response to low oxygen levels in blood. It stimulates the bone marrow to produce more red blood cells, which increases the oxygen-carrying capacity of the blood.
This capacity is essential in treating certain types of anemia, especially those caused by cancer. While it has uses in medical treatments, EPO is a banned substance in most organized sports due to its potential for misuse as a performance-enhancing drug.