Question

when the oxygen content of the blood is low.______is released by the kidneys to increase red blood cell production in the red bone marrow.

Explanation:

Erythropoietin is a hormone. When blood oxygen is low, the kidneys detect this and release erythropoietin. This hormone acts on the red bone marrow to stimulate the production of red blood cells. Red blood cells carry oxygen, so increasing their production helps to raise the oxygen - carrying capacity of the blood.

Answer:

Erythropoietin