Final answer:
B) Low, kidneys is the correct answer. The kidneys release EPO when there is a low oxygen level in the blood, stimulating the production of red blood cells in the bone marrow.
Step-by-step explanation:
When blood oxygen levels are low, the kidneys release erythropoietin (EPO), a hormone critical for the production of red blood cells. The hormone EPO stimulates the bone marrow to produce more erythrocytes, the oxygen-carrying cells in the blood. This physiological response is essential for maintaining adequate oxygen levels throughout the body. While the liver can also secrete EPO, the kidneys are the primary producers, accounting for 85 percent of circulating EPO.
Conditions that might trigger increased EPO production include living at high altitudes, where there is less atmospheric pressure and oxygen, starting an aerobic exercise program that requires more oxygen, or experiencing conditions such as severe bleeding or malnutrition that reduce erythrocyte numbers. However, it's important to note that renal failure can lead to anemia due to the lack of EPO production, which poses significant challenges for the body's oxygen supply.