Red blood cell production occurs to increase oxygen delivery when oxygen levels are low.

• A. Increased capillarisation
• B. Oxygen debt
• C. Cardiac output
• D. Red blood cell production

Answer: (D)

When tissues experience low oxygen, the body boosts red blood cell production to raise the blood’s oxygen-carrying capacity. This happens because the kidneys detect hypoxia and release erythropoietin, which signals the bone marrow to produce more red blood cells. More red blood cells mean more hemoglobin available to bind and transport oxygen, so oxygen delivery to tissues improves during low-oxygen conditions.

Other options don’t directly increase the blood’s oxygen-carrying capacity. Increasing capillarisation helps oxygen diffuse from blood to muscles but doesn’t raise the amount of oxygen the blood can carry. Oxygen debt describes the extra oxygen needed after exercise to restore conditions, not a change in RBC production. Cardiac output is the volume of blood the heart pumps; it affects delivery but not the blood’s inherent capacity to carry oxygen. Red blood cell production directly enhances oxygen delivery, making it the best answer.