Which factor describes how muscle cells extract oxygen from capillary blood to produce energy?

• A. The heart's pumping ability
• B. The oxygen-carrying capacity of the blood
• C. The capillarization within a muscle
• D. The ability of the muscle cells to extract oxygen and use it

Answer: (D)

Amount of energy you can generate aerobically depends on how well muscle cells extract oxygen from the blood and use it in the mitochondria. This extraction is the arteriovenous difference: as blood passes through active muscle, the cells pull oxygen out and burn it to produce ATP. The muscle’s own capacity to do this hinges on mitochondrial density, oxidative enzyme activity, and related factors like myoglobin content, which together determine how efficiently oxygen is used. External delivery factors—how hard the heart can pump and how much oxygen the blood can carry—affect how much oxygen reaches the muscle, and capillarization increases diffusion, but the key determinant for producing energy from the available oxygen is the muscle cells’ ability to extract and utilize it.