Which statement best describes the adaptations that increase oxygen utilization?

• A. Increase capillary density around muscle fiber, enzyme activity, and mitochondria in muscle cells
• B. Decrease capillary density
• C. Decrease mitochondria number
• D. Increase only heart size

Answer: (A)

Improving oxygen utilization comes from muscle-level adaptations that enhance how efficiently oxygen is delivered and used for energy. Training increases capillary density around each muscle fiber, which shortens the path oxygen must travel from blood to the muscle cells and improves delivery. It also boosts the number of mitochondria inside muscle cells, giving more sites for aerobic respiration, and raises the activity of oxidative enzymes that drive the processes converting fuels into ATP using oxygen. Together, these changes raise the muscle’s aerobic capacity and its ability to use oxygen during work. The other options don’t support this combined improvement: fewer capillaries would limit oxygen delivery, fewer mitochondria would reduce oxidative capacity, and increasing heart size alone doesn’t directly raise the muscle’s capacity to utilize oxygen without the peripheral adaptations.