Final answer:
During exercise of skeletal muscles, aerobic respiration is used to generate ATP. Oxygen is needed for aerobic respiration and regular aerobic exercise improves efficiency of the circulatory system.
Step-by-step explanation:
Cellular respiration is the process through which cells convert carbohydrates and oxygen into energy in the form of ATP. During exercise of skeletal muscles, aerobic respiration is the primary process used to generate ATP. Oxygen is needed to sustain aerobic respiration, and a steady supply of oxygen is provided to the muscles through the circulatory system. Additionally, regular aerobic exercise improves the efficiency of the circulatory system and allows for longer periods of oxygen supply to the muscles.