When you exercise, your muscle cells use oxygen to convert the energy stored in glucose into the energy stored in ATP, which they then use to drive muscle contractions. When you exercise, your muscles need more oxygen. Therefore, to maintain an adequate oxygen level in all of the tissues in your body, you increase respiration rate and depth. This allows you to take in more oxygen. Your heart rate increases, which allows it to deliver more oxygenated blood to your muscles and other organs that will need more oxygen. Is this an example of a positive or negative feedback loop?