Cellular respiration is the process in which your muscles use oxygen to produce ATP energy. The process is simple. Your body obtains oxygen from the ambient air you breathe. It enters the blood stream and is carried to your muscles, where some of it is used immediately, and the rest is stored by a compound called myoglobin. Whether you’re exercising or not, the oxygen in your body is used to break down glucose and create the fuel for your muscles called ATP.