If you are reading this, it is because your body is supplying adequate energy to your brain, and you are able to interpret these letter combinations as words. The process by which your brain receives energy is called aerobic respiration. Your brain and heart need aerobic respiration to keep you alive.
Aerobic means "with oxygen." The process of aerobic respiration occurs when adequate oxygen is available. On a daily basis, this system supplies your cells with energy. Your body breaks down the fat and sugar you eat to produce a substance called adenosine triphosphate, or ATP.
ATP is transferred into your cells to provide you with energy. Every cell in your body uses ATP. Some cells can store more ATP than others. For example, your muscle cells have enough storage to provide energy so you can move; your body uses aerobic respiration to continuously produce ATP so that your muscles do not fatigue.
Heart and Brain
Unlike your muscle cells, the cells of your heart and brain do not have adequate storage space. Your heart needs to receive oxygen at all times to fill its cells with energy. This is reinforced when you are told the first thing to do in CPR, when someone's heart stops beating, is to circulate the oxygenated blood by breathing into a person and pumping his heart. Aerobic respiration is at work day and night to keep you alive.
When you exercise, the ATP needs to be delivered into the cells at a faster pace. Throughout your workout, aerobic respiration is working for you by circulating the oxygen and transporting energy into your cells. This process allows you to exercise for 30 to 60 minutes. If the energy were not in the cells, you would not be able to continue.
Your body uses different enzymes at different exercise levels to produce ATP. When you exercise at a low intensity, such as walking, your body is mainly using fat-metabolizing enzymes for ATP production. Carbohydrate-metabolizing enzymes are also at work, but their contribution increases as the intensity of your movement increases.