During aerobic exercise, when you increase your activity level and work your large muscle groups, your muscles require more fuel. Oxygen is the raw material or fuel used to make muscle food, a substance called ATP (adenosine triphosphate, or adenosine-5'-triphosphate). When you exercise, your muscles rapidly pull the oxygen out of your blood. Your brain senses the low oxygen levels and stimulates your body to respond very quickly by making you breathe more rapidly and more deeply. This maximizes the amount of oxygen in your lungs. Oxygen (which makes up 21 percent of the air we breathe) is then passed from your lungs into your blood, where it latches on to the hemoglobin molecules. It is then ready for transport to the muscle groups screaming for more fuel.