Aerobic literally means ‘‘with air’’; that is, oxygen is required. Aerobic exercise involves the rhythmic contrac¬tion and relaxation of large muscle groups and movement of joints, for example, brisk walking, swimming, cycling, jogging, and dancing.
Oxygen is obtained in the lungs and transported by the blood. Exercising muscles require an increasing supply of oxygen, glucose, and other constituents. The body gets the additional oxygen by increasing the rate of breathing, the heart rate, and the output of blood from the heart. The amount of blood pumped each minute (cardiac output) increases from the average resting value of about 5 L to around 20 L. A large part of this output goes to the exercising muscles.
During aerobic exercise oxygen is used by the muscles. Oxygen converts energy in glucose and fatty acids to the energy form, adenosine triphosphate (ATP). Muscle fibers require a direct source of energy such as ATP to contract and cause limbs to move during exercise. The quantity of oxygen consumed during aerobic exercise tells us how many calories of sugar and fats were used and quantifies the caloric cost of the exercise. . Walking uses much less oxygen than a slow jog, which requires less oxygen than a fast run.

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