Cardiorespiratory fitness (aerobic fitness or "cardio") is the ability of your heart to pump stronger and more efficiently and your muscles to use oxygen more efficiently. As you get more aerobically fit, your heart will pump more blood and oxygen with each beat (this is called "stroke volume") and your muscles will extract (or consume) more oxygen. For instance, if you have 100 oxygen molecules floating around in your bloodstream, a conditioned muscle might consume 75 molecules, whereas a deconditioned muscle might only consume 30, or even fewer than that. In fact, elite distance runners can be as much as three times more efficient at consuming oxygen than sedentary individuals. Running improves your aerobic fitness by increasing the activity of enzymes and hormones that stimulate the muscles and the heart to work more efficiently.
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