When you do aerobic exercise your heart pumps blood throughout your heart more rapidly, which has the effect of strengthening your heart muscle. A bigger, stronger heart muscle works more efficiently, taking in more blood per beat and distributing larger volumes of fresh oxygen throughout the body. As a result, a stronger, more efficient heart doesn’t need to beat as quickly during exercise; this is why elite athletes have resting heart rates as low as 40 beats per minute -- 20 to 40 beats lower than the average heart rate.
The more aerobic exercise you do, the more efficient your muscles become at receiving greater volumes of oxygen. This is a result of a rise in the number of enzymes working to deliver oxygen to your muscles. With a steady, rich supply of oxygen available, muscles don’t tire as quickly, and this is why regular aerobic exercise improves your fitness endurance over time.
Your muscles are home to millions of microscopic mitochondria that use oxygen as fuel to break down proteins, fats and carbohydrates. This, in turn, becomes a viable fuel for your muscles to function properly. Rich oxygen supplies increase the density of mitochondria active in your muscle cells, which not only drives up athletic performance but also helps you to burn more fat and carbohydrates. Finally, the more oxygen present, the more fat you burn; fat has nine calories per gram and carbs only have four, so fat provides more energy to your muscles, but it requires more oxygen to burn into fuel for your body; fat is denser than carbohydrates. The cycle continues: Efficient, oxygen-rich muscles burn more fuel, more fuel means you can exercise longer, and the more you exercise the more fit you will be.