With aerobic exercise, your heart's left ventricle—the chamber that pumps blood out to your body—grows larger and holds more blood. The average adult left ventricle holds about 120 milliliters (about half a cup), and pumps out 55-60 percent of its contents with each beat—66-72 milliliters (about a quarter of a cup). Aerobically fit hearts pump out 65 percent, a bonus teaspoon to a tablespoon with every beat.
The ventricle wall also thickens proportionally to the increase in the chamber's size. A strong biceps allows you to lift more weight and doesn't tire as easily. A strong heart muscle pumps more blood, so it requires fewer beats, hence the lower heart rate of athletes. And it doesn't tire as fast.
Fewer beats also mean the heart itself receives more blood, because the blood vessels that serve your heart can only fill when it's relaxed—between beats. The relaxation phase of a beating heart takes up two-thirds of each cycle, the contraction about one-third. This means your heart gets oxygen 70 percent of the time and is deprived of it 30 percent of the time. When you start to exercise, at first your heart rate goes up and the contraction phase takes up half your heart's time. Now your heart is getting oxygen only 50 percent of the time—quite a challenge, especially when you consider that your heart's work increases sharply with exercise. The increased demands placed on the untrained heart early in an exercise program coupled with the reduced amount of time the heart itself gets blood are among the reasons exercise can produce symptoms for people with silent heart disease.
As you continue to exercise, however, your body naturally regulates your heart. Your heart grows and thickens and pumps a greater volume of blood per beat. In time it accomplishes so much more with each beat that it requires fewer beats. Your heart rate goes down. With fewer beats, your heart is relaxed more often and thus receives more oxygen. For this reason, you need to start exercise slowly and build gradually to give your heart time to build a better system.
Exercise also improves your second heart—the mechanism by which blood is returned to your heart, primarily through the squeezing action of your muscles. With exercise, your muscles squeeze better and send more blood back to your heart. The amount of blood your muscles return to your heart determines how well you refill your left ventricle, and that determines how much blood gets pumped to your body.
Without your second heart at work, your left ventricle doesn't refill adequately and your heart cannot supply adequate blood to your body. Think about a bridesmaid in a wedding or a soldier in formation, standing completely still for prolonged periods, with no muscle action to return blood to the heart. Less blood to the heart means less output to the brain. And this can cause blood pressure—and the bridesmaid or the soldier—to suddenly drop.
( Eric Heiden, M.D., a five-time Olympic gold medalist speed skater, is now an orthopedic surgeon in Utah. He co-authored "Faster, Better, Stronger: Your Fitness Bible" (HarperCollins) with exercise performance physician Max Testa, M.D., and DeAnne Musolf. Visit www.fasterbetterstronger.com.)