Whether you're a world-class athlete pounding out miles during an Ironman competition or a middle-age executive just trying to stave off the battle of the bulge, the most important organ of your body is your heart. Small by most standards, the human heart is about the size of an average adult fist and circulates about 5 liters of blood every minute while at rest. But as you become more fit, this tiny muscle increases both its efficiency and strength to meet the demands of a 100-mile century ride or a 2-mile swim in open water. Here's what's going on inside.
Heart as a Pump
As magnificent as it is, the heart is nothing more than a four-chambered pump. The two top chambers of the heart are called atria (atrium is one chamber), and the two lower chambers are called ventricles. Deoxygenated blood returning from the body enters the right atrium from the inferior and superior vena cava. From there, it passes into the right ventricle, where it gets sent to the lungs for oxygenation via the pulmonary artery. After the blood has become oxygenated from the lungs, it returns to the left atrium via the left and right pulmonary veins, enters the left ventricle, then leaves via the aorta and supplies blood to the rest of the body.
Heart at Rest
Even at rest, the heart must continue to supply the entire body with a constant, circulating supply of blood. Whether you're a couch potato or highly trained athlete, the resting heart does the same job: pumping approximately 5 liters of blood throughout the entire body, once every minute. This is called the cardiac output and represents how much blood leaves the heart with every contraction. To insure that the heart has all of the blood that it needs, it is equipped with a series of coronary arteries and a dense network of over 2,000 capillaries per cubic inch.
Effects of Exercise on the Resting Heart
While most people know that the human heart becomes more efficient during exercise, it also becomes more efficient at rest. The cardiac output of the heart can be calculated as the stroke volume (or the amount of blood ejected each time the left ventricle contracts) multiplied by the heart rate. For instance, the average stroke volume is 70 ml/beat, and the average heart beats 70 times per minute. Multiplying those results in slightly less than 5,000 ml/min. As the athlete become fitter, the heart becomes stronger, increasing its stroke volume. Theoretically, if an athlete's stroke volume increases from 70 to 90 ml/beat, the heart has become 20 percent more efficient. To eject the same 5,000 ml/min, the reduction would be made up by the lower resting heart rate. It's not entirely that simple, but the basic principle applies.
Submaximal Cardiac Output
If the resting heart is becoming more efficient from exercise, it also makes sense that the exercising heart rate becomes more efficient as well. During submaximal exercise, the heart responds by increasing cardiac output. Remember that cardiac output is the product of the stroke volume multiplied by the heart rate. Since the trained athlete has both a lower heart rate response and a higher stroke volume, he can accomplish the same amount of work more efficiently than his lesser-trained counterpart--up to a point.
Maximal Responses to Exercise
Regardless of your fitness level, everyone has a maximum heart rate that declines with age. So a world-class athlete and a couch potato of the same age, let's say two 40-year-old males, should both have the same maximum heart rate. You can compute it by subtracting your age from 220. For instance, the two previously described 40-year-old men would have the same maximum heart rate of 180. That's on paper. The only way to determine your actual maximum heart rate is to exercise on a treadmill until you literally run out of gas. And even this has a number of limitations, such as how well you feel on that day, how motivated you are, how well the test is conducted, what type of exercise test is conducted and a host of other variables. For everyday purposes, the formula of 220 minus your age works well enough for most circumstances.
Responses of Training
If the fit and the unfit have the same amount of blood to circulate throughout their bodies and their maximum heart rate doesn't change with training, what does? The important thing to remember is that as the heart muscle becomes stronger and more efficient with training, each time it contracts, it squeezes out a larger volume of blood per beat, at a lower heart rate. This translates to more work being done at a more efficient heart rate. At the same time, the heart has more time to fill the left ventricle before it pumps the blood out to the body. Another added bonus is increased blood volume. With regular training, athletes increased blood volume and added red blood cell count. A higher red blood cell count means more oxygen-carrying capacity in the blood.
