The heart is a muscular structure about the size of your fist, connected to the rest of your body by a 60,000-mile network of blood vessels. Shaped more like a cone than the Valentine we picture, the heart lies slightly to the left of the center of your chest, protected by the breastbone (sternum) in front, the spinal column in back, and the lungs and ribs on both sides. The heart is positioned so that the tip of the cone points toward your left hip.
In a lifetime of 68 years, the human heart will typically beat 2.5 billion times. That amounts to about once a second, every minute of your life--considerably faster during exercise and slower when you sleep. Although the heart only weighs between 7 and 15 ounces (depending on your size and weight), it can pump five or more quarts of blood a minute. Each day, your heart pumps about 2,000 gallons of blood throughout your body--enough blood in a lifetime to fill more than three supertankers--and is strong enough to drive a single drop of blood throughout your entire body in about 24 seconds.
The heart's pumping action consists of squeezing blood out of its chambers, (contracting) and then expanding to allow blood to flow back in (relaxation). The action is as simple as squeezing water out of a soft plastic bottle while holding it under water and then releasing your grasp so water is sucked back into the bottle as it expands. This cycle of contraction and relaxation, the heartbeat, creates the pulse you can feel in your wrist. Doctors look at two measures to determine the strength of the heart muscle: ejection fraction and cardiac output.
Ejection fraction: No matter how forceful your heart's contraction, it doesn't pump all the blood out of the ventricles with each beat. The portion of blood pumped out of a filled ventricle is referred to as the ejection fraction. A normal ejection fraction of about 60 to 65 percent means that about two-thirds of the blood in the ventricles is pumped out with each beat. The ejection fraction is a good indicator of the overall function of the heart. In a healthy person, the ejection fraction increases about 5 percent with exercise. However, when the ventricles are diseased, as a result of a heart attack or other heart disorders, the ejection fraction can fall to 30 percent.
Cardiac output: The actual amount of blood pumped by the left ventricle during one contraction is called the stroke volume. The stroke volume and the heart rate determine the cardiac output, which is the amount of blood the heart pumps through the entire circulatory system in one minute.