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The Heart

The heart's duties are much broader than simply pumping blood continuously throughout life. The heart must also be able to respond to changes in the body's demand for oxygen. The heart works very differently during sleep, for example, than in the middle of a marathon run. The heart and the rest of the circulatory system can respond almost instantaneously to constantly shifting situations—when a person stands up or lies down, for example, or when a person is faced with a potentially dangerous situation.

To begin with, the heart is really a muscle. It's located a little to the left of the middle of your chest, and it's about the size of your fist. The heart is made up of four different blood-filled areas, and each of these areas is called a chamber. There are two chambers on each side of the heart. One chamber is on the top and one chamber is on the bottom. The two chambers on top are called the atria. If you're talking only about one, call it an atrium. The atria are the chambers that fill with the blood returning to the heart from the body and lungs. The heart has a left atrium and a right atrium. The heart has a left ventricle and a right ventricle. Their job is to squirt out the blood to the body and lungs. Running down the middle of the heart is a thick wall of muscle called the septum. The septum's job is to separate the left side and the right side of the heart.

Although the right and left halves of the heart are separate, they both contract in unison, producing a single heartbeat. The sequence of events from the beginning of one heartbeat to the beginning of the next is called the cardiac cycle. The cardiac cycle has two phases: diastole, when the heart's chambers are relaxed, and systole, when the chambers contract to move blood. During the systolic phase, the atria contract first, followed by contraction of the ventricles. This sequential contraction ensures efficient movement of blood from atria to ventricles and then into the arteries. If the...