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 atria and ventricles contracted simultaneously, the heart would not be able to move as much blood with each beat.

During diastole, both atria and ventricles are relaxed, and the atrioventricular valves are open. Blood pours from the veins into the atria, and from there into the ventricles. In fact, most of the blood that enters the ventricles simply pours in during diastole. Systole then begins as the atria contract to complete the filling of the ventricles. Next, the ventricles contract, forcing blood out through the semi lunar valves and into the arteries, and the atrioventricular valves close to prevent blood from flowing back into the atria. As pressure rises in the arteries, the semi lunar valves snap shut to prevent blood from flowing back into the ventricles. Diastole then begins again as the heart muscle relaxes-the atria first, followed by the ventricles-and blood begins to pour into the heart once more.

An instrument known as a stethoscope is used to detect internal body sounds, including the sounds produced by the heart as it is beating. The characteristic heartbeat sounds are made by the valves in the heart--not by the contraction of the heart muscle itself. The sound comes from the leaflets of the valves slapping together. The closing of the atrioventricular valves, just before the ventricles contract, makes the first heart sound. The second heart sound is made when the semi lunar valves snap closed. The first heart sound is generally longer and lower than the second.

Blood pressure, the pressure exerted on the walls of blood vessels by the flowing blood, also varies during different phases of the cardiac cycle. Blood pressure in the arteries is higher during systole, when the ventricles are contracting, and lower during diastole, as the blood ejected during systole moves into the body's capillaries. Blood pressure is measured in millimeters of mercury using a sphygmomanometer, an instrument that consists of a pressure recording device and an inflatable cuff that is usually placed around the upper arm. Normal blood pressure in an adult is about 120 mm of mercury during systole, and about 80 mm of mercury during diastole. Blood pressure is usually noted as a ratio of systolic pressure to diastolic pressure--for example, 120/80. A person's blood pressure may increase for a short time during moments of stress or strong emotions. However, a prolonged or constant elevation of blood pressure, a condition known as hypertension, can increase a person's risk for heart attack, stroke, heart and kidney failure, and other health problems.

Unlike most muscles, which rely on nerve impulses to cause them to contract, heart muscle can contract of its own accord. Certain heart muscle cells have the ability to contract spontaneously, and these cells generate electrical signals that spread to the rest of the heart and cause it to contract with a regular, steady beat. The heartbeat begins with a small group of specialized muscle cells located in the upper right-hand corner of the right atrium. This area is known as the sinoatrial (SA) node. Cells in the SA node generate their electrical signals more frequently than cells elsewhere in the heart, so the electrical signals generated by the SA node synchronize the electrical signals traveling to the rest of the heart. For this reason, the SA node is also known as the heart's pacemaker.

Impulses generated by the SA node spread rapidly throughout the atria, so that all the muscle cells of the atria contract virtually in unison. Electrical impulses cannot be conducted through the partition between the atria and ventricles, which is primarily made of fibrous connective tissue rather than muscle cells. The impulses from the SA node are carried across this connective tissue partition by a small bridge of muscle called the atrioventricular conduction system. The first part of this system is a group of cells at the lower margin of the right atrium, known as the atrioventricular (AV) node. Cells in the AV node conduct impulses relatively slowly, introducing a delay of about two-tenths of a second before an impulse reaches the ventricles. This delay allows time for the blood in the atria to empty into the ventricles before the ventricles begin contracting.

After making its way through the AV node, an impulse passes along a group of muscle fibers called the bundle of HIS, which span the connective tissue wall separating the atria from the ventricles. Once on the other side of that wall, the impulse spreads rapidly among the muscle cells that make up the ventricles. The impulse travels to all parts of the ventricles with the help of a network of fast-conducting fibers called Purkinje fibers. These fibers are necessary because the ventricular walls are so thick and massive. If the impulse had to