The Heart
As you can feel with your hand, your heart is located near the center of your chest. The heart, which is composed almost entirely of muscle, is a hollow organ that is about the size of your clenched fist. The heart is enclosed in a protective sac of tissue called the pericardium (pehr-ih-KAHR-dee-um). In the walls of the heart, there are two thin layers of epithelial and connective tissue that form a sandwich around a thick layer of muscle called the myocardium. The powerful contractions of the myocardium pump blood through the circulatory system.
The heart muscle contracts on average 72 times a minute, pumping about 70 milliliters of blood with each contraction. This means that during one year, an average person's heart pumps more than enough blood to fill an Olympic-sized swimming pool. (An Olympic-sized swimming pool is about 2,000,000 liters: 0.07 liters × 4320 beats per hour × 24 hours × 365 days = 2,649,024 liters.)
Dividing the right side of the heart from the left side of the heart is the septum. The septum prevents the mixing of oxygen-poor and oxygen-rich blood. On each side of the septum are two chambers. The upper chamber, which receives the blood, is the atrium (plural: atria). The lower chamber, which pumps blood out of the heart, is the ventricle. The heart has four chambers—two atria and two ventricles.
Circulation Through the Body The heart functions as two separate pumps. The figure at right shows the circulation of blood through the body. The right side of the heart pumps blood from the heart to the lungs. This pathway is known as pulmonary circulation. In the lungs, carbon dioxide leaves the blood and oxygen is absorbed. The oxygen-rich blood then flows into the left side of the heart and is pumped to the rest of the body. This pathway is called systemic circulation. Blood that returns to the right side of the heart is oxygen-poor because cells have absorbed much of the oxygen and loaded the blood with carbon dioxide. At this point, it is ready for another trip to the lungs.
Circulatory Pathways
Human Circulation
Circulation Through the Heart Blood enters the heart through the right and left atria, as shown in the figure at right. As the heart contracts, blood flows into the ventricles and then out from the ventricles to either the body or the lungs. There are flaps of connective tissue called valves between the atria and the ventricles. Blood moving from the atria holds the valves open. When the ventricles contract, the valves close, which prevents blood from flowing back into the atria.
Structures of the Heart
At the exits from the right and left ventricles, there are valves that prevent blood that flows out of the heart from flowing back in. This system of valves keeps blood moving through the heart in one direction, like traffic on a one-way street. The one-way flow increases the pumping efficiency of the heart. The valves are so important to heart function that surgeons often attempt to repair or replace a valve that has been damaged due to disease. Click the activity at right to test your knowledge of the structures of the heart.
Heartbeat There are two networks of muscle fibers in the heart, one in the atria and one in the ventricles. When a single fiber in either network is stimulated, all the fibers are stimulated and the network contracts as a unit. Each contraction begins in a small group of cardiac muscle cells—the sinoatrial node—located in the right atrium. Because these cells “set the pace” for the heart as a whole by starting the wave of muscle contraction through the heart, they are also called the pacemaker.
Artificial Pacemakers
As shown in the figure at right, the impulse spreads from the pacemaker (SA node) to the network of fibers in the atria. It is picked up by a bundle of fibers called the atrioventricular node and carried to the network of fibers in the ventricles. When the network in the atria contracts, blood in the atria flows into the ventricles. When the muscles in the ventricles contract, blood flows out of the heart. This two-step pattern of contraction makes the heart a more efficient pump.
The Heartbeat
Your heart can beat faster or more slowly, depending on your body's need for oxygen-rich blood. During vigorous exercise, your heart rate may increase to about 200 beats per minute. Although the heartbeat is not directly controlled by the nervous system, the autonomic nervous system does influence heart rate. Neurotransmitters released by the sympathetic nervous system increase heart rate. Those released by the parasympathetic nervous system decrease heart rate.