THE HEART

Heart is a hollow muscular organ that pumps blood through the body. The heart, blood, and blood vessels make up the circulatory system, which is responsible for distributing oxygen and nutrients to the body and carrying away carbon dioxide and other waste products. The heart is the circulatory system's power supply. It must beat ceaselessly because the body's tissues-especially the brain and the heart itself-depend on a constant supply of oxygen and nutrients delivered by the flowing blood. If the heart stops pumping blood for more than a few minutes, death will result.

The human heart is shaped like an upside-down pear and is located slightly to the left of center inside the chest cavity. About the size of a closed fist, the heart is made primarily of muscle tissue that contracts rhythmically to propel blood to all parts of the body. This rhythmic contraction begins in the developing embryo about three weeks after conception and continues throughout an individual's life. The muscle rests only for a fraction of a second between beats. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters (179 million quarts) of blood.

STRUCTURE OF THE HEART

The human heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. The atria are sometimes known as auricles. They collect blood that pours in from veins, blood vessels that return blood to the heart. The heart's lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles propel blood into arteries, blood vessels that carry blood away from the heart.

A wall of tissue separates the right and left sides of the heart. Each side pumps blood through a different circuit of blood vessels: The right side of the heart pumps oxygen-poor blood to the lungs, while the left side of the heart pumps oxygen-rich blood to the body. Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body's tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart.

The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood that is oxygen-rich returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta, the body's largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.

•A. THE HEART VALVES

Four valves within the heart prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction, the valves close. Two valves, known as atrioventricular valves, are located between the atria and ventricles. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve. The left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two heart valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve.

•B. THE MYOCARDIUM

Muscle tissue, known as myocardium or cardiac muscle, wraps around a scaffolding of tough connective tissue to form the walls of the heart's chambers. The atria, the receiving chambers of the heart, have relatively thin walls compared to the ventricles, the pumping chambers. The left ventricle has the thickest walls-nearly 1 cm (0.5 in) thick in an adult-because it must work the hardest to propel blood to the farthest reaches of the body.

•C. THE PERICARDIUM

A tough, double-layered sac known as the pericardium surrounds the heart. The inner layer of the pericardium, known as the epicardium, rests directly on top of the heart muscle. The outer layer of the pericardium attaches to the breastbone and other structures in the chest cavity and helps hold the heart in place. Between the two layers of the pericardium is a thin space filled with a watery fluid that helps prevent these layers from rubbing against each other when the heart beats.

•D. THE ENDOCARDIUM

The inner surfaces of the heart's chambers are lined with a thin sheet of shiny, white tissue known as the endocardium. The same type of tissue, more broadly referred to as endothelium, also lines the body's blood vessels, forming one continuous lining throughout the circulatory system. This lining helps blood flow smoothly and prevents blood clots from forming inside the circulatory system.

•E. THE CORONARY ARTERIES

The heart is nourished not by the blood passing through its chambers but by a specialized network of blood vessels. Known as the coronary arteries, these blood vessels encircle the heart like a crown. About 5 percent of the blood pumped to the body enters the coronary arteries, which branch from the aorta just above where it emerges from the left ventricle. Three main coronary arteries-the right, the left circumflex, and the left anterior descending-nourish different regions of the heart muscle. From these three arteries arise smaller branches that enter the muscular walls of the heart to provide a constant supply of oxygen and nutrients. Veins running through the heart muscle converge to form a large channel called the coronary sinus, which returns blood to the right atrium.

FUNCTION OF THE HEART

The heart's duties are much broader than simply pumping blood continuously throughout life. The heart must also 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 5-km (3-mi) run. Moreover, the heart and the rest of the circulatory system can respond almost instantaneously to shifting situations-when a person stands up or lies down, for example, or when a person is faced with a potentially dangerous situation

THE CARDIAC CYCLE

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 semilunar 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 semilunar 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.

A health-care professional uses an instrument known as a stethoscope 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 semilunar valves snap closed. The first heart sound is generally longer and lower than the second, producing a heartbeat that sounds like lub-dup, lub-dup, lub-dup.

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 (mm) 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 less than 120 mm of mercury during systole, and less than 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.

SOURCE: Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.