A look at cardiac anatomy

Cardiac anatomy includes the location of the heart; the structure of the heart, heart wall, chambers, and valves; and the layout and structure of coronary circulation.

Outside the heart

The heart is a cone-shaped, muscular organ. It’s located in the chest, behind the sternum in the mediastinal cavity (or mediastinum), between the lungs, and in front of the spine. The heart lies tilted in this area like an upside-down triangle. The top of the heart, or its base, lies just below the second rib; the bottom of the heart, or its apex, tilts forward and down, toward the left

side of the body, and rests on the diaphragm. (See Location of the pediatric heart, page 4.)

The mediastinum is home to the heart.

CARDIAC ANATOMY AND PHYSIOLOGY

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The heart varies in size depending on the person’s body size, but the organ is roughly 5 (12.5 cm) long and 31/2 (9 cm) wide, or about the size of the person’s fist. The heart’s weight, typically 9 to 12 oz (255 to 340 g), varies depending on the person’s size, age, sex, and athletic conditioning. An athlete’s heart usually weighs more than that of the average person, and an elderly person’s heart weighs less. (See The older adult heart.)

Layer upon layer

The heart’s wall is made up of three layers: the epicardium, myocardium, and endocardium. (See Layers of the heart wall.) The epicardium, the outer layer (and the visceral layer of the serous pericardium), is made up of squamous epithelial cells overlying connective tissue. The myocardium, the middle layer, makes up the largest portion of the heart’s wall. This layer of muscle tissue contracts with each heartbeat. The endocardium, the heart’s innermost layer, contains endothelial tissue with small blood vessels and bundles of smooth muscle.

A layer of connective tissue called the pericardium surrounds the heart and acts as a tough, protective sac. It consists of the fibrous pericardium and the serous pericardium. The fibrous pericardium, composed of tough, white, fibrous tissue, fits loosely around the heart, protecting it. The fibrous pericardium attaches to the great vessels, diaphragm, and sternum. The serous pericardium, the thin, smooth, inner portion, has two layers:

•the parietal layer, which lines the inside of the fibrous pericardium

•the visceral layer, which adheres to the surface of the heart.

Between the layers

The pericardial space separates the visceral and parietal layers and contains 10 to 20 ml of thin, clear pericardial fluid that lubricates the two surfaces and cushions the heart. Excess pericardial fluid, a condition called pericardial effusion, compromises the heart’s ability to pump blood.

Inside the heart

The heart contains four chambers—two atria and two ventricles. (See Inside a normal heart, page 6.) The right and left atria serve as volume reservoirs for blood being sent into the ventricles. The right atrium receives deoxygenated blood returning from the body through the inferior and superior vena cavae and from the heart through the coronary sinus. The left atrium receives oxygenated blood from the lungs through the four pulmonary veins. The interatrial septum divides the chambers and helps them contract. Contraction of the atria forces blood into the ventricles below.

Ages

and stages

Location of the pediatric heart

The heart of an infant is positioned more horizontally in the chest cavity than that of the adult. As a result, the apex is at the fourth left intercostal space. Until age 4, the apical impulse is to the left of the midclavicular line. By age 7, the heart is located in the same position as the adult heart.

I rest on the diaphragm.

Layers of the heart wall

This cross section of the heart wall shows its various layers.

Pericardial space

Fibrous pericardium

Parietal pericardium

Epicardium

Myocardium

Endocardium

Pump up the volume

The right and left ventricles serve as the pumping chambers of the heart. The right ventricle receives blood from the right atrium and pumps it through the pulmonary arteries to the lungs, where it picks up oxygen and drops off carbon dioxide. The left ventricle receives oxygenated blood from the left atrium and pumps it through the aorta and then out to the rest of the body. The interventricular septum separates the ventricles and also helps them to pump.

The thickness of a chamber’s walls depends on the amount of high-pressure work the chamber does. Because the atria collect blood for the ventricles and don’t pump it far, their walls are considerably thinner than the walls of the ventricles. Likewise, the left ventricle has a much thicker wall than the right ventricle because the left ventricle pumps blood against the higher pressures in

the body’s arterial circulation, whereas the right ventricle pumps blood against the lower pressures in the lungs.

Ages

and stages

The older adult heart

As a person ages, his heart usually becomes slightly smaller and loses its contractile strength and efficiency (although exceptions occur in people with hypertension or heart disease). By age 70, cardiac output at rest has diminished by 30% to 35% in many people.

Irritable with age

As the myocardium of the aging heart becomes more irritable, extra systoles may occur, along with sinus arrhythmias and sinus bradycardias. In addition, increased fibrous tissue infiltrates the sinoatrial node and internodal atrial tracts, which may cause atrial fibrillation and flutter.

A LOOK AT CARDIAC ANATOMY

Cardiac cords

The mitral valve has two cusps, or leaflets, and the tricuspid valve has three. The cusps are anchored to the papillary muscles in the heart wall by fibers called chordae tendineae. These cords work together to prevent the cusps from bulging backward into the atria during ventricular contraction. If damage occurs, blood can flow backward into a chamber, resulting in a heart murmur.

Under pressure

The semilunar valves are the pulmonic valve and the aortic valve. These valves are called semilunar because the cusps resemble three half-moons. Because of the high pressures exerted on the valves, their structure is much simpler than that of the AV valves.

They open due to pressure within the ventricles and close due to the back pressure of blood in the pulmonary arteries and

aorta, which pushes the cusps closed. The pulmonic valve, located where the pulmonary artery meets the right ventricle, permits blood to flow from the right ventricle to the pulmonary artery

and prevents blood backflow into that ventricle. The aortic valve, located where the left ventricle meets the aorta, allows blood to flow from the left ventricle to the aorta and prevents blood backflow into the left ventricle.

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When valves close, heart sounds are heard.

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Getting into circulation

Like the brain and all other organs, the heart needs an adequate supply of blood to survive. The coronary arteries, which lie on the surface of the heart, supply the heart muscle with blood and oxygen. Understanding coronary blood flow can help you provide better care for a patient with a myocardial infarction (MI) because

you’ll be able to predict which areas of the heart would be affected by a blockage in a particular coronary artery.

Open that ostium

The coronary ostium, an opening in the aorta that feeds blood to the coronary arteries, is located near the aortic valve. During systole, when the left ventricle is pumping blood through the

aorta and the aortic valve is open, the coronary ostium is partially covered. During diastole, when the left ventricle is filling with blood, the aortic valve is closed and the coronary ostium is open, enabling blood to fill the coronary arteries.

With a shortened diastole, which occurs during periods of tachycardia, less blood flows through the ostium into the coronary arteries. Tachycardia also impedes coronary blood flow because contraction of the ventricles squeezes the arteries and lessens blood flow through them.

That’s right, Coronary

The right coronary artery, as well as the left coronary artery (also known as the left main artery), originates as a single branch off the ascending aorta from the area known as the sinuses of Valsalva. The right coronary artery supplies blood to the right atrium, the right ventricle, and part of the inferior and posterior surfaces of the left ventricle. In about 50% of the population, the artery also supplies blood to the sinoatrial (SA) node. The bundle of His and the AV node also receive their blood supply from the right coronary artery.

What’s left, Coronary?

The left coronary artery runs along the surface of the left atrium, where it splits into two major branches, the left anterior descending and the left circumflex arteries. The left anterior descending artery runs down the surface of the left ventricle toward the apex and supplies blood to the anterior wall of the left ventricle, the interventricular septum, the right bundle branch, and the left anterior fasciculus of the left bundle branch. The branches of the left anterior descending artery—the septal perforators and the diagonal arteries—help supply blood to the walls of both ventricles.

Knowing about coronary blood flow can help me predict which areas of the heart would be affected by a blockage in a particular coronary artery.