The heart is a hollow, muscular organ about the size of a closed fist. It’s about 5″ (12.5 cm) long, 3½″ (9 cm) in diameter at its widest point and weighs 1 to 1¼ 1b (453.5 to 567 g). The heart is located between the lungs in the mediastinum, behind and to the left of the sternum.

The heart spans the area from the second to the fifth intercostal space. The heart’s right border lines up with the right border of the sternum. The heart’s left border lines up with the left midclavicular line. The heart’s base, its posterior surface, is the widest part of the heart. It’s formed mainly by the left atrium, which is located at the top of the heart. The apex, formed mainly by the left ventricle, is located at the fifth left intercostal space at the bottom of the heart. The apex is the point of maximum impulse, where heart sounds are the loudest. (See Locating the heart, page 2.)

Leading into and out of the heart are the great vessels:

Like the brain and other organs, the heart needs an adequate supply of oxygenated blood to survive. The main coronary arteries lie on the surface of the heart, with smaller arterial branches penetrating the surface into the cardiac muscle mass. The heart receives its blood supply almost entirely through these arteries. Only a small percentage of the heart’s endocardial surface can obtain sufficient amounts of nutrition directly from the blood in the cardiac chambers. (See A look at coronary circulation, page 6.)

Understanding coronary blood flow can help you provide better care to a patient with coronary artery disease because you’ll be able to predict which areas of the heart will be affected by a narrowing or occlusion of a particular coronary artery.

The left main and right coronary arteries arise from the coronary ostia, small orifices located just above the aortic valve cusps. The right coronary artery fills the groove between the atria and ventricles, giving
rise to the acute marginal artery and ending as the posterior descending artery. The right coronary artery supplies blood to:

The posterior descending artery supplies the posterior wall of the left ventricle in most people.

The left main coronary artery varies in length from a few millimeters to a few centimeters. It splits into two major branches, the left anterior descending (also known as the interventricular) and the left circumflex
arteries. The left anterior descending artery runs down the anterior surface of the heart toward the apex. This artery and its branches—the diagonal arteries and the septal perforators—supply blood to:

The circumflex artery circles the left ventricle, ending on its posterior surface. The obtuse marginal artery arises from the circumflex artery. The circumflex artery provides oxygenated blood to:

In most people, the right coronary artery is the dominant vessel, meaning the right coronary artery supplies the posterior wall via the posterior descending artery. This is called right coronary dominance or a dominant right coronary artery. Likewise, when the left coronary artery supplies the posterior wall via the posterior descending artery, the terms left coronary dominance or dominant left coronary artery are used.

When two or more arteries supply the same region, they usually connect through anastomoses, junctions that provide alternative routes of blood flow. This network of smaller arteries, called collateral circulation, provides blood to capillaries that directly feed the heart muscle. Collateral circulation often becomes so strong that even if major coronary arteries become narrowed with plaque, collateral circulation can continue to supply blood to the heart.

In contrast to the other vascular beds in the body, the heart receives its blood supply primarily during ventricular relaxation or diastole, when the left ventricle is filling with blood. This is because the coronary ostia lie near the aortic valve and become partially occluded when the aortic valve opens during ventricular contraction or systole. However, when the aortic valve closes, the ostia are unobstructed, allowing blood to fill the coronary arteries. Because diastole is the time when the coronary arteries receive their blood supply, anything that shortens diastole, such as periods of increased heart rate or tachycardia, will also decrease coronary blood flow.

In addition, the left ventricle compresses intramuscular blood vessels during systole. During diastole, the cardiac muscle relaxes, and blood flow through the left ventricular capillaries is no longer obstructed.

Just like the other parts of the body, the heart has its own veins, which remove oxygen-depleted blood from the myocardium. About three quarters of the total coronary venous blood flow leaves the left ventricle by way of the coronary sinus, an enlarged vessel that returns blood to the right atrium. Most of the venous blood from the right ventricle flows directly into the right atrium through the small anterior cardiac veins, not by way of the coronary sinus. A small amount of coronary blood flows back into the heart through the thebesian veins, minute veins that empty directly into all chambers of the heart.

Valves in the heart keep blood flowing in only one direction through the heart, preventing blood from traveling the wrong way. Healthy valves open and close passively as a result of pressure changes within the four chambers.