The heart is located in the mediastinum, the cavity between the lungs. The heart is tilted so that its pointed end, the apex, points downward toward the left hip, while the broad end, the base, faces upward toward the right shoulder. The heart is surrounded by the pericardium, a sac characterized by the following two layers:
- The outer fibrous pericardium anchors the heart to the surrounding structures.
- The inner serous pericardium consists of an outer parietal layer and an inner visceral layer. A thick layer of serous fluid, the pericardial fluid, lies between these two layers to provide a slippery surface for the movements of the heart.
The wall of the heart consists of three layers:
- The epicardium is the visceral layer of the serous pericardium.
- The myocardium is the muscular part of the heart that consists of contracting cardiac muscle and noncontracting Purkinje fibers that conduct nerve impulses. Cardiac cells (cardiomyocytes) are in this layer.
- The endocardium is the thin, smooth, endothelial, inner lining of the heart, which is continuous with the inner lining of the blood vessels.
As blood travels through the heart, it enters a total of four chambers and passes through four valves. The two upper chambers, the right and left atria, are separated longitudinally by the interatrial septum. The two lower chambers, the right and left ventricles, are the pumping machines of the heart and are separated longitudinally by the interventricular septum. A valve follows each chamber and prevents the blood from flowing backward into the chamber from which the blood originated.
Two prominent grooves are visible on the surface of the heart:
- The coronary sulcus (atrioventricular groove) marks the junction of the atria and ventricles.
- The anterior interventricular sulcus and posterior interventricular sulcus mark the junction of the ventricles on the anterior and posterior sides of the heart, respectively.
The pathway of blood through the chambers and valves of the heart is described as follows (see Figure 1):
- The right atrium, located in the upper right side of the heart, and a small appendage, the right auricle, act as a temporary storage chamber so that blood will be readily available for the right ventricle. Deoxygenated blood from the systemic circulation enters the right atrium through three veins: the superior vena cava, the inferior vena cava, and the coronary sinus. During the interval when the ventricles are not contracting, blood passes down through the right atrioventricular (AV) valve into the next chamber, the right ventricle. The AV valve is also called the tricuspid valve because it consists of three flexible cusps (flaps).
- The right ventricle is the pumping chamber for the pulmonary circulation. The ventricle, with walls thicker and more muscular than those of the atrium, contracts and pumps deoxygenated blood through the three‐cusped pulmonary semilunar valve and into a large artery, the pulmonary trunk. The pulmonary trunk immediately divides into two pulmonary arteries, which lead to the left and right lungs, respectively. The following events occur in the right ventricle:
When the right ventricle contracts, the right AV valve closes and prevents blood from moving back into the right atrium. Small tendonlike cords, the chordae tendineae, are attached to papillary muscles at the opposite, bottom side of the ventricle. These cords limit the extent to which the AV valve can be forced closed, preventing it from being pushed through and into the atrium.
When the right ventricle relaxes, there is less pressure in the right ventricle and more pressure in the pulmonary trunk. This high pressure in the pulmonary trunk causes the valve to close, thereby preventing the backflow of blood and the return of blood to the right ventricle.
- The left atrium and its auricle appendage receive oxygenated blood from the lungs through four pulmonary veins (two from each lung). The left atrium, like the right atrium, is a holding chamber for blood in readiness for its flow into the left ventricle. When the ventricles relax, blood leaves the left atrium and passes through the left AV valve into the left ventricle. The left AV valve is also called the mitral or bicuspid valve, the only heart valve with two cusps.
- The left ventricle is the pumping chamber for the systemic circulation. Because a greater blood pressure is required to pump blood through the much more extensive systemic circulation than through the pulmonary circulation, the left ventricle is larger and its walls are thicker than those of the right ventricle. When the left ventricle contracts, it pumps oxygenated blood through the aortic semilunar valve, into a large artery, the aorta, and throughout the body. The following events occur in the left ventricle, simultaneously and analogously with those of the right ventricle:
When the left ventricle contracts, the left AV valve closes and prevents blood from moving back into the right atrium. As in the right AV valve, the chordae tendineae prevent overextension of the left AV valve.
When the left ventricle relaxes, this results in less pressure in the left ventricle and higher pressure in the aorta. This high pressure causes the aortic semilunar valve to close, thus preventing the return of blood to the left ventricle.
Figure 1. The pathway of blood through the chambers and valves of the heart.
Two additional passageways are present in the fetal heart:
- The foramen ovale is an opening across the interatrial septum. It allows blood to bypass the right ventricle and the pulmonary circuit while the nonfunctional fetal lungs are still developing. The opening, which closes at birth, leaves a shallow depression called the fossa ovalis in the adult heart.
The coronary circulation consists of blood vessels that supply oxygen and nutrients to the tissues of the heart. Blood entering the chambers of the heart cannot provide this service because the endocardium is too thick for effective diffusion (and only the left side of the heart contains oxygenated blood). Instead, the following two arteries that arise from the aorta and encircle the heart in the artioventricular groove provide this function:
- The ductus arteriosus is a connection between the pulmonary trunk and the aorta. Blood that enters the right ventricle is pumped out through the pulmonary trunk. Although some blood enters the pulmonary arteries (to provide oxygen and nutrients to the fetal lungs), most of the blood moves directly into the aorta through the ductus arteriosus.
- The left coronary artery has the following two branches: the anterior interventricular artery (left anterior descending, or LAD, artery) and the circumflex artery.
- The right coronary artery has the following two branches: the posterior interventricular artery and the marginal artery.
Blood from the coronary circulation returns to the right atrium by way of an enlarged blood vessel, the coronary sinus. Three veins, the great cardiac vein, the middle cardiac vein, and the small cardiac vein, feed the coronary sinus.