The heart is a four-chambered muscular organ that lies in the thorax, posterior to the sternum and intercostal cartilages. It sits on the superior surface of the diaphragm and is the largest organ in the mediastinum, weighing approximately 2 g per lb of ideal body weight. The wall of the heart is composed of three layers. The most superficial of these layers is the epicardium. The epicardium contains an outer fibrous connective tissue and an inner serous pericardium. The space between these layers, the pericardial sac, contains about 25 mL of fluid that acts to reduce friction of the beating heart. Just underneath the pericardium is the muscular middle layer of the heart, the myocardium, which in addition to its contractile properties has the capacity to conduct electrical impulses to its cells. The endocardium, a sheet of endothelium resting on a thin layer of connective tissue, is the innermost layer of the heart and extends outward to include the valves of the heart.

The four chambers of the heart are the right and left atria, superiorly, and the right and left ventricles, inferiorly. The two sides of the heart are divided internally by the interventricular septum. Atrioventricular (AV) valves are made up of leaflets of connective tissue connected to fibrous cords called chordae tendineae. They connect to muscular projections (papillary muscles) from the inner surface of the heart wall. The AV valves sit between the atria and the ventricles and function to prevent backward blood flow. Specifically, the tricuspid valve contains three leaflets and lies between the right atrium and the right ventricle. The mitral or bicuspid valve contains two leaflets and sits between the left atrium and the left ventricle. The two semilunar valves are made up of leaflets attached to a fibrous ring of endothelium: The pulmonic semilunar valve lies between the right ventricle and the pulmonary trunk, whereas the aortic semilunar valve sits between the left ventricle and the aorta (see Fig. 9-1).

The right atrium is the receiving chamber for deoxygenated blood returning from systemic circulation and is fed by the superior vena cava, inferior vena cava, and coronary sinus. Deoxygenated blood enters the right atrium and flows through the tricuspid valve into the right ventricle. The right ventricle pumps the blood through the pulmonic semilunar valve and into the lungs for oxygenation. Once oxygenated, the blood then returns to the left atrium of the heart. Here, it
passes through the mitral valve into the left ventricle. With each beat of the heart, the left ventricle pumps this oxygen-rich blood through the aortic valve into the aorta and out to the body, where it delivers a fresh supply of oxygen to the tissues.

The heart itself is nourished with a rich blood supply by coronary arteries extending over the surface of the epicardium. The right and left coronary arteries arise from the base of the aorta and surround the heart like a crown. The left coronary artery passes posterior to the pulmonary trunk and then divides into two. The left anterior descending (LAD) artery runs anterior and downward toward the apex of the heart. In its path, the LAD artery branches to supply the free wall of the right and left ventricles, anterior papillary muscle, anterior two-thirds of the septum, and much of the conduction tissue, namely, the bundle of His and right and left bundle branches. The circumflex artery follows the coronary sulcus, the groove between the left atrium and ventricle, and runs posteriorly to supply the left atrium and the posterior and lateral portions of the left ventricle. In approximately 45% of people, the circumflex branch also supplies blood to the sinoatrial (SA) node. The right coronary artery (RCA) travels in the coronary sulcus between the right atrium and right ventricle on the anterior surface of the heart. At the top of the ventricular septum posteriorly, the RCA becomes the posterior descending branch and runs parallel to the ventricular septum, supplying most of the right atrium and right ventricle. At the inferior border of the heart, the RCA branches off to become the marginal artery. It then sends a penetrating branch to the AV node and part of the bundle of His (see Fig. 9-2).

The coronary veins return deoxygenated blood from the heart wall into the right atrium. Specifically, the great (anterior), middle (posterior), and small (inferior) cardiac veins drain into the coronary sinus, located in the posterior coronary sulcus. Coronary artery anatomy can vary significantly among people. The anatomic distribution has no physiologic implications in the normal healthy heart. However, in those people with disease of the coronary arteries, a blockage in an artery that supplies a great deal of cardiac tissue can be fatal.

The conducting system of the heart includes the SA node, the AV node, the bundle of His, right and left bundle branches, and the Purkinje fibers. This specialized network of cells making up the conduction system
is the cardiac muscle cells modified for spontaneous excitation and rapid conduction of an action potential. The primary pacemaker of the heart, the SA node, is located in the posterior wall of the right atrium near the opening to the superior vena cava. All of the cells in the conducting system have automaticity, that is, the ability of self-excitation. However, those in the SA node fire spontaneously at a faster rate than the others, at a frequency between 60 and 80 bpm.

Once an impulse is generated at the SA node, the action potential spreads quickly through the cardiac muscle cells and internodal pathways of the right atrium and left atria, causing simultaneous depolarization and contraction. At the same time, the impulse is conducted through the conduction pathway to the AV node located in the interatrial septum. Because the AV node is the only connection of impulse between the atria and the ventricles, the action potential slows here, causing a slight delay and allowing time for atrial kick and ventricular filling. This delay in conduction at the AV node also has the protective function of preventing excessive impulse rates from entering the ventricles. Consequently, the AV node also can serve as a backup pacemaker should the SA node fail to initiate an impulse. Thus, the AV node has a significant role in preserving cardiac function.

The action potential continues from the point of origin to the bundle of His, a fibrous projection of cells, which enters the interventricular septum and divides into right and left bundle branches. About 1 cm down the septum, the bundle branches become bundles of Purkinje fibers. These fibers approach the apex of the heart and turn superiorly into the ventricular walls, extending into the myocardium on the inner surfaces of the ventricles. Once an electrical impulse enters the bundle of His, the conduction through the bundle branches and Purkinje fibers is extremely fast, about 1 to 4 ms. This rapid conduction of the impulse to the apex of the heart allows simultaneous depolarization of both ventricles, with a wave of contraction traveling superiorly, effectively pumping blood up and out through the great vessels (see Fig. 9-3).

Cardiac muscle fibers generate spontaneous contractions that are coordinated into a functional heartbeat by the electrical conduction mechanisms inherent in the heart itself. However, the activity of the heart is also influenced by inputs from the nervous system.

The heart receives two opposing neural inputs belonging to the autonomic nervous system. Both send motor neurons to certain cardiac muscle cells. One input comes from the cells of the parasympathetic nervous system, whose synaptic terminals in the heart release the neurotransmitter acetylcholine. The effect of acetylcholine is to decrease the rate of depolarization during the pacemaker potential of the SA node. This has the effect of increasing the interval between successive action potentials, thereby decreasing the rate at which the pacemaker drives the rate of the heartbeat.

The second neural input to the heart comes from cells of the sympathetic nervous system, whose synaptic terminals release the neurotransmitter norepinephrine. Activation of this input increases the heart rate. This effect is also mediated through the pacemaker potential, which depolarizes more rapidly after the activation of the sympathetic input. Both factors accentuate the depolarizing trend during the pacemaker potential.

A cardiac muscle cell is made up of contractile units known as sarcomeres. Within the sarcomere are long thread-like structures called myofibrils. Myofibrils are made up of two types of contractile proteins, or myofilaments, called actin and myosin. Cross-bridges that project from myosin fibers link themselves to the thinner actin filaments, giving the muscle cell a striated appearance. When cardiac muscle cells receive an electrical impulse, the actin filaments slide over myosin filaments, shortening the sarcomere. The shortening of adjacent sarcomeres effectively shortens the entire muscle cell.

The plasma membranes of adjacent myocardial cells form a structure known as intercalated discs. These junctions contain desmosomes, which join adjacent cells, and gap junctions, which allow electrical signals to pass directly from cell to cell. This complex network of electrically linked cells, through which an impulse can travel quickly, sets the environment for the simultaneous contraction of many cells. This is crucial for the heart to pump efficiently.

The cardiac cycle describes the mechanical events of the heart involving pressure and volume changes and path of blood flow. One cardiac cycle is one beat of the heart and lasts approximately 0.8 seconds in duration during a normal resting heart rate. The cardiac cycle can be divided into two phases: systole, the contractile phase, and diastole, the filling or relaxation phase.