Chapter 4. Heart

Big Picture

The pericardium is a sac that encloses the heart, akin to the pleura that encloses the lungs. The pericardium and the heart are located in the middle of the thorax, between T4 and T8 vertebrae. The pericardium has parietal and visceral layers.

Pericardial Sac

The parietal pericardium is composed of an external fibrous layer (fibrous pericardium) and an internal serous layer (serous pericardium) (Figure 4-1A and B).

• Fibrous pericardium. A strong, dense collagenous tissue that blends with the tunica externa of the great vessels and the central tendon of the diaphragm.
• Serous pericardium. Lines the inner surface of the fibrous pericardium.
• Pericardial space. Lies between the serous layer of the parietal pericardium and the visceral pericardium.
• Visceral pericardium (epicardium). A serous layer that intimately follows the contours of the heart surface. At the root of the heart, the visceral pericardium is contiguous with the serous pericardium, analogous to the visceral and parietal pleura of the hilum of each lung.

The pericardial sinuses are subdivisions of the pericardial sac and consist of the following spaces:

• Transverse sinus. Lies posterior to the ascending aorta and the pulmonary trunk and anterosuperior to the left atrium and the pulmonary veins.
• Oblique sinus. Lies posterior to the heart and is surrounded by the reflection of the serous pericardium around the right and left pulmonary veins and the inferior vena cava.

The pericardium receives its blood supply from the pericardiacophrenic vessels and branches from the bronchial and esophageal vessels. It is innervated by visceral sensory fibers from the phrenic and vagus nerves and the sympathetic trunks.

Big Picture

The heart has the following three layers (Figure 4-1B):

• Epicardium. The outer layer, also known as the visceral pericardium.
• Myocardium. The middle layer, consisting of cardiac muscle responsible for contraction of the heart.
• Endocardium. The inner layer, consisting of endothelial cells that line the lumen of the four chambers.

Anatomy of the Heart

The heart, including its left and right sides, surfaces, borders, and sulci (Figure 4-1C to F), art described in a variety of ways.

Right and Left Sides of the Heart

The expression “right side of the heart” refers to the right atrium and the right ventricle, which collect systemic deoxygenated blood and pump it into the lungs. By comparison, the “left side of the heart” refers to the left atrium and the left ventricle, which collect oxygenated blood from the pulmonary circulation and pump it into the body.

Surfaces of the Heart

The shape of the heart is roughly that of a cone with a flat base and a pointed apex. The surfaces of the heart consist of the following:

• Base. Is flat, faces posteriorly, and is formed mainly by the pulmonary veins of the left atrium. The base faces posteriorly toward the T6–T9 vertebral bodies, intervened by the pericardium, the esophagus, and the aorta.
• Sternocostal surface. Is part of the heart's cone shape and is named for its location deep to the sternum and costal cartilages of the five uppermost ribs.
• Diaphragmatic surface. Is also part of the heart's cone shape and is named for the heart surface directly superior to the central tendon of the diaphragm. Note that the heart, as it lies within the thorax, rests upon the diaphragmatic surface but not on its base.

The apex is formed by the tip of the left ventricle and points anteriorly, inferiorly, and to the left. The apex lies deep to the left fifth intercostal space medial to the midclavicular line. In an adult, the apex of the heart is approximately one hand's breadth from the median plane, near the midclavicular (nipple) line. This location is useful clinically because auscultation of the mitral valve is best heard over the apex.

Borders of the Heart

• The superior border is formed by the right and left auricles. The ascending aorta and pulmonary trunk emerge from this border, and the superior vena cava enters the right atrium.
• The right ventricle forms the inferior border.
• The left border is formed by the left ventricle and auricle of the left atrium.
• The right border is formed by the superior vena cava, right atrium, and inferior vena cava.

Sulci of the Heart

Sulci (grooves) mark the outer surface of the heart at the sites where one chamber meets another. The cardiac sulci are as follows:

• Sulcus terminalis. A groove on the external surface of the right atrium. This sulcus marks the junction of the primitive sinus venosus with the atrium in the embryo, and corresponds to a ridge on the internal surface of the right atrium, the crista terminalis.
• Atrioventricular (AV) groove (coronary sulcus). An external demarcation of the atria from the ventricles. The AV groove contains the right coronary, the left coronary, and the circumflex coronary arteries and the coronary sinus.
• Anterior interventricular groove. Is demarcates the left ventricle from the right ventricle on the anterior surface of the heart and contains the left anterior descending artery and the great cardiac vein.
• Posterior interventricular groove. Is demarcates the left ventricle from the right ventricle on the posterior surface of the heart and contains the posterior interventricular artery and the middle cardiac vein.

Big Picture

Although blood fills the chambers of the heart, the myocardium is so thick that it requires its own artery–capillary–vein system, called the “coronary circulation,” to deliver and remove blood to and from the myocardium. The vessels that supply oxygenated blood to the myocardium are known as coronary arteries. The vessels that remove the deoxygenated blood from the heart muscle are known as cardiac veins.

Coronary Arteries and Associated Branches

The coronary arteries and branches course along the epicardium in the cardiac sulci and interventricular grooves (Figure 4-2). Each coronary artery sends branches to the heart muscle.

Left Coronary Artery

The left coronary artery arises from the aorta, superior to the left cusp of the aortic valve, and is shorter than the right coronary artery. However, the branches from the left coronary artery distribute blood to a larger area of myocardium. The left coronary artery supplies most of the left ventricle, left atrium, bundle of His, and the anterior aspect of the interventricular septum. The left coronary artery gives rise to the following two sizable branches:

• Left anterior descending artery. Also called the anterior interventricular artery and is often referred to by the acronym LAD. It supplies the anterior region of the left ventricle, including the anterolateral myocardium, apex, anterior interventricular septum, and the anterolateral papillary muscle.
• Left circumflex artery. Wraps around the left side to the posterior side of the heart. The circumflex artery supplies the posterolateral side of the left ventricle and gives off the left marginal branches which also supply the left ventricle.

Right Coronary Artery

The right coronary artery arises from the aorta, superior to the right cusp of the aortic valve. The right coronary artery travels along the right AV groove, between the root of the pulmonary trunk and the right auricle, and supplies the right atrium, right ventricle, the sinuatrial (SA) node, and the AV node. The right coronary artery gives rise to the following branches:

• Posterior descending artery. Supplies the inferior wall, posterior interventricular septum, and the posteromedial papillary muscle. In a few cases, the circumflex artery gives off the posterior descending artery.
• Right marginal artery. Supplies the right ventricular wall.
• SA nodal artery. Passes between the right atrium and the opening of the superior vena cava and supplies the SA node. In a few cases, the circumflex artery supplies the SA nodal artery.

Coronary Dominance

Coronary dominance is determined by which of the left and right coronary arteries gives rise to the posterior interventricular artery, which supplies the posterior interventricular semptum and part of the left ventricle. The dominant artery is usually the right coronary artery.

Cardiac Veins

The cardiac veins and associated tributaries are the major veins of the coronary circulation and run parallel to the arteries (Figure 4-2). They drain blood from the heart wall. The cardiac veins are as follows:

• The coronary sinus is the largest vein draining the heart muscle and lies in the coronary sulcus. The coronary sinus collects most of the venous return from the great, middle, and small cardiac veins and returns the venous blood to the right atrium. The coronary sinus opening in the right atrium is superior to the septal leaflet of the tricuspid valve.
  • Great cardiac vein. Begins at the apex of the heart and ascends in the anterior interventricular groove, parallel to the left anterior descending artery, and drains into the coronary sinus.
  • Middle cardiac vein. Begins at the apex of the heart and ascends in the posterior interventricular sulcus, parallel to the posterior interventricular artery, and drains into the coronary sinus.
  • Small cardiac vein. Courses along the acute margin of the heart, along with the marginal artery, and then courses posteriorly into the coronary sinus.
• The anterior cardiac veins drain the anterior portion of the right ventricle, cross the coronary groove, and empty directly into the right atrium. Anterior cardiac veins do not drain into the coronary sinus.

Big Picture

The human heart is a four-chambered pump composed of cardiac muscle. Two of the heart chambers, called atria, are relatively thin walled. The function of atria is to receive blood from organs outside the heart and pump it under low pressure to the corresponding ventricle. The two ventricles have thicker muscular walls that function to pump blood to organs, through capillary networks, which add resistance to blood flow.

Right Atrium

The right atrium (Figure 4-3A) receives deoxygenated venous blood from the systemic circulation via the superior and inferior venae cavae and from the coronary circulation via the coronary sinus. The right atrium pumps blood into the right ventricle through the tricuspid (right AV) valve. The tricuspid valve, located between the right atrium and the right ventricle, ensures unidirectional flow of blood from the right atrium to the right ventricle. In the fetus, the foramen ovale is an opening in the interatrial septum, which allows blood entering the right atrium from the venae cavae to pass directly to the left side of the heart, bypassing the lungs. Blood flow to the lungs is bypassed because the placenta is responsible for gas exchange in utero. In an adult, this foramen, termed the fossa ovalis, is shut.

On the superior aspect of the right atrium is the auricle, which is an outpouching of tissue derived from the fetal atrium, with rough myocardium on its internal surface known as pectinate muscles. The sinus venarum, which has a smooth wall, is on the internal surface of the remainder of the right atrium. The crista terminalis is the internal vertical ridge that separates the rough portion from the smooth portion of the right atrium. The crista terminalis extends vertically from the superior to the inferior vena cava. The SA node of the conducting system of the heart is located in the superior part of the crista terminalis.

Right Ventricle

The right ventricle (Figure 4-3B) forms the greater part of the anterior surface of the heart and lies anterior to much of the left ventricle. The right ventricle receives blood from the right atrium via the tricuspid valve. Outflow during systole is to the pulmonary trunk via the pulmonary semilunar valve. The function of the pulmonary valve is to ensure unidirectional flow of blood from the right ventricle to the pulmonary trunk, thus preventing retrograde return of blood from the pulmonary trunk back into the right ventricle. The internal surface of the right ventricle has projecting ridges called trabeculae carneae, with papillary muscles attached to the ventricular wall. Fibrous chords called chordae tendineae attach papillary muscles to the cusps of the tricuspid valve, preventing prolapse.

The moderator band (septomarginal trabecula) is a large ridge of heart muscle that is attached from the anterior and septal papillary muscles to the septal and anterior ventricular walls. When present (which occurs in approximately 60% of hearts), the moderator band conveys within it the right branch of the AV bundle, which is part of the conducting system of the heart.

Left Atrium

The left atrium (Figure 4-3C) forms most of the base of the heart. The left atrium is in contact with the esophagus. The left atrium receives oxygenated blood from two left and two right pulmonary veins. The blood in the left atrium is pumped into the left ventricle through the mitral (bicuspid, or left AV) valve. The function of the mitral valve is to ensure unidirectional flow of blood from the left atrium to the left ventricle during diastole, thus preventing retrograde return of blood from the left ventricle into the left atrium during systole.

Left Ventricle

The left ventricle (Figure 4-3C) is longer and more conically shaped than the right ventricle and lies anterior to the left atrium. Most of the left ventricle is located on the posterior side of the heart. The left ventricle receives oxygenated blood from the left atrium through the mitral valve, and it pumps blood into the aorta through the aortic valve. The function of the aortic valve is to ensure unidirectional flow of blood from the left ventricle to the aortic arch during systole and to prevent retrograde return of blood from the aortic arch back into the left ventricle during diastole. The myocardial wall of the left ventricle is much thicker than that of the right ventricle, which is necessary to generate higher pressure to pump blood to the organs, muscles, and skin.

Big Picture

Control of heart contractions is mediated through a combination of autonomic innervation via the cardiac plexus as well as the conduction pathway through the heart muscle itself. To understand heart innervation, we must study the autonomic contribution to the cardiac plexus and how it influences the conduction system of the heart.

Cardiac Plexus

The cardiac plexus is divisible into a superficial cardiac plexus, between the aortic arch and the pulmonary artery, and a deep cardiac plexus, between the aortic arch and the tracheal bifurcation. The plexuses receive a combination of both sympathetic and parasympathetic fibers in the manner discussed below (Figure 4-4).

Sympathetic Contribution

Preganglionic sympathetic fibers originate bilaterally in the lateral horns of the gray matter of the spinal cord between the T1 and the T5 spinal cord levels and enter the sympathetic chain via the white rami communicantes. After entering the sympathetic chain, fibers travel to the cardiac plexus via two possible routes:

• Preganglionic sympathetic fibers synapse in the superior parts of the thoracic sympathetic chain and send postganglionic sympathetic fibers directly from the sympathetic ganglia to the cardiac plexuses via thoracic cardiac nerves.
• The preganglionic sympathetic fibers ascend through the sympathetic chain and synapse in either the superior, middle, or inferior cervical ganglia before sending off postganglionic sympathetic fibers via cervical cardiac nerves to the cardiac plexuses.

Parasympathetic Contribution

Preganglionic parasympathetic (vagal) fibers in the left and right vagus nerves originate in the medulla oblongata and descend through the neck and into the thorax to the cardiac plexuses. The synapse of vagal preganglionic and postganglionic parasympathetic fibers occurs either in the cardiac plexus or in the walls of the heart near the SA node of the right atrium. Therefore, the cardiac plexus serves as a conduit not only for parasympathetic preganglionic and postganglionic and visceral sensory fibers but also for sympathetic postganglionic fibers.

In summary, mixed nerves from the cardiac plexus supply the heart with sympathetic fibers, which increase the heart rate and the force of contraction and cause dilation of the coronary arteries, and parasympathetic fibers, which decrease the heart rate, reduce the force of contraction, and constrict the coronary arteries.

Conducting System of the Heart

The autonomic branches from the cardiac plexus help to regulate the rate and force of heart contractions, through influencing the SA node and the AV node, as follows:

• SA node. The rhythm of the heart is normally controlled by the SA node, a group of automatically depolarizing specialized cardiac muscle cells located at the superior end of the crista terminalis, where the right atrium meets the superior vena cava. The SA node is considered the “pacemaker of the heart” and initiates the heart beat, which can be altered by autonomic nervous stimulation (sympathetic stimulation speeds it up, whereas vagal stimulation slows it down). The wave of depolarization sweeps down the walls of the atria, stimulating them to contract, and eventually reaches the AV node.
• AV node. The AV node is located in the interatrial septum just superior to the opening of the coronary sinus. This node receives impulses from the SA node and passes them to the AV bundle (of His).
• AV bundle (of His). The AV bundle begins at the AV node and descends through the fibrous skeleton of the heart before dividing into the left and right bundles (of His), corresponding to the left and right ventricles, respectively. This divergent pathway ensures that ventricular contraction begins in the region of the apex. Conduction ends near the aortic and pulmonic valves. Impulses also pass from the left and right bundle branches to the papillary muscles in the corresponding ventricles. In the right ventricle, the moderator band (septomarginal trabeculum) contains the right bundle branch.

Cardiac Cycle

The flow of blood through the heart is described in the following steps (Figure 4-5):

1. All blood returning to the heart from the tissues enters the atria. Blood from the systemic and coronary circulation fills the right atrium, whereas blood from the pulmonary circulation fills the left atrium.

2. As the left and right atria are filled, pressure rises against the AV valves. The atria contract simultaneously and both the right and left AV valves are forced open (diastole).

3. Blood fills the left and right ventricle.

4. Ventricles contract (systole), forcing the blood against the AV valve cusps and closing them, which causes the “lub” heart sound. The chordae tendineae and papillary muscles prevent prolapse of the valve leaflets into the atria.

5. Intraventricular pressure continues to rise and forces the pulmonary and aortic semilunar valves open. As a result blood flows into the pulmonary trunk and aorta, respectively.

6. As the ventricles relax (diastole) and intraventricular pressure decreases, blood flows back into the pulmonary trunk and aorta, filling the cusps of their semilunar valves and forcing the semilunar valves to close, which results in the “dub” heart sound.

7. The cycle begins again and repeats itself.