Chapter 21

### Structure of the Heart

The heart is composed of (1) the endocardium, which lines the internal surfaces of the cardiac chambers and valves; (2) the myocardium; (3) the pericardium, composed of an inner visceral layer covering the heart and an outer parietal layer completing the pericardial sac; and (4) a specialized conducting system consisting of the sinoatrial (SA) and atrioventricular (AV) nodes, the bundle of His, and the arborizing Purkinje fibers.

The muscular interatrial and interventricular septa divide the heart longitudinally into a right side, which accepts the deoxygenated systemic venous return and pumps it into the low-pressure pulmonary circulation; and a left side, which accepts the oxygenated pulmonary venous blood and pumps it into the aorta (Figure 21-1). The left side and the systemic arterial circulation are at much higher hydrostatic pressures than the right side and the pulmonary arterial circulation.

###### Figure 21–1.

Normal pressures and oxygen saturation in the different chambers of the heart and great vessels.

Each side of the heart is further divided by the atrioventricular valves into an atrium and a ventricle. The muscular atrial walls normally have a thickness not exceeding 2 mm. The right ventricle pumps blood into the relatively low-pressure pulmonary circulation (systolic pressure 15–30 mm Hg) and has a wall thickness of less than 0.5 cm. The left ventricle develops a systolic pressure of 100–140 mm Hg to maintain the high-pressure systemic circulation (Figure 21-2). The left ventricle normally has a wall thickness of up to 1.5 cm.

###### Figure 21–2.

Cycle of contraction and valve action for the left side of the heart. The right side of the heart acts in parallel except that pressures are lower and valve motions are not quite synchronous.

The pump mechanism of the heart is made possible by the presence of valves that isolate one part of the heart from another when they close (Figure 21-2). The cardiac valves are thin, translucent fibrous membranes that are attached circumferentially to the valve ring. Blood flow through normal open valves is nonturbulent and laminar and therefore not perceived by auscultation. When a valve closes, the free edges come firmly into apposition, effectively closing the orifice.

The atrioventricular valves are composed of two (mitral valve) or three (tricuspid valve) cusps. The free edges of the atrioventricular valves are attached to the papillary muscles of the ventricle by fibrous cords (chordae tendineae). Closure of the valves in systole prevents regurgitation of blood into the atrium.

The semilunar (aortic and pulmonary) valves remain closed during diastole, preventing regurgitation of blood from the great vessels into the ventricles.

### Myocardial Function

The myocardium is composed ...

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