ANATOMY OF THE HEART
The heart is composed of three layers (Fig. 26-1). The inner layer, or endocardium, consists of endothelial tissue and lines the inside of the heart and valves. The middle layer, or myocardium, is made up of muscle fibers and is responsible for the pumping action. The exterior layer of the heart is called the epicardium.
The heart is encased in a thin, fibrous sac called the pericardium, which is composed of two layers. Adhering to the epicardium is the visceral pericardium. Enveloping the visceral pericardium is the parietal pericardium, a tough fibrous tissue that attaches to the great vessels, diaphragm, sternum, and vertebral column and sup-ports the heart in the mediastinum. The space between these two layers (pericardial space) is filled with about 30 mL of fluid, which lubricates the surface of the heart and reduces friction during systole.
The four chambers of the heart constitute the right- and left-sided pumping systems. The right side of the heart, made up of the right atrium and right ventricle, distributes venous blood (deoxygenated blood) to the lungs via the pulmonary artery (pulmonary circulation) for oxygenation. The right atrium re-ceives blood returning from the superior vena cava (head, neck, and upper extremities), inferior vena cava (trunk and lower ex-tremities), and coronary sinus (coronary circulation).
The left side of the heart, composed of the left atrium and left ventricle, distributes oxygenated blood to the remainder of the body via the aorta (systemic circulation). The left atrium receives oxy-genated blood from the pulmonary circulation via the pulmonary veins. The relationships of the four heart chambers are shown in Figure 26-1.
The varying thicknesses of the atrial and ventricular walls re-late to the workload required by each chamber. The atria are thin-walled because blood returning to these chambers gener-ates low pressures. In contrast, the ventricular walls are thicker because they generate greater pressures during systole. The right ventricle contracts against low pulmonary vascular pres-sure and has thinner walls than the left ventricle. The left ven-tricle, with walls two-and-a-half times more muscular than those of the right ventricle, contracts against high systemic pressure.
Because the heart lies in a rotated position within the chest cavity, the right ventricle lies anteriorly (just beneath the ster-num) and the left ventricle is situated posteriorly. The left ven-tricle is responsible for the apex beat or the point of maximum impulse (PMI), which is normally palpable in the left midclavic-ular line of the chest wall at the fifth intercostal space.
The four valves in the heart permit blood to flow in only one di-rection. The valves, which are composed of thin leaflets of fibrous tissue, open and close in response to the movement of blood and pressure changes within the chambers. There are two types of valves: atrioventricular and semilunar.
The valves that separate the atria from the ventricles are termed atrioventricular valves. The tricuspid valve, so named because it is composed of three cusps or leaflets, separates the right atrium from the right ventricle. The mitral, or bicuspid (two cusps) valve, lies between the left atrium and the left ventricle (see Fig. 26-1).
Normally, when the ventricles contract, ventricular pressure rises, closing the atrioventricular valve leaflets. Two additional structures, the papillary muscles and the chordae tendineae, maintain valve closure. The papillary muscles, located on the sides of the ventricular walls, are connected to the valve leaflets by thin fibrous bands called chordae tendineae. During systole, contrac-tion of the papillary muscles causes the chordae tendineae to be-come taut, keeping the valve leaflets approximated and closed.
The two semilunar valves are composed of three half-moon-like leaflets. The valve between the right ventricle and the pulmonary artery is called the pulmonic valve; the valve between the left ven-tricle and the aorta is called the aortic valve.
The left and right coronary arteries and their branches (Fig. 26-2) supply arterial blood to the heart. These arteries originate from the aorta just above the aortic valve leaflets. The heart has large metabolic requirements, extracting approximately 70% to 80% of the oxygen delivered (other organs consume, on average, 25%). Unlike other arteries, the coronary arteries are perfused during diastole. An increase in heart rate shortens diastole and can de-crease myocardial perfusion. Patients, particularly those with coronary artery disease (CAD), can develop myocardial ischemia (inadequate oxygen supply) when the heart rate accelerates.
The left coronary artery has three branches. The artery from the point of origin to the first major branch is called the left main coronary artery. Two bifurcations arise off the left main coronary artery. These are the left anterior descending artery, which courses down the anterior wall of the heart, and the circumflex artery, which circles around to the lateral left wall of the heart.
The right side of the heart is supplied by the right coronary artery, which progresses around to the bottom or inferior wall of the heart. The posterior wall of the heart receives its blood sup-ply by an additional branch from the right coronary artery called the posterior descending artery.
Superficial to the coronary arteries are the coronary veins. Venous blood from these veins returns to the heart primarily through the coronary sinus, which is located posteriorly in the right atrium.
The myocardium is composed of specialized muscle tissue. Micro-scopically, myocardial muscle resembles striated (skeletal) mus-cle, which is under conscious control. Functionally, however, myocardial muscle resembles smooth muscle because its contrac-tion is involuntary. The myocardial muscle fibers are arranged in an interconnected manner (called a syncytium) that allows for co-ordinated myocardial contraction and relaxation. The sequential pattern of contraction and relaxation of individual muscle fibers ensures the rhythmic behavior of the myocardium as a whole and enables it to function as an effective pump.