Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

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eral left ventricle on the frontal plane of the heart (Fig. 17-4B).When the hand is placed on the thorax, the main impact of the heart’s contraction is felt against the chest wall at a point between the fifth and sixth ribs, a little below the nipple and approximately 3 inches to the left of the midline. This is called the point of maximum impulse. The wall of the heart is composed of an outer epicar- dium, which lines the pericardial cavity; the myocardium or muscle layer; and the smooth endocardium, which lines the chambers of the heart (Fig. 17-5). A fibrous skeleton separates the atria and ventricles and forms a rigid support for attachment of the heart valves. The interatrial and interventricular septa divide the heart into a right and a left pump, each composed of two muscular chambers: a thin-walled atrium, which serves as a reservoir for blood coming into the heart, and a thick-walled ventricle, which pumps blood out of the heart. The increased thickness of the left ventricular wall compared to the right ventricle (Fig. 17-4C) results from the additional work this ventricle is required to perform. Pericardium The pericardium forms a fibrous covering around the heart, holding it in a fixed position in the thorax and providing physical protection and a barrier to infec- tion. The pericardium consists of a tough outer fibrous layer and a thin inner serous layer. The outer fibrous layer is attached to the great vessels that enter and leave the heart, the sternum, and the diaphragm. The fibrous pericardium is highly resistant to distention; it prevents acute dilatation of the heart chambers and exerts a restraining effect on the left ventricle. The inner serous layer consists of a visceral layer and a parietal layer. The visceral layer, also known as the visceral pericardium or epicardium, covers the entire heart and great vessels and then folds over to form the parietal layer that lines the fibrous pericardium (see Fig. 17-5). Between the visceral and parietal layers is the pericardial cavity, a potential space that contains 30 to 50 mL of serous fluid. This fluid acts as a lubricant to minimize friction between the two layers as the heart contracts and relaxes. Myocardium The myocardium, or muscular portion of the heart, forms the walls of the atria and ventricles. Cardiac mus- cle cells, like skeletal muscle, are striated and composed of sarcomeres that contain actin and myosin filaments (see Chapter 1). They are smaller and more compact than skeletal muscle cells and contain many large mito- chondria, reflecting their continuous energy needs. The contractile properties of cardiac muscle are simi- lar to those of skeletal muscle, except the contractions are involuntary and the duration of contraction is much longer. Unlike the orderly longitudinal arrangement of skeletal muscle fibers, cardiac muscle cells are arranged as an interconnecting latticework, with their fibers divid- ing, recombining, and then dividing again (Fig. 17-6A). The fibers are separated from neighboring cardiac muscle cells by dense structures called intercalated disks . The intercalated disks, which are unique to cardiac muscle,

SUMMARY CONCEPTS (continued)

The Heart as a Pump The heart is a four-chambered muscular pump approxi- mately the size of a fist that beats an average of 70 times each minute, 24 hours each day, 365 days each year for a lifetime. In 1 day, this pump moves more than 1800 gallons of blood throughout the body, and the work per- formed by the heart over a lifetime would lift 30 tons to a height of 30,000 ft. Functional Anatomy of the Heart The heart, which is enclosed in a loose-fitting sac called the pericardium , is located between the lungs in the medi- astinal space of the intrathoracic cavity. It is located pos- terior to the sternum and anterior to the vertebral column and extends about 5 inches from the second to fifth ver- tebrae (Fig. 17-4A). The heart is suspended by the great vessels, with its broader side (i.e., base) facing upward and its tip (i.e., apex) pointing downward, forward, and to the left. The heart is positioned obliquely, so that the right side of the heart is almost fully in front of the left side of the heart, with only a small portion of the lat- intraluminal pressure, wall tension becomes greater as the radius of a vessel increases; and more pressure will be needed to overcome the contractile tension in a vessel wall as the diameter decreases. Wall tension is also affected by wall thickness, increasing as the wall becomes thinner and decreasing as it becomes thicker. ■■ The velocity or speed of blood flow through a vessel is greatly affected by its cross-sectional area, increasing as the cross-sectional area decreases and decreasing as it increases. High velocity can create turbulent blood flow, in which the blood moves crosswise and lengthwise in blood vessels; as opposed to laminar or layered flow, in which the blood components are arranged so that the plasma is adjacent to the smooth surface of the inner lining of the vessel wall and the blood components are in the center of the bloodstream. ■■ Vascular compliance or capacitance reflects the distensibility of blood vessels and total quantity of blood that can be stored in a given part of the circulatory system for a given change in pressure. It is greater in the thin-walled vessels of the venous system than in the thick-walled vessels of the arterial system. ( text continued from page 379 )