Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

inotropic drugs that enhance cardiac contractility by increasing the free intracellular calcium concentration in cardiac muscle. Endocardium The endocardium is a thin, three-layered membrane that lines the heart and covers the valves. The inner- most layer, which lines the heart chambers, consists of smooth endothelial cells supported by a thin layer of connective tissue. The endothelial lining of the endocar- dium is continuous with the lining of the blood vessels that enter and leave the heart. The middle layer consists of dense connective tissue with elastic fibers. The outer layer, which is composed of irregularly arranged con- nective tissue cells, contains blood vessels and branches of the conduction system and is continuous with the myocardium. Heart Valves and Fibrous Skeleton An important structural feature of the heart is its fibrous skeleton, which consists of four interconnecting valve rings and surrounding connective tissue (Fig. 17-7). The fibrous skeleton separates the atria and ventricles and forms a rigid support for attachment of the valves and insertion of the cardiac muscle. The tops of the valve rings are attached to the muscle tissue of the atria,

Fibrous pericardium

Pericardial cavity

Myocardium

Endocardium

Parietal pericardium

Visceral pericardium (epicardium)

FIGURE 17-5. Layers of the heart, showing the visceral pericardium, the pericardial cavity, and the parietal pericardium.

Although cardiac muscle cells require calcium for contraction, they have a less well-defined sarcoplasmic reticulum for storing calcium than skeletal muscle cells. Thus, cardiac muscle relies more heavily than skeletal muscle on an influx of extracellular calcium ions for contraction. The cardiac glycosides (e.g., digoxin) are

Muscle fiber

Nucleus

Striations

Intercalated disk A

Tricuspid valve

Mitral valve

B

Longitudinal portion (contains large gap junctions)

Aortic valve Pulmonic valve FIGURE 17-7. Fibrous skeleton of the heart, which forms the four interconnecting valve rings and support for attachment of the valves and insertion of cardiac muscle.

FIGURE 17-6. (A) Cardiac muscle fibers showing their branching structure. (B) Area indicated where cell junctions lie in the intercalated disks.