Porth's Essentials of Pathophysiology, 4e

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Control of Cardiovascular Function

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External jugular vein

Internal jugular vein

Subclavian vein

A

Superior vena cava

Aortic arch

Right atrium

Left atrium

Left coronary artery

Left ventricle

Pericardium

Right ventricle

Pleura

Right coronary artery

Posterior

B

Left ventricle

Right ventricle

Interventricular septum

Anterior C FIGURE 17-4. (A) The heart in relation to the sternum, ribs, and lungs. (B) Anterior view of the heart and great vessels (note that the lungs, which normally fold over part of the heart’s anterior, have been pulled back). (C) Cross-section of the heart showing the increased thickness of the left ventricle compared to the right.

contain gap junctions that serve as low-resistance path- ways for passage of ions and electrical impulses from one cardiac cell to another (Fig. 17-6B). Thus, the myo- cardium behaves as a single unit, or syncytium , rather than as a group of isolated units, as does skeletal muscle. When one myocardial cell becomes excited, the impulse travels rapidly so the heart can beat as a unit. As in skeletal muscle, cardiac muscle cells con- tain actin and myosin filaments, which interact and slide along one another during muscle contraction.

A number of important proteins regulate the interaction between the actin and myosin filaments. These include the tropomyosin and the troponin complex, which consists of three subunits (troponin T, troponin I, and troponin C) that regulate calcium-mediated muscle con- traction (see Chapter 1, Fig. 1-19). In clinical practice, the serum levels of specific cardiac forms of troponin T and troponin I, released from injured heart muscle, are used in the diagnosis of myocardial infarction (see Chapter 19).