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are compressed. Blood flow in the left coronary artery during cardiac systole is only 10 to 30% of that during diastole, when the heart musculature is relaxed and most of the blood flow occurs. The compression effect of systole on blood flow is minimal in the right ventricle, probably as a result of the lower pressures developed by a smaller muscle mass (Fig. 17.1). Changes in blood flow during the cardiac cycle in healthy people have no obvious deleterious effects even during maximal exercise,- however, in people with compromised coronary arteries, an increased heart rate decreases the time spent in diastole, impairing coronary blood flow.

The heart musculature is perfused from the epicardial (outside) surface to the endocardial (inside) surface. Mi-crovascular pressures are dissipated by blood flow friction as the vessels pass through the heart tissue. Therefore, the mechanical compression of systole has more effect on the blood flow through the endocardial layers where compres-sive forces are higher and microvascular pressures are lower. This problem occurs particularly in heart diseases of all types, and most kinds of tissue impairment affect the subendocardial layers.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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