Anatomy & Physiology

Anatomy & Physiology Study Guide

Total Peripheral Resistance Total peripheral resistance of the cardiovascular system indicates several factors: vascular resistance, blood viscosity, and turbulence. Va cular Resistance Vascular resistance, the resistance of the blood vessels, is the largest component. The most important factor in vascular resistance is friction between blood and the vessel walls. The amount of friction depends on two factors: vessel length and vessel diameter. Blood Viscosity Viscosity is the resistance to flow generated by interactions among molecules and suspended materials in a liquid. Liquids of low viscosity, such as water (viscosity 1.0), flow at low pressures; thick, syrupy fluids, such as molasses (viscosity 300), flow only under higher pressures. Whole blood has a viscosity about five times that of water, due to the presence of plasma proteins and blood cells. Under normal conditions, the viscosity of blood remains stable, but anemia, polycythemia, and other disorders that affect the hematocrit also change blood viscosity, and thus peripheral resistance. Turbulence High flow rates, irregular surfaces, and sudden changes in vessel diameter upset the smooth flow of blood, creating eddies and swirls. This phenomenon, called turbulence, increases resistance and slows blood flow. Turbulence normally occurs when blood flows between the atria and the ventricles, and between the ventricles and the aortic and pulmonary trunks. It also develops in large arteries, such as the aorta, when cardiac output and arterial flow rates are very high. However, turbulence seldom occurs in smaller vessels unless their walls are damaged. For example, the development of an atherosclerotic plaque creates abnormal turbulence and restricts blood flow. As a consequence of the distinctive sound, or bruit, produced by turbulence, the presence of plaques in large blood vessels can often be detected with a stethoscope. Arterial Blood Pressure Arterial pressure is important because it maintains blood flow through capillary beds. To do this, it must always be high enough to overcome the peripheral resistance. Arterial pressure is not constant; it rises during ventricular systole and falls during ventricular diastole. The peak blood pressure measured during ventricular systole is called systolic pressure , and the minimum blood pressure at the end of ventricular diastole is called diastolic pressure . In recording blood pressure, we separate systolic and diastolic pressures by a slash, as in “120/80” (“one-twenty over eighty”) or “110/75.” A pulse is a rhythmic pressure oscillation that accompanies each heartbeat. The difference between systolic and diastolic pressures is the pulse pressure . A normal range of systolic and diastolic pressures occurs in healthy individuals. When pressures shift outside of the normal range, clinical problems develop. Abnormally high blood pressure is termed hypertension ; abnormally low blood pressure, hypotension . Hypertension is much more common, and in fact, many cases of hypotension result from overly aggressive drug treatment for hypertension. The usual criterion established by the American Heart Association for hypertension in adults is a blood pressure greater than 140/90.

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