Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

sympathetic-mediated acceleration of heart rate and blood vessel tone, are called the vasomotor center . The third, which controls parasympathetic-mediated slow- ing of heart rate, is called the cardioinhibitory center . These brain stem centers receive information from many areas of the nervous system, including the hypothala- mus. The arterial baroreceptors and chemoreceptors provide the medullary cardiovascular center with con- tinuous information regarding changes in blood pres- sure (see Chapter 18). The sympathetic nervous system serves as the final common pathway for controlling the smooth muscle tone of the blood vessels. Most of the sympathetic pre- ganglionic fibers that control vessel function originate in the vasomotor center of the brain stem, travel down the spinal cord, and exit in the thoracic and lumbar (T1 to L2) segments. The sympathetic neurons that supply the blood vessels maintain them in a state of tonic activity, so that even under resting conditions, the blood vessels are partially constricted. Vessel constriction and relaxation are accomplished by altering this basal input. Increasing sympathetic activity causes constriction of some vessels, such as those of the skin, the gastrointestinal tract, and the kidneys. Blood vessels in skeletal muscle are supplied by both vasoconstrictor and vasodilator fibers. Activation of sympathetic vasodilator fibers causes vessel relaxation and provides the muscles with increased blood flow dur- ing exercise. Although the parasympathetic nervous sys- tem contributes to the regulation of heart function, it has little or no control over blood vessels. The actions of the ANS are mediated by chemical neurotransmitters. Acetylcholine is the postganglionic neurotransmitter for parasympathetic neurons and norepinephrine is the main postganglionic neurotrans- mitter for sympathetic neurons. Sympathetic neurons also respond to epinephrine, which is released into the bloodstream by the adrenal medulla. The neurotrans- mitter dopamine can also act as a neurotransmitter for some sympathetic neurons. The synthesis, release, and inactivation of the autonomic neurotransmitters are dis- cussed in Chapter 34.

R E V I EW E X E R C I S E S 1. In persons with atherosclerosis of the coronary arteries, symptoms of myocardial ischemia do not usually occur until the vessel has been 75% occluded. Use Poiseuille law to explain. 2. Once an arterial aneurysm has begun to form, it will continue to enlarge as the result of the increased tension in its wall. A. Explain the continued increase in size using the law of Laplace. B. Using information related to cross-sectional area and velocity of flow, explain why there is stasis of blood flow with the tendency to form clots in aneurysms with a large cross-sectional area. ■■ Neural control of the circulation is vested in the autonomic nervous system, with both the sympathetic and parasympathetic nervous systems exerting control over heart rate and the sympathetic nervous system controlling cardiac contractility and blood vessel tone. ■■ The arterial system is a high-pressure system that delivers blood to the tissues. It relies on the intermittent ejection of blood from the left ventricle and the generation of arterial pressure pulsations that move blood toward the capillaries where the exchange of gases, nutrients, and wastes occur. ■■ The venous system is a low-pressure system that collects blood from the capillaries. It relies on the presence of valves in the veins of the extremities to prevent retrograde flow and on the milking action of the skeletal muscles that surround the veins to return blood to the right heart. ■■ The arterioles, capillaries, and venules of the microcirculation facilitate the exchange of gases, nutrients, and metabolic waste-products between body tissues and the circulatory system. Local control of blood flow in the microcirculation is governed largely by the metabolic needs of the tissues and is regulated by local tissue factors such as lack of oxygen and the accumulation of metabolites, endothelial- derived vasodilators and vasoconstrictors, and humoral factors such as histamine, bradykinin, and the prostaglandins. ■■ Collateral circulation, which involves the development of collateral channels between smaller arteries, is a mechanism for long-term regulation of blood flow in areas where larger vessels have become occluded.

SUMMARY CONCEPTS

■■ The systemic circulation consists of arteries and arterioles, capillaries, and venules and veins.The walls of the blood vessels, except the capillaries, are composed of three layers: an outer layer, the tunica externa, composed of large collagen fibers that protect the vessel and anchor it to the surrounding structures; a middle layer, the tunica media, composed of smooth muscle that constricts to regulate vessel diameter; and an inner layer, the tunica intima, of flattened endothelial cells that provide a smooth and slippery surface for blood flow.