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Unformatted text preview: Lecture 22: Cardiovascular System - Circulation and Blood Pressure II Reading: chapter 10, section: capillaries, pgs 351-358 (pgs 357-364, if using 5 th edition) Arterioles - ( AR- TEIR- E- OLZ ) these small arterial branches form when arteries enter the organ they are supplying. They serve as the main source of vascular resistance in the circulatory system, because their small diameter greatly increases the resistance to blood flow and this leads to a marked drop in mean pressure. The large pressure differential encourages blood flow, and the marked drop in pressure converts the pulsatile pressure changes in the arteries to a non- fluctuating pressure in the capillaries (Figure 10-9) . Arteriolar radius can be regulated intrinsically and/or extrinsically by vasoconstriction and vasodilation (Figure 10-10) to control cardiac output and arterial pressure (Figure 10- 12,10-14) . ( 1 ) intrinsic control (last lecture) ( 2) extrinsic control - factors extrinsic to an organ or tissue that control arteriolar radius. These influences are both neural and hormonal and can act to influence the total peripheral resistance to blood flow. They can be counteracted by local chemical and physical influences. (1) sympathetic ( NERVOUS SYSTEM ) control - acts through norepinephrine (NE) release to contract arteriolar smooth muscle leading to vasoconstriction. The action of NE is mediated by alpha adrenergic receptors on the arteriolar smooth muscle cells. This action of NE can be overridden by local metabolic controls. The cardiovascular control center in the brainstem is the main region responsible for regulating sympathetic...
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- Winter '08