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Unformatted text preview: Chapter 34. THE PERIPHERAL CIRCULATORY SYSTEM AND CARDIOVASCULAR REGULATIO N The primary function of the circulatory system is to provide for the blood perfusion needs of tissues. The driving force clearly derives from the pumping function of the heart. Less obvious without giving consideration to the fundamentals of hydrodynamics (detailed in Chapter 18, Section D) is the fact that flow is a consequence of the pressure differential between that of the great arteries and that of the venous side of the circulation. Central arterial pressure reflects the opposing influences of cardiac output and total peripheral resistance of the blood vessels (Figure 34-1). Because systoles are discrete events that are separated so that the heart can fill between each ejection phase, another device is needed to sustain some degree of arterial pressure between systoles. The architecture of the arterial vessels changes with distance from the heart, as shown in Figure 34-2. A. The vascular tree The major arteries have thick fibroelastic walls, while arteries of smaller caliber have proportionately more smooth muscle. Blood is ejected from the ventricles at a faster rate than it is able to leave the major arteries. The peak arterial pressure during each systole ( systolic pressure ) causes these fibroelastic vessels to be deformed outward, generating a transient stretching or tension in the vessel wall (the underlying Figure 34-1. Determinants of central arterial pressure. The pressure within the great arteries is a result of the pumping of the heart into an essentially closed vascular system which poses resistance to flow. When blood enters the arteries faster than it can leave, the result is an increase in pressure within those vessels. Pressure changes if cardiac output or total peripheral resistance changes. Both are subject to multiple levels of control. principles were outlined in relation to the Laplace Law in Box 18-1). During diastole, when there is no further distending influence from the ventricles, the walls of these stretched vessels recoil and, in so doing, they transmit the recoil potential back onto the blood contained within the lumen. This translates to an incomplete maintenance of pressure ( diastolic pressure ) within the vessel. Sufficient diastolic pressure remains to sustain blood flow, between each systole. The cycle of stretching then recoil of larger arteries serves to conduct pulse pressure along the vessels: this is what is palpated when one's pulse is counted. The smooth muscle in the walls of the smaller arteries and arterioles are subject to control by neural and chemical influences. The state of contraction of the circular layer of muscle in the vessel wall, or degree of constriction, determines the diameter of the lumen of the vessel. Pressure within these vessels is drastically reduced compared to that in the trunk arteries, but extensive branching results in the total cross-sectional area for flow being markedly increased....
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- Fall '08