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The measured pulmonary capillary wedge pressure is an approximation of left ventricular enddiastolic pressure.110Cardiovascular Physiology120Leftventricularpressure10080RightventricularpressureAorticpressure6040Left atrialpressure20Right atrialpressure0120Pressure (mm Hg)100806040200LeftatriumLeftventricleAortaLargearteriesSmall ArteriolesarteriesCapillariesVeinsRightatriumRightPulmonaryventricle arteriesFigure 10.2 Pulse Pressures through the Circulation Changes in pressure pulses are illustratedas blood flows from the left ventricle (LV) to the aorta, through the systemic circulation, and back to theright ventricle (RV) and pulmonary artery (PA).Pressure Difference (⌬P)Blood Flow (Q)Vessel wallR = Resistance to flow⌬P = P1 – P2P1Radius (r)R⌬PP2RLength (L)Resistance to Flow (R)Poiseuille’s LawR=xLx8r4 = viscosityL = vessel lengthr = vessel radius1,000100Resistance (R)per unit length10mm Hg1ArterialsideRFlow (Q) = ⌬P/RRBloodRVenousside(mm3/sec)/m0.10.0129 25 21 17 13 9 3 9 13 17 21 25 29Vessel Radius (m)Figure 10.3 Poiseuille’s Law Under normal physiological conditions, flow through vessels is governed by Poiseuille’s law.
By far, the greatest resistance to flow occurs in the smallest vessels constitutingthe microcirculation, particularly the arterioles. This is due to the small radius of these vessels, and theinverse relationship between resistance and the fourth power of the radius of a vessel.RFlow, Pressure, and ResistanceCross-Sectional Area (A)and Flow Velocity (V)V2V1V2V2V2Small area (A1)High velocity (V1)V2V2ArteriolesCapillaries40600Velocity(V)5003040020300200Area (A)Velocity (cm/sec)V2Cross-Sectional Area (cm2)V2V2Arteries11110100Large area (A2)Low velocity (V2)AortaArteriesArteriolesVenulesCapillariesVeinsVena cavaFigure 10.4 Relationship between Velocity of Blood Flow and Cross-Sectional Area Proceeding downstream from the aorta, branching of arterial vessels increases total cross-sectional area andthus results in diminished velocity of blood flow from the aorta to the capillaries. Velocity increases fromthe capillaries to the large veins with the confluence of vessels and the resulting decrease in total crosssectional area.Turbulent flowLaminar flowFlowTurbulentLaminarPressure Difference (⌬P)Figure 10.5 Laminar and Turbulent Flow Normally, laminar, or streamlined, flow occurs throughoutmost of the vascular system.
Pathological conditions such as coarctation (narrowing of a vessel), valvularabnormalities, and low blood viscosity (as in anemia) produce turbulence, and are associated with murmursin the heart or great vessels, or vascular bruits heard by auscultation. Turbulence produces an increase inthe pressure gradient required to produce flow.Key relationships to remember are as follows:■■■Resistance is directly proportional to the length of thetube.Resistance is directly proportional to the viscosity of thefluid.Resistance is inversely proportional to the fourth powerof the radius of the tube.Flow (Q) can also be related to cross-sectional area (A) andlinear velocity of the flow (V) in this formula:Q = VAIn the cardiovascular system, the overall flow rate (cardiacoutput) is the same at every level of the circulation, becausethe circulation is in series.
Thus, based on the Q = VArelationship, the velocity of blood flow is slowest in the capillaries, which have the greatest aggregate cross-sectional area(600 cm2) in the vascular system, and fastest in the aorta,where the cross-sectional area is only about 4 cm2 (Fig. 10.4).The low velocity in capillaries is beneficial in terms of exchangeof dissolved substances between blood and tissues.Flow through vessels is generally laminar, with the greatestvelocity of flow in the center of the vessel and lowest velocitynear the vascular wall (Fig.
10.5). This streamlining of flow iscaused by shear stress produced as blood flows past the stationary vessel wall. Turbulent flow, on the other hand, ischaracterized by irregularities in flow patterns, such as whorls,vortices, and eddies. Vascular disease often is associated withsites of turbulence.Reynolds’ Number (Re) relates the factors associated withturbulence:Re ⫽VD⭸112Cardiovascular Physiologywhere V is velocity of flow, D is the diameter of the tube, ∂ isthe density of the fluid, and η is the viscosity of the fluid.When Re is below 2000, flow is usually laminar; values above3000 result in turbulence. In the cardiovascular system, densityof blood is always close to 1.0 and is not a factor; changes inthe other variables are important, along with a number ofother factors. Turbulence in blood vessels is associated withaudible murmurs and is promoted by:■■■■■high velocity of blood flowlarge vessel diameterlow viscosity of blood (low hematocrit)abrupt changes in vessel diameter, for example in aneurysms(see “Clinical Correlate”) or coarctation (narrowing)vascular branch pointsCLINICAL CORRELATEAortic AneurysmAn aneurysm is a saclike enlargement in the wall of an artery,caused by weakening of the vessel wall.
With bulging, wall stressis further increased, through increased radius of the vessel anddecreased wall thickness, according to Laplace’s equation (T =Ptr). Risk factors for aortic aneurysm include hypertension,smoking, obesity, atherosclerosis, and hypercholesterolemia.Wall tension (T) is another important biophysical parameterin this system and can be conceptualized as the force necessaryto hold together a theoretical slit occurring in a vessel wall.This is defined by Laplace’s law:T = Ptrwhere Pt is the transmural pressure (the difference betweenpressure inside and outside the vessel, in other words, thepressure gradient across the vascular wall), and r is the vesselradius.
Because of the small radii of capillaries and venules,they are protected against rupture despite the transmuralpressure gradient. Wall tension is an important considerationin large arteries, where transmural pressures are high and radiiare large, especially when the vascular wall is diseased (see“Clinical Correlate”).Rupture of an aortic aneurysm is a medical emergency involvingprofuse internal hemorrhage and is often fatal.
Dissection of ananeurysm is defined as bleeding into the vascular wall through atear in the inner layer of the vessel (media intima), a commoncomplication in thoracic aortic aneurysms. Aneurysms are treatedby replacement of the affected aortic segment by a synthetic graft,or by endovascular implantation of a graft in the area of theaneurysm.Ascending aortaRupture of aneurysmDissection ofaortic wallDescending aortaRuptured and Dissecting Aortic Aneurysm113CHAPTER11The Cardiac PumpBased on the biophysical principles described in Chapter 10,blood flows through the systemic and pulmonary circulations,with the energy for this flow being provided by the pumpingaction of the heart.
Like early mechanical pumps, the heartfunctions in a cyclic manner, with periods of filling (diastole)and periods of contraction and ejection of blood (systole).THE CARDIAC CYCLEThe cardiac cycle (or Wiggers diagram) consists of one cycleof ventricular systole and diastole.
At rest the cycle is 0.86seconds in duration if the heart rate is 70 beats per minute.The changes in ventricular pressure and volume, aortic pressure and flow, atrial pressure, venous pulse, electrocardiogram, and phonocardiogram are all interdependent, andunderstanding the interrelationships between these variablesis a key step in comprehending the complexities of hemodynamics (Fig. 11.1).Left ventricular volume and pressures in the left heart andaorta are illustrated in Figure 11.1.
Tracings for the right ventricle and pulmonary artery are similar in shape but substantially lower than those for the left ventricle and aorta. Withinthis cardiac cycle diagram, there are two short intervals knownas periods of isovolumetric contraction and isovolumetricrelaxation. During these periods, all of the valves of the heartare closed. The isovolumetric contraction period for the leftheart begins with mitral valve closure (the aortic valve isalready closed) and ends with opening of the aortic valve; theisovolumetric relaxation period begins with aortic valveclosure and ends when the mitral valve reopens.There is some slight asynchrony between the left heart and theright heart in terms of valve opening and closure, althoughthe sequences are the same.
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