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In doing so, the myocardium of theleft ventricle, and in particular, the subendocardium, mustgenerate an extravascular tissue pressure that is higher thanleft ventricular and arterial pressures. This intramyocardialpressure impedes left coronary flow during systole, when thenet coronary perfusion pressure is arterial pressure minus theintramyocardial pressure. Thus, left coronary artery flow islow during systole (see Fig. 12.9). During isovolumetric contraction, there is a rapid decrease in flow because tissue pressure has exceeded arterial pressure.
During the remainder ofsystole, the shape of the left coronary flow curve is similar tothe aortic pressure curve, but remains low due to high intramyocardial pressure.the work of the heart, and thus metabolic vasodilation largelyoverrides sympathetic coronary vasoconstriction. However, itis believed that sympathetic vasoconstriction may limit theextent of metabolic vasodilation, and thus, in a compromisedheart (for example, in coronary heart disease), sympatheticstimulation may contribute to ischemia.Autoregulation of coronary blood flow can be demonstratedexperimentally but is a less important factor in normal regulation of coronary flow than metabolic factors or the effects ofintramyocardial pressure.Blood Flow in Exercise and Effects onSpecific CirculationsThe importance of regional differences in regulatory mechanisms governing blood flow can be gleaned by considering thechanges in flow that occur during exercise.
Regulation of arterial blood pressure and flow during dynamic exercise is acomplex process, involving large changes in cardiac outputand regional resistances (Fig. 12.10). Dynamic exercise, alsocalled aerobic exercise, involves rhythmic contraction/relaxation of large skeletal muscle groups, for example duringjogging, swimming, or aerobics.
Young, fit individuals areable to exercise at levels associated with cardiac outputs of20 to 30 L/min, by increasing both heart rate and strokevolume. Sympathetic nervous system activation results invasoconstriction and reduced flow (as a proportion of cardiacoutput) in many, but not all, regional circulations (seeFig. 12.10):■■With the onset of isovolumetric relaxation, left coronaryartery flow increases markedly due to two factors. Duringsystole, when flow is low, metabolites (H+, CO2, K+, prostaglandins, lactic acid, adenosine, and others) accumulate, andO2 tension falls.
These changes produce coronary vasodilation; adenosine is believed to be the major factor involved.The vasodilation, combined with the fall in intramyocardialpressure, results in the large increase in left coronary flowduring diastole.Flow through the right coronary artery is less affected byintramyocardial pressure, because tissue pressure in the rightventricle does not nearly reach that of the left ventricle, anddoes not exceed arterial pressure. Thus, the shape of the rightcoronary artery blood flow curve (see Fig. 12.9) resembles theaortic pressure curve, with the highest flow occurring duringsystole.Sympathetic nerves innervate coronary arteries and can affectblood flow, but to a lesser extent than in most other circulations.
Activation of the sympathetic nervous system increases137■Coronary blood flow increases, mainly as a result ofmetabolic vasodilation.Skin blood flow is reduced at first, but will eventuallyrise as a result of thermoregulatory mechanisms.Skeletal muscle flow increases dramatically, as constrictor effects of catecholamines are overridden bymetabolic vasodilation (and to some extent, activationof β-adrenergic receptors).Total peripheral resistance is reduced as a result of thesechanges. Thus, aerobic exercise is associated with high cardiacoutput and low resistance.
Constriction of veins results inincreased central venous pressure, augmenting cardiac fillingpressure and stroke volume. Arterial blood pressure is elevated during dynamic exercise, but it is mainly systolic pressure (and thus pulse pressure) that rises, due to the largestroke volume. Diastolic pressure may be slightly elevated orslightly reduced, because low peripheral resistance results ina rapid fall in arterial pressure during diastole.Fetal CirculationAlthough the fetal circulation is not a special circulation inthe same sense as the cerebral and coronary circulations, there138Cardiovascular PhysiologyCLINICAL CORRELATEMyocardial IschemiaMyocardial ischemia is reduced blood flow to heart muscle, resulting in poor oxygenation. Angina, arrhythmias, myocardial infarction, and sudden death may result.
In patients with coronary heartdisease, buildup of atherosclerotic plaque in epicardial coronaryarteries results in reduced perfusion of the affected vessels. Functionally, ischemia and infarction may result in reduced contractility of the heart, arrhythmias, electrocardiogram (EKG) changes,wall motion abnormalities, and even ventricular aneurysm. Myocardial infarction is one of the major causes of congestive heartfailure. Severe cases of coronary occlusion are treated by angioplasty or coronary artery bypass graft surgery. In bypass surgery,a section of vessel, typically a piece of saphenous vein, is graftedto bypass the blockage.
In angioplasty, a balloon catheter isemployed to open the occluded artery. To reduce the occurrenceof restenosis (reocclusion), a stent is often deployed.A. Coronary angioplasty and stent deploymentAs the first step,a coronary guidewire is advancedacross the stenoticatheroscleroticplaque.A balloon cathetercontaining the stentis placed in thedilated area.A double-lumencatheter with aballoon is slidover the guidewire; the balloonis inflated tocompress theplaque andopen theobstruction.The balloon isexpanded,deployingthe stent.Once the stenthas beendeployed, thecatheter andthe guide wireare removed.B. Manifestations of myocardial infarctionFirst andsecond daysAfter 2or 3 daysTransmural infarction nearlycomplete.
Some ischemiaand injury may be presentat borders.R wave gone ornearly goneSignificantQ waveKeyTransmuralinfarctioncomplete.T-waveNoinversion R wavebeginningST elevationmay decreaseMyocardialischemiaInfarcted tissue replaced byfibrous scar, sometimesbulging (ventricularaneurysm).Deep T-waveinversionSome R wavemay returnST may beat baselineMarkedQ waveMyocardialinjuryFirstseveral daysAfter severalweeks or monthsSignificantQ waveusuallypersistsMyocardialdeath(infarction)T waveoften lessinvertedST elevationmay persistif aneurysmdevelopsFibrosisAfter severalweeks or monthsSome subendocardial muscledies, but lesion does not extendthrough entire heart wall.R wave persistsbut may diminishsomewhatT-waveinversionmay occurST oftenQ wave not returns tosignificant baselineLesion heals.
Somesubendocardial fibrosismay occur but does notinvolve entire thicknessof heart wall.may orST segment may notand T wave returnto normalQ wavenot significantThe Peripheral CirculationAnticipation of exercisestimulates cardioregulatorycenters, increasing heart rate139Sympathetic inhibition by baroreceptormechanism is overwhelmed by generalizedstimulation of sympatheticsVagus nerve (X)Sympatheticcardiac nervesSympatheticcardiacnervesBaroreceptors stimulated by rise inblood pressure; fall in blood pressuredecreases tonic sympathetic inhibitionCatecholamine output by suprarenal medullaepromoted by sympathetic stimulationSympathetic nervestimulation and circulating catecholamines, plusrelative decrease in vagaltone, accelerate SA nodedischarge rateRight sideof heartLungLeft sideof heartSympathetic nerves andcirculating catecholaminesact directly on heart muscle,increasing force of contractionIncreased venous return due toaction of muscle pump andrespiratory movementsIncreasedrate ofcontractionLiver and splanchnic beds:blood flow diminishesIncreasedcardiacoutputKidneys: blood flowdiminishesSkin: vasoconstriction atfirst, then dilation for heatdissipationMuscle: initial compression followed bymarked vasodilation due to release ofmetabolites and circulating epinephrineFigure 12.10 Circulatory Response to Exercise Dynamic (aerobic) exercise elicits an integratedcirculatory response.
The autonomic nervous system is important in the neural regulation of this response,affecting heart rate, contractility, and vascular tone. During exercise, skeletal muscle blood flow is greatlyelevated due mainly to the production of vasodilatory metabolites.140Cardiovascular PhysiologyPrenatal circulationAortaPulmonary trunkDuctus arteriosusSuperior vena cavaLeft pulmonary arteryRight pulmonary arteryLeft pulmonary veinRight pulmonary veinForamen ovaleInferior vena cavaHepatic veinAortaDuctus venosusCeliac trunkLiverHepatic portal veinSuperior mesenteric arteryUmbilical veinKidneyLigamentum arteriosum (obliteratedductus arteriosus)GutUmbilical arteriesFossa ovalis(obliteratedforamenovale)VesselPO2Umbilical veinUmbilical arteryInferior vena cava(below ductus venosus)Foramen ovaleAortaDuctus arteriosusPulmonary arteryPulmonary veins8058276762525242Ligamentum venosum(obliterated ductus venosus)Ligamentum teres(round ligament) of liver(obliterated umbilical vein)Medial umbilical ligaments(occluded part of umbilical arteries)Postnatal circulationFigure 12.11 Fetal and Neonatal Circulation The fetal circulation is specifically adapted to efficiently exchange gases, nutrients, and wastes through placental circulation.
Upon birth, the shunts (foramenovale, ductus arteriosus, and ductus venosus) close and the placental circulation is disrupted, producingthe series circulation of blood through the lungs, left atrium, left ventricle, systemic circulation, right heart,and back to the lungs.are several significant differences between the prenatal andpostnatal circulations that merit discussion. The fetus, enveloped in the amniotic sac and floating in the amniotic fluid, isdependent on the placental circulation for exchange of gasesand nutrients. To adapt the circulation to prenatal life, thereare six structures in the fetus not normally seen in adults(Fig. 12.11):■Two large umbilical arteries, which branch from thesystemic arterial circulation and supply the placentalThe Peripheral Circulation■■■■circulation, where gas exchange and nutrient and wasteexchange occur.The umbilical vein, which returns placental blood to thesystemic venous circulation, supplying O2 and nutrientsto the fetus.The ductus venosus, a shunt between the umbilical veinand the inferior vena cava.
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