Y khoa, y dược - The cardiovascular system: blood vessels: Part A

19The Cardiovascular System: Blood Vessels: Part ABlood VesselsDelivery system of dynamic structures that begins and ends at the heartArteries: carry blood away from the heart; oxygenated except for pulmonary circulation and umbilical vessels of a fetusCapillaries: contact tissue cells and directly serve cellular needsVeins: carry blood toward the heartFigure 19.2Large veins(capacitancevessels)LargelymphaticvesselsArteriovenousanastomosisLymphaticcapillaryPostcapillaryvenuleSinusoidMetarterioleTerminal arterioleArterioles(resistance vessels)Muscular arteries(distributingvessels)Elastic arteries(conductingvessels)Small veins(capacitancevessels)LymphnodeCapillaries(exchange vessels)Precapillary sphincterThoroughfarechannelLymphaticsystemVenous systemArterial systemHeart Structure of Blood Vessel WallsArteries and veinsTunica intima, tunica media, and tunica externaLumenCentral blood-containing space CapillariesEndothelium with sparse basal laminaFigure 19.1bTunica media(smooth muscle andelastic fibers)Tunica externa(collagen fibers)LumenArteryLumenVeinInternal elastic laminaExternal elastic laminaValve(b)Endothelial cellsBasement membraneCapillarynetworkCapillaryTunica intima• Endothelium• Subendothelial layerTunicsTunica intimaEndothelium lines the lumen of all vesselsIn vessels larger than 1 mm, a subendothelial connective tissue basement membrane is presentTunicsTunica mediaSmooth muscle and sheets of elastinSympathetic vasomotor nerve fibers control vasoconstriction and vasodilation of vesselsTunicsTunica externa (tunica adventitia)Collagen fibers protect and reinforce Larger vessels contain vasa vasorum to nourish the external layerTable 19.1 (1 of 2)Table 19.1 (2 of 2)Elastic (Conducting) ArteriesLarge thick-walled arteries with elastin in all three tunicsAorta and its major branchesLarge lumen offers low-resistance Act as pressure reservoirs—expand and recoil as blood is ejected from the heartMuscular (Distributing) Arteries and ArteriolesDistal to elastic arteries; deliver blood to body organsHave thick tunica media with more smooth muscleActive in vasoconstrictionArteriolesSmallest arteriesLead to capillary bedsControl flow into capillary beds via vasodilation and vasoconstrictionCapillariesMicroscopic blood vessels Walls of thin tunica intima, one cell thickPericytes help stabilize their walls and control permeabilitySize allows only a single RBC to pass at a timeCapillariesIn all tissues except for cartilage, epithelia, cornea and lens of eyeFunctions: exchange of gases, nutrients, wastes, hormones, etc.Figure 19.2Large veins(capacitancevessels)LargelymphaticvesselsArteriovenousanastomosisLymphaticcapillaryPostcapillaryvenuleSinusoidMetarterioleTerminal arterioleArterioles(resistance vessels)Muscular arteries(distributingvessels)Elastic arteries(conductingvessels)Small veins(capacitancevessels)LymphnodeCapillaries(exchange vessels)Precapillary sphincterThoroughfarechannelLymphaticsystemVenous systemArterial systemHeartCapillariesThree structural typesContinuous capillariesFenestrated capillariesSinusoidal capillaries (sinusoids)Continuous CapillariesAbundant in the skin and musclesTight junctions connect endothelial cells Intercellular clefts allow the passage of fluids and small solutesContinuous capillaries of the brainTight junctions are complete, forming the blood-brain barrierFigure 19.3aRed bloodcell in lumenIntercellularcleftEndothelialcellEndothelialnucleusTight junctionPinocytoticvesiclesPericyteBasementmembrane(a) Continuous capillary. Least permeable, and most common (e.g., skin, muscle).Fenestrated CapillariesSome endothelial cells contain pores (fenestrations)More permeable than continuous capillariesFunction in absorption or filtrate formation (small intestines, endocrine glands, and kidneys)Figure 19.3bRed bloodcell in lumenIntercellularcleftFenestrations(pores)EndothelialcellEndothelialnucleusBasement membraneTight junctionPinocytoticvesicles(b) Fenestrated capillary. Large fenestrations (pores) increase permeability. Occurs in special locations (e.g., kidney, small intestine).Sinusoidal CapillariesFewer tight junctions, larger intercellular clefts, large lumensUsually fenestratedAllow large molecules and blood cells to pass between the blood and surrounding tissuesFound in the liver, bone marrow, spleen Figure 19.3cNucleus ofendothelialcell Red bloodcell in lumenEndothelialcellTight junctionIncompletebasementmembraneLargeintercellularcleft(c) Sinusoidal capillary. Most permeable. Occurs in special locations (e.g., liver, bone marrow, spleen).Capillary BedsInterwoven networks of capillaries form the microcirculation between arterioles and venulesConsist of two types of vesselsVascular shunt (metarteriole—thoroughfare channel):Directly connects the terminal arteriole and a postcapillary venuleCapillary BedsTrue capillaries10 to 100 exchange vessels per capillary bedBranch off the metarteriole or terminal arterioleBlood Flow Through Capillary BedsPrecapillary sphincters regulate blood flow into true capillariesRegulated by local chemical conditions and vasomotor nerves Figure 19.4(a) Sphincters open—blood flows through true capillaries.(b) Sphincters closed—blood flows through metarteriole thoroughfare channel and bypasses true capillaries.PrecapillarysphinctersMetarterioleVascular shuntTerminal arteriolePostcapillary venuleTerminal arteriolePostcapillary venuleThoroughfare channelTrue capillaries VenulesFormed when capillary beds uniteVery porous; allow fluids and WBCs into tissuesPostcapillary venules consist of endothelium and a few pericytesLarger venules have one or two layers of smooth muscle cellsVeinsFormed when venules convergeHave thinner walls, larger lumens compared with corresponding arteriesBlood pressure is lower than in arteriesThin tunica media and a thick tunica externa consisting of collagen fibers and elastic networksCalled capacitance vessels (blood reservoirs); contain up to 65% of the blood supplyFigure 19.1aArteryVein(a)Figure 19.5Heart 8%Capillaries 5%Systemic arteriesand arterioles 15%Pulmonary bloodvessels 12%Systemic veinsand venules 60%VeinsAdaptations that ensure return of blood to the heartLarge-diameter lumens offer little resistance Valves prevent backflow of blood Most abundant in veins of the limbsVenous sinuses: flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)Vascular AnastomosesInterconnections of blood vesselsArterial anastomoses provide alternate pathways (collateral channels) to a given body regionCommon at joints, in abdominal organs, brain, and heartVascular shunts of capillaries are examples of arteriovenous anastomosesVenous anastomoses are commonPhysiology of Circulation: Definition of TermsBlood flowVolume of blood flowing through a vessel, an organ, or the entire circulation in a given periodMeasured as ml/minEquivalent to cardiac output (CO) for entire vascular systemRelatively constant when at restVaries widely through individual organs, based on needsPhysiology of Circulation: Definition of TermsBlood pressure (BP)Force per unit area exerted on the wall of a blood vessel by the blood Expressed in mm HgMeasured as systemic arterial BP in large arteries near the heartThe pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areasPhysiology of Circulation: Definition of TermsResistance (peripheral resistance)Opposition to flow Measure of the amount of friction blood encountersGenerally encountered in the peripheral systemic circulationThree important sources of resistanceBlood viscosityTotal blood vessel lengthBlood vessel diameterResistanceFactors that remain relatively constant:Blood viscosityThe “stickiness” of the blood due to formed elements and plasma proteins Blood vessel lengthThe longer the vessel, the greater the resistance encounteredResistance Frequent changes alter peripheral resistanceVaries inversely with the fourth power of vessel radiusE.g., if the radius is doubled, the resistance is 1/16 as muchResistance Small-diameter arterioles are the major determinants of peripheral resistanceAbrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance Disrupt laminar flow and cause turbulenceRelationship Between Blood Flow, Blood Pressure, and ResistanceBlood flow (F) is directly proportional to the blood (hydrostatic) pressure gradient (P) If P increases, blood flow speeds upBlood flow is inversely proportional to peripheral resistance (R)If R increases, blood flow decreases: F = P/RR is more important in influencing local blood flow because it is easily changed by altering blood vessel diameterSystemic Blood PressureThe pumping action of the heart generates blood flow Pressure results when flow is opposed by resistanceSystemic pressureIs highest in the aortaDeclines throughout the pathwayIs 0 mm Hg in the right atriumThe steepest drop occurs in arteriolesFigure 19.6Systolic pressureMean pressureDiastolic pressureArterial Blood PressureReflects two factors of the arteries close to the heartElasticity (compliance or distensibility)Volume of blood forced into them at any timeBlood pressure near the heart is pulsatile Arterial Blood PressureSystolic pressure: pressure exerted during ventricular contraction Diastolic pressure: lowest level of arterial pressure Pulse pressure = difference between systolic and diastolic pressureArterial Blood PressureMean arterial pressure (MAP): pressure that propels the blood to the tissuesMAP = diastolic pressure + 1/3 pulse pressure Pulse pressure and MAP both decline with increasing distance from the heartCapillary Blood PressureRanges from 15 to 35 mm HgLow capillary pressure is desirableHigh BP would rupture fragile, thin-walled capillariesMost are very permeable, so low pressure forces filtrate into interstitial spacesVenous Blood PressureChanges little during the cardiac cycleSmall pressure gradient, about 15 mm Hg Low pressure due to cumulative effects of peripheral resistanceFactors Aiding Venous ReturnRespiratory “pump”: pressure changes created during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand Muscular “pump”: contraction of skeletal muscles “milk” blood toward the heart and valves prevent backflowVasoconstriction of veins under sympathetic control Figure 19.7Valve (open)ContractedskeletalmuscleValve (closed)VeinDirection ofblood flowMaintaining Blood PressureRequiresCooperation of the heart, blood vessels, and kidneysSupervision by the brainMaintaining Blood PressureThe main factors influencing blood pressure:Cardiac output (CO)Peripheral resistance (PR)Blood volumeMaintaining Blood PressureF = P/PR and CO = P/PRBlood pressure = CO x PR (and CO depends on blood volume)Blood pressure varies directly with CO, PR, and blood volumeChanges in one variable are quickly compensated for by changes in the other variablesCardiac Output (CO)Determined by venous return and neural and hormonal controlsResting heart rate is maintained by the cardioinhibitory center via the parasympathetic vagus nervesStroke volume is controlled by venous return (EDV)Cardiac Output (CO)During stress, the cardioacceleratory center increases heart rate and stroke volume via sympathetic stimulationESV decreases and MAP increasesFigure 19.8Venous returnExerciseContractility of cardiac muscleSympathetic activityParasympathetic activityEpinephrine in bloodEDVESVStroke volume (SV)Heart rate (HR)Cardiac output (CO = SV x HRActivity of respiratory pump(ventral body cavity pressure)Activity of muscular pump(skeletal muscles)Sympathetic venoconstrictionBP activates cardiac centers in medullaInitial stimulusResultPhysiological responseControl of Blood PressureShort-term neural and hormonal controlsCounteract fluctuations in blood pressure by altering peripheral resistanceLong-term renal regulationCounteracts fluctuations in blood pressure by altering blood volumeShort-Term Mechanisms: Neural ControlsNeural controls of peripheral resistanceMaintain MAP by altering blood vessel diameterAlter blood distribution in response to specific demandsShort-Term Mechanisms: Neural ControlsNeural controls operate via reflex arcs that involveBaroreceptorsVasomotor centers and vasomotor fibersVascular smooth muscleThe Vasomotor CenterA cluster of sympathetic neurons in the medulla that oversee changes in blood vessel diameterPart of the cardiovascular center, along with the cardiac centersMaintains vasomotor tone (moderate constriction of arterioles)Receives inputs from baroreceptors, chemoreceptors, and higher brain centersShort-Term Mechanisms: Baroreceptor-Initiated ReflexesBaroreceptors are located inCarotid sinusesAortic archWalls of large arteries of the neck and thoraxShort-Term Mechanisms: Baroreceptor-Initiated ReflexesIncreased blood pressure stimulates baroreceptors to increase input to the vasomotor centerInhibits the vasomotor center, causing arteriole dilation and venodilationStimulates the cardioinhibitory centerFigure 19.9 Baroreceptors in carotid sinusesand aortic archare stimulated. Baroreceptorsin carotid sinusesand aortic archare inhibited. Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratorycenter) and inhibit vasomotorcenter. Impulses from baroreceptors stimulatecardioacceleratory center (and inhibit cardioinhibitorycenter) and stimulate vasomotor center. CO and Rreturn bloodpressure tohomeostatic range. CO and Rreturn blood pressureto homeostatic range. Rate ofvasomotor impulsesallows vasodilation,causing R Vasomotorfibers stimulatevasoconstriction,causing R Sympatheticimpulses to heartcause HR, contractility, and CO. Sympatheticimpulses to heartcause HR, contractility, and CO. Stimulus: Blood pressure(arterial bloodpressure falls belownormal range). Stimulus: Blood pressure(arterial bloodpressure rises abovenormal range).32154a4bHomeostasis: Blood pressure in normal range4b32154aFigure 19.9 step 1 Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).1Homeostasis: Blood pressure in normal rangeFigure 19.9 step 2Baroreceptorsin carotidsinuses and aortic arch are stimulated. Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).21Homeostasis: Blood pressure in normal rangeFigure 19.9 step 3Baroreceptorsin carotidsinuses and aortic arch are stimulated. Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).231Homeostasis: Blood pressure in normal rangeFigure 19.9 step 4a Sympatheticimpulses to heartcause HR, contractility, and CO.Baroreceptorsin carotidsinuses and aortic arch are stimulated. Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).2314aHomeostasis: Blood pressure in normal rangeFigure 19.9 step 4b Sympatheticimpulses to heartcause HR, contractility, and CO. Rate ofvasomotor impulsesallows vasodilation,causing RBaroreceptorsin carotidsinuses and aortic arch are stimulated. Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).2314b4aHomeostasis: Blood pressure in normal rangeFigure 19.9 step 5 Sympatheticimpulses to heartcause HR, contractility, and CO. CO and R returnblood pressureto homeostaticrange. Rate ofvasomotor impulsesallows vasodilation,causing RBaroreceptorsin carotidsinuses and aortic arch are stimulated. Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratory center) and inhibit vasomotor center. Stimulus: Blood pressure(arterial bloodpressure risesabove normalrange).2314b4a5Homeostasis: Blood pressure in normal rangeFigure 19.9 step 1 Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).1Homeostasis: Blood pressure in normal rangeFigure 19.9 step 2 Baroreceptorsin carotid sinusesand aortic archare inhibited. Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).21Homeostasis: Blood pressure in normal rangeFigure 19.9 step 3 Baroreceptorsin carotid sinusesand aortic archare inhibited. Impulses from baroreceptorsstimulate cardioacceleratory center(and inhibit cardioinhibitory center)and stimulate vasomotor center. Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).231Homeostasis: Blood pressure in normal rangeFigure 19.9 step 4a Baroreceptorsin carotid sinusesand aortic archare inhibited. Impulses from baroreceptorsstimulate cardioacceleratory center(and inhibit cardioinhibitory center)and stimulate vasomotor center. Sympatheticimpulses to heartcause HR, contractility, and CO. Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).2314aHomeostasis: Blood pressure in normal rangeFigure 19.9 step 4b Baroreceptorsin carotid sinusesand aortic archare inhibited. Impulses from baroreceptorsstimulate cardioacceleratory center(and inhibit cardioinhibitory center)and stimulate vasomotor center. Vasomotorfibers stimulatevasoconstriction,causing R Sympatheticimpulses to heartcause HR, contractility, and CO. Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).2314b4aHomeostasis: Blood pressure in normal rangeFigure 19.9 step 5 Baroreceptorsin carotid sinusesand aortic archare inhibited. Impulses from baroreceptorsstimulate cardioacceleratory center(and inhibit cardioinhibitory center)and stimulate vasomotor center. CO and Rreturn bloodpressure tohomeostaticrange. Vasomotorfibers stimulatevasoconstriction,causing R Sympatheticimpulses to heartcause HR, contractility, and CO. Stimulus: Blood pressure(arterial bloodpressure fallsbelow normalrange).2314b4a5Homeostasis: Blood pressure in normal rangeFigure 19.9 Baroreceptors in carotid sinusesand aortic archare stimulated. Baroreceptorsin carotid sinusesand aortic archare inhibited. Impulses from baroreceptorsstimulate cardioinhibitory center(and inhibit cardioacceleratorycenter) and inhibit vasomotorcenter. Impulses from baroreceptors stimulatecardioacceleratory center (and inhibit cardioinhibitorycenter) and stimulate vasomotor center. CO and Rreturn bloodpressure tohomeostatic range. CO and Rreturn blood pressureto homeostatic range. Rate ofvasomotor impulsesallows vasodilation,causing R Vasomotorfibers stimulatevasoconstriction,causing R Sympatheticimpulses to heartcause HR, contractility, and CO. Sympatheticimpulses to heartcause HR, contractility, and CO. Stimulus: Blood pressure(arterial bloodpressure falls belownormal range). Stimulus: Blood pressure(arterial bloodpressure rises abovenormal range).32154a4bHomeostasis: Blood pressure in normal range4b32154aShort-Term Mechanisms: Baroreceptor-Initiated ReflexesBaroreceptors taking part in the carotid sinus reflex protect the blood supply to the brainBaroreceptors taking part in the aortic reflex help maintain adequate blood pressure in the systemic circuitShort-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors are located in theCarotid sinusAortic archLarge arteries of the neckShort-Term Mechanisms: Chemoreceptor-Initiated Reflexes Chemoreceptors respond to rise in CO2, drop in pH or O2Increase blood pressure via the vasomotor center and the cardioacceleratory centerAre more important in the regulation of respiratory rate (Chapter 22)Influence of Higher Brain CentersReflexes that regulate BP are integrated in the medullaHigher brain centers (cortex and hypothalamus) can modify BP via relays to medullary centersShort-Term Mechanisms: Hormonal ControlsAdrenal medulla hormones norepinephrine (NE) and epinephrine cause generalized vasoconstriction and increase cardiac outputAngiotensin II, generated by kidney release of renin, causes vasoconstrictionShort-Term Mechanisms: Hormonal ControlsAtrial natriuretic peptide causes blood volume and blood pressure to decline, causes generalized vasodilationAntidiuretic hormone (ADH)(vasopressin) causes intense vasoconstriction in cases of extremely low BPLong-Term Mechanisms: Renal RegulationBaroreceptors quickly adapt to chronic high or low BPLong-term mechanisms step in to control BP by altering blood volumeKidneys act directly and indirectly to regulate arterial blood pressureDirect renal mechanism Indirect renal (renin-angiotensin) mechanism Direct Renal MechanismAlters blood volume independently of hormonesIncreased BP or blood volume causes the kidneys to eliminate more urine, thus reducing BPDecreased BP or blood volume causes the kidneys to conserve water, and BP risesIndirect MechanismThe renin-angiotensin mechanism Arterial blood pressure  release of reninRenin production of angiotensin II Angiotensin II is a potent vasoconstrictorAngiotensin II  aldosterone secretionAldosterone  renal reabsorption of Na+ and  urine formation Angiotensin II stimulates ADH releaseFigure 19.10Arterial pressureBaroreceptorsIndirect renalmechanism (hormonal)Direct renalmechanismSympathetic stimulationpromotes renin releaseKidneyRenin releasecatalyzes cascade,resulting in formation ofADH releaseby posteriorpituitaryAldosteronesecretion byadrenal cortexWaterreabsorptionby kidneys Blood volume FiltrationArterial pressureAngiotensin IIVasoconstriction( diameter of blood vessels)Sodiumreabsorptionby kidneysInitial stimulusPhysiological responseResultFigure 19.11Activity ofmuscularpump andrespiratorypumpReleaseof ANPFluid loss fromhemorrhage,excessivesweatingCrisis stressors:exercise, trauma, bodytemperatureBloodbornechemicals:epinephrine,NE, ADH,angiotensin II; ANP releaseBody sizeConservationof Na+ andwater by kidneyBlood volumeBlood pressureBlood pH, O2, CO2Dehydration,high hematocritBloodvolumeBaroreceptorsChemoreceptorsVenousreturnActivation of vasomotor and cardiacacceleration centers in brain stemHeartrateStrokevolumeDiameter ofblood vesselsCardiac outputInitial stimulusResultPhysiological responseMean systemic arterial blood pressureBloodviscosityPeripheral resistanceBlood vessellength