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

19 The Cardiovascular System: Blood Vessels: Part BMonitoring Circulatory EfficiencyVital signs: pulse and blood pressure, along with respiratory rate and body temperaturePulse: pressure wave caused by the expansion and recoil of arteriesRadial pulse (taken at the wrist) routinely usedFigure 19.12Common carotidarteryBrachial arteryRadial arteryFemoral arteryPopliteal arteryPosterior tibialarteryDorsalis pedisarterySuperficial temporalarteryFacial arteryMeasuring Blood PressureSystemic arterial BPMeasured indirectly by the auscultatory method using a sphygmomanometerPressure is increased in the cuff until it exceeds systolic pressure in the brachial arteryMeasuring Blood PressurePressure is released slowly and the examiner listens for sounds of Korotkoff with a stethoscopeSounds first occur as blood starts to spurt through the artery (systolic pressure, normally 110–140 mm Hg)Sounds disappear when the artery is no longer constricted and blood is flowing freely (diastolic pressure, normally 70–80 mm Hg)Variations in Blood PressureBlood pressure cycles over a 24-hour periodBP peaks in the morning due to levels of hormonesAge, sex, weight, race, mood, and posture may vary BP Alterations in Blood Pressure Hypotension: low blood pressureSystolic pressure below 100 mm HgOften associated with long life and lack of cardiovascular illnessHomeostatic Imbalance: HypotensionOrthostatic hypotension: temporary low BP and dizziness when suddenly rising from a sitting or reclining position Chronic hypotension: hint of poor nutrition and warning sign for Addison’s disease or hypothyroidismAcute hypotension: important sign of circulatory shockAlterations in Blood Pressure Hypertension: high blood pressureSustained elevated arterial pressure of 140/90 or higherMay be transient adaptations during fever, physical exertion, and emotional upsetOften persistent in obese peopleHomeostatic Imbalance: HypertensionProlonged hypertension is a major cause of heart failure, vascular disease, renal failure, and stroke Primary or essential hypertension90% of hypertensive conditionsDue to several risk factors including heredity, diet, obesity, age, stress, diabetes mellitus, and smokingHomeostatic Imbalance: HypertensionSecondary hypertension is less commonDue to identifiable disorders, including kidney disease, arteriosclerosis, and endocrine disorders such as hyperthyroidism and Cushing’s syndromeBlood Flow Through Body TissuesBlood flow (tissue perfusion) is involved inDelivery of O2 and nutrients to, and removal of wastes from, tissue cells Gas exchange (lungs)Absorption of nutrients (digestive tract)Urine formation (kidneys)Rate of flow is precisely the right amount to provide for proper functionFigure 19.13BrainHeartSkeletalmusclesSkinKidneyAbdomenOtherTotal blood flow during strenuousexercise 17,500 ml/minTotal bloodflow at rest5800 ml/minVelocity of Blood FlowChanges as it travels through the systemic circulationIs inversely related to the total cross-sectional areaIs fastest in the aorta, slowest in the capillaries, increases again in veinsSlow capillary flow allows adequate time for exchange between blood and tissuesFigure 19.14Relative cross-sectional area ofdifferent vesselsof the vascular bedTotal area(cm2) of thevascularbedVelocity ofblood flow(cm/s)AortaArteriesArteriolesCapillariesVenulesVeinsVenae cavaeAutoregulationAutomatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in timeIs controlled intrinsically by modifying the diameter of local arterioles feeding the capillariesIs independent of MAP, which is controlled as needed to maintain constant pressure AutoregulationTwo types of autoregulationMetabolicMyogenicMetabolic ControlsVasodilation of arterioles and relaxation of precapillary sphincters occur in response toDeclining tissue O2 Substances from metabolically active tissues (H+, K+, adenosine, and prostaglandins) and inflammatory chemicalsMetabolic ControlsEffectsRelaxation of vascular smooth muscleRelease of NO from vascular endothelial cellsNO is the major factor causing vasodilationVasoconstriction is due to sympathetic stimulation and endothelins Myogenic ControlsMyogenic responses of vascular smooth muscle keep tissue perfusion constant despite most fluctuations in systemic pressurePassive stretch (increased intravascular pressure) promotes increased tone and vasoconstrictionReduced stretch promotes vasodilation and increases blood flow to the tissueFigure 19.15MetaboliccontrolspHSympathetica Receptorsb ReceptorsEpinephrine,norepinephrineAngiotensin IIAntidiuretichormone (ADH)Atrialnatriureticpeptide (ANP)DilatesConstrictsProstaglandinsAdenosineNitric oxideEndothelinsStretchO2CO2K+Amounts of:Amounts of:NervesHormonesMyogeniccontrolsIntrinsic mechanisms(autoregulation)• Distribute blood flow to individual organs and tissues as neededExtrinsic mechanisms• Maintain mean arterial pressure (MAP)• Redistribute blood during exercise and thermoregulationLong-Term AutoregulationAngiogenesisOccurs when short-term autoregulation cannot meet tissue nutrient requirementsThe number of vessels to a region increases and existing vessels enlarge Common in the heart when a coronary vessel is occluded, or throughout the body in people in high-altitude areasBlood Flow: Skeletal MusclesAt rest, myogenic and general neural mechanisms predominateDuring muscle activityBlood flow increases in direct proportion to the metabolic activity (active or exercise hyperemia)Local controls override sympathetic vasoconstriction Muscle blood flow can increase 10 or more during physical activity Blood Flow: BrainBlood flow to the brain is constant, as neurons are intolerant of ischemiaMetabolic controlsDeclines in pH, and increased carbon dioxide cause marked vasodilationMyogenic controlsDecreases in MAP cause cerebral vessels to dilate Increases in MAP cause cerebral vessels to constrict Blood Flow: BrainThe brain is vulnerable under extreme systemic pressure changes MAP below 60 mm Hg can cause syncope (fainting)MAP above 160 can result in cerebral edemaBlood Flow: SkinBlood flow through the skinSupplies nutrients to cells (autoregulation in response to O2 need)Helps maintain body temperature (neurally controlled) Provides a blood reservoir (neurally controlled)Blood Flow: SkinBlood flow to venous plexuses below the skin surfaceVaries from 50 ml/min to 2500 ml/min, depending on body temperatureIs controlled by sympathetic nervous system reflexes initiated by temperature receptors and the central nervous systemTemperature RegulationAs temperature rises (e.g., heat exposure, fever, vigorous exercise)Hypothalamic signals reduce vasomotor stimulation of the skin vesselsHeat radiates from the skinTemperature RegulationSweat also causes vasodilation via bradykinin in perspirationBradykinin stimulates the release of NO As temperature decreases, blood is shunted to deeper, more vital organsBlood Flow: LungsPulmonary circuit is unusual in thatThe pathway is shortArteries/arterioles are more like veins/venules (thin walled, with large lumens)Arterial resistance and pressure are low (24/8 mm Hg)Blood Flow: LungsAutoregulatory mechanism is opposite of that in most tissuesLow O2 levels cause vasoconstriction; high levels promote vasodilationAllows for proper O2 loading in the lungsBlood Flow: HeartDuring ventricular systoleCoronary vessels are compressedMyocardial blood flow ceasesStored myoglobin supplies sufficient oxygenAt rest, control is probably myogenicBlood Flow: HeartDuring strenuous exerciseCoronary vessels dilate in response to local accumulation of vasodilatorsBlood flow may increase three to four times Blood Flow Through CapillariesVasomotionSlow and intermittent flowReflects the on/off opening and closing of precapillary sphinctersCapillary Exchange of Respiratory Gases and NutrientsDiffusion ofO2 and nutrients from the blood to tissuesCO2 and metabolic wastes from tissues to the bloodLipid-soluble molecules diffuse directly through endothelial membranesWater-soluble solutes pass through clefts and fenestrationsLarger molecules, such as proteins, are actively transported in pinocytotic vesicles or caveolaeFigure 19.16 (1 of 2)Red bloodcell in lumenEndothelial cellIntercellular cleftFenestration(pore)Endothelial cell nucleusTight junctionBasement membranePinocytotic vesiclesFigure 19.16 (2 of 2)BasementmembraneEndothelialfenestration(pore)IntercellularcleftPinocytoticvesiclesCaveolae4 Transportvia vesicles orcaveolae (largesubstances)3 Movementthroughfenestrations (water-soluble substances)2 Movementthrough intercellular clefts (water-soluble substances)1 Diffusionthrough membrane (lipid-soluble substances)LumenFluid Movements: Bulk FlowExtremely important in determining relative fluid volumes in the blood and interstitial spaceDirection and amount of fluid flow depends on two opposing forces: hydrostatic and colloid osmotic pressuresHydrostatic PressuresCapillary hydrostatic pressure (HPc) (capillary blood pressure)Tends to force fluids through the capillary walls Is greater at the arterial end (35 mm Hg) of a bed than at the venule end (17 mm Hg)Interstitial fluid hydrostatic pressure (HPif)Usually assumed to be zero because of lymphatic vesselsColloid Osmotic PressuresCapillary colloid osmotic pressure (oncotic pressure) (OPc) Created by nondiffusible plasma proteins, which draw water toward themselves~26 mm HgInterstitial fluid osmotic pressure (OPif)Low (~1 mm Hg) due to low protein contentNet Filtration Pressure (NFP)NFP—comprises all the forces acting on a capillary bedNFP = (HPc—HPif)—(OPc—OPif)At the arterial end of a bed, hydrostatic forces dominateAt the venous end, osmotic forces dominateExcess fluid is returned to the blood via the lymphatic systemFigure 19.17HP = hydrostatic pressure• Due to fluid pressing against a wall• “Pushes”• In capillary (HPc) • Pushes fluid out of capillary • 35 mm Hg at arterial end and 17 mm Hg at venous end of capillary in this example• In interstitial fluid (HPif) • Pushes fluid into capillary • 0 mm Hg in this example OP = osmotic pressure• Due to presence of nondiffusible solutes (e.g., plasma proteins)• “Sucks”• In capillary (OPc) • Pulls fluid into capillary • 26 mm Hg in this example• In interstitial fluid (OPif) • Pulls fluid out of capillary • 1 mm Hg in this exampleArterioleCapillaryInterstitial fluidNet HP—Net OP(35—0)—(26—1)Net HP—Net OP(17—0)—(26—1)VenuleNFP (net filtration pressure)is 10 mm Hg; fluid moves outNFP is ~8 mm Hg;fluid moves inNetHP35mmNetOP25mmNetHP17mmNetOP25mmCirculatory ShockAny condition in whichBlood vessels are inadequately filledBlood cannot circulate normally Results in inadequate blood flow to meet tissue needsCirculatory ShockHypovolemic shock: results from large-scale blood loss Vascular shock: results from extreme vasodilation and decreased peripheral resistanceCardiogenic shock results when an inefficient heart cannot sustain adequate circulationFigure 19.18Signs and symptomsAcute bleeding (or other events that causeblood volume loss) leads to:1. Inadequate tissue perfusion resulting in O2 and nutrients to cells2. Anaerobic metabolism by cells, so lactic acid accumulates3. Movement of interstitial fluid into blood, so tissues dehydrateInitial stimulusResultPhysiological responseChemoreceptors activated(by in blood pH)Baroreceptor firing reduced(by blood volume and pressure)Hypothalamus activated(by pH and blood pressure)Major effectMinor effectBrainActivation ofrespiratory centersCardioacceleratory andvasomotor centers activatedSympathetic nervoussystem activatedADHreleasedNeuronsdepressedby pHIntense vasoconstriction(only heart and brain spared)Heart rateCentralnervous systemdepressedAdrenalcortexKidneyRenin releasedRenal blood flowAldosteronereleasedKidneys retainsalt and waterAngiotensin IIproduced in bloodWaterretentionUrine outputRate anddepth ofbreathingTachycardia,weak, threadypulseSkin becomescold, clammy,and cyanoticThirstRestlessness(early sign)Coma(late sign)CO2 blownoff; bloodpH risesBlood pressure maintained;if fluid volume continues todecrease, BP ultimatelydrops. BP is a late sign.Circulatory PathwaysTwo main circulationsPulmonary circulation: short loop that runs from the heart to the lungs and back to the heartSystemic circulation: long loop to all parts of the body and back to the heartFigure 19.19aR. pulmon-ary veinsPulmonarytrunkPulmonary capillariesof the R. lungPulmonary capillariesof the L. lungR. pulmonaryarteryL. pulmonaryarteryTosystemic circulationL. pulmonaryveins(a) Schematic flowchart.FromsystemiccirculationRARVLVLAFigure 19.20AzygossystemVenousdrainageArterialbloodThoracicaortaInferiorvenacavaAbdominalaortaInferiorvenacavaSuperiorvena cavaCommoncarotid arteriesto head andsubclavianarteries toupper limbsAortic archAortaRARVLVLACapillary beds ofhead andupper limbsCapillary beds ofmediastinal structuresand thorax wallsDiaphragmCapillary beds ofdigestive viscera,spleen, pancreas,kidneysCapillary beds of gonads,pelvis, and lower limbsArteriesVeinsDeliveryBlood pumped into single systemic artery—the aortaBlood returns via superior and interior venae cavae and the coronary sinusLocationDeep, and protected by tissuesBoth deep and superficialPathwaysFairly distinctNumerous interconnectionsSupply/drainagePredictable supplyUsually similar to arteries, except dural sinuses and hepatic portal circulationDifferences Between Arteries and VeinsDevelopmental AspectsEndothelial lining arises from mesodermal cells in blood islandsBlood islands form rudimentary vascular tubes, guided by cues such as vascular endothelial growth factorThe heart pumps blood by the 4th week of developmentDevelopmental AspectsFetal shunts (foramen ovale and ductus arteriosus) bypass nonfunctional lungsDuctus venosus bypasses the liverUmbilical vein and arteries circulate blood to and from the placentaDevelopmental AspectsVessel formation occursTo support body growthFor wound healingTo rebuild vessels lost during menstrual cyclesWith aging, varicose veins, atherosclerosis, and increased blood pressure may arisBlood Circulation: Study Guide