hemodynamic & av pressures 2008

7/24/2019 Hemodynamic & Av Pressures 2008

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INTERRELATIONSHIPS AMONG VESSELSTRUCTURE, BLOOD PROPERTIES, WALLCOMPLIANCE IN VASCULAR FUNCTION.

INTERRELATIONSHIPS AMONG CARDIAC

OUTPUT, VASCULAR RESISTANCE ANDPRESSURE.

Laminar flow

Turbulent

flow


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Cardiovascular Physiology 2008,HEMODYNAMICS, ARTERIAL & VENOUS PRESSURESpage 2/26

READING REFERENCES:Chapters 5 Cardiovascular Physiology Concepts, Richard E. Klabunde, LippincottWilliams and Wilkins, 2005

CVS OVERVIEW, HEMODYNAMICS, ARTERIAL & VENOUS PRESSURES

OBJECTIVES HIGHLIGHTS:

Describe the componentparts and enumeratesimilarities anddifferences between thecentral and peripheralcirculations.

Central circulation also known as thepulmonary circulationbeginsat the pulmonary artery and ends at the pulmonary veins. This is alow pressure system which perfuses only the lungs.

Peripheral circulation also known as the systemic circulationbegins at the aorta and ends at the superior and inferior vena cava.This is a high pressure systemand perfuses the rest of the tissues.Tissues in this circuit are arranged in a parallel disposition to insure

that a similar pressure gradient(driving force) will be available foreach tissue.

The pulmonary and the systemic circulations are situated betweentwo pumps: the right and left hearts. The two sides of the heart are inserieswith the two circulations. All blood pumped from the rightventricle enters the pulmonary circulation and then into the leftventricle which pumps blood into the systemic circulation before itreturn back to the right heart. This organization requires that theoutput of each side closely matches the output of the other.


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CVS OVERVIEW, HEMODYNAMICS, ARTERIAL & VENOUS PRESSURESOBJECTIVES HIGHLIGHTS:

Identify the meaning of

blood flow velocity , itsdistribution along thecardiovascular systemand the hemodynamicprinciple explaining it(i.e.relationship betweenblood flow, vessel areaand velocity).

Velocity is the distancean object (solid, liquid or gas) moves with respect

to time (i.e., the distance traveled per unit of time). In the case of bloodflowing in a vessel, velocity is often expressed in the units of cm/sec.

Blood flow velocity is highestat the aorta and lowestat the exchangevessels because flow (Q) is the product of the cross-sectional area (A) andvelocity (V) in a closed hydraulic system.

Q = A x V

Since in such a closed system, flow is constant, if area increases, flowvelocity has to decrease to maintain flow constant. In other words velocityis inversely proportional to cross-sectional area in a system of constantflow.

Identify the distribution ofresistance along thecardiovascular systemand its relationship with

pressure changes.

The main generator of pressure at the systemic circulation is the LEFVENTRICLE. When the pressure in the left ventricle exceeds that in the aortathe aortic semilunar valve opens and the pressure in both places rises. Thrise in pressure expands the great arteries because of their elastic fibers an

because blood enters the arterial tree faster than it leaves through the smaarterioles. When the left ventricle relaxes, its internal pressure drops near zeroThe pressure in the arteries on the other hand, fall slowly because when thvessels recoil back to their relaxed situation they help maintain the forwardmovement of blood during the relaxation period of the heart. Thus, thpressure drop from the root of aorta to the point of origin of arterioles irelatively small.


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The major loss of pressureoccurs at the ARTERIOLES because of the higresistance they offer to blood flow The resistance is regulated by the caliber othe small arterioles which in turn is controlled by the state of contraction of thethick smooth muscle covering. The pressure in the capillaries and veindecreases and becomes to approximately zeroin the great veins entering thright atrium.

The right side of the heart(along with the pulmonary circulation) generates pressure pattern similar, but of lesser magnitude (6 times less) than thaobserved in the left heart and the systemic circulation. Pressure fallcontinuously from the arterial to the venous side of the vascular bed.Pressurdrop is greater where the resistance is higher.

Pressure

(mmH

g)

Time

RIGHT HEART

LEFT HEART

Pressure

(mmH

g)

Time

RIGHT HEART

LEFT HEART


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Identify the distribution of

blood volume along thecardiovascular system.

The largest volume is concentrated in the low pressureareas: systemicveins (64%),due to the properties of their walls which permit them todistend. Capillaries, on the other hand have a low blood volume (4%)due to their small size.

Therefore, if the normal blood voume is rapidly expanded (i.e. bloodtransfusion) or contracted (i.e. hemorrhage) most of the volume change isaccomodated in the low pressure venoussystem.

Describe the physicalfactors that govern bloodflow.

Hemodynamics concerns the physical factors governing blood flowwithin the circulatory system. Blood flow (Q) through an organ or anyvascular network is driven by a perfusion pressure( i.e., pressure

gradient or difference) that is normally represented by the differencebetween the arterial and venous pressures across the organ (P). Theactual blood flow at any given perfusion pressure is determined by theresistance to blood flow.

The relationship between flow, pressure, and resistanceis given bythe following equation (Poiseuille equation)

Q = PA PV or Q = P

R R

where Q = flow, PAand PV= mean arterial and venouspressures, respectively, and R = resistance to flow (termedvascular resistance).

From which it is observed that Pis the variable that promotes flow(directly proportional) and R is that variable that opposes flow(inverselyproportional).


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Distinguish between

laminar and turbulentflow; define Reynoldnumber and how itpredicts the type offlow present in avessel.

Laminar flow is the normal (silent)flow of a fluid that behaves as if it wereformed by many individual concentric layers (laminaes) which slide onepast the other (parallel) down the length of a blood vessel . As one movesfrom the walls to the axis, the laminaes will move with greater velocitygenerating a parabolic gradient of velocities.

Therefore, the highest velocity is at the center of the vessel. The lowest(zero velocity will be next to the walls of the vessel.

If several factors (which are summarized in what is known as the Reynoldsnumber (Re)) increase above a limit (2000 or 1000 if radius is used insteaof diameter), laminar flow will be transformed into a turbulent flow.Turbulent flowloses the parabolic profile of flow velocities, is noisyandwill represent a higher resistanceto blood flow.

Re = (v D

) /

Where v= mean velocity, D = vessel diameter, =blood

density, and =blood viscosity. Turbulence generates soundwaves(e.g., ejection murmurs, carotid bruits) that can be heardwith a stethoscope. Turbulence alters the relationship betweenflow and perfusion pressure such that the relationship is no

longer linearas described by the Poiseuille relationship.

Instead, a greaterperfusion pressure is required to propelbloodat a given flow rate.


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Describe the primary

factors that determinethe resistance to bloodflow within a singlevessel.

Vascular resistance (R) is due to both

vessel properties: radius (r), length (L)

blood properties: viscosity ()

The influence of each factor in vascular resistance has beenexperimentally determined and is summarized in the followingequation:

From which it is appreciated that the most important factor whichdetermines vascular resistance is the vessel radiussince its value israised to the fourth power(r4). Therefore, small changes in vesselradius will affect vascular resistance greatly. If the vessel radiusincreases, resistance decreases dramatically. If the vessel radiusdecreases, resistance will increase dramatically.

Vessel length does not change appreciably in vivoand, therefore, cangenerally be considered as a constant. Although blood viscositynormally does not change very much, it can be significantly altered bychanges in hematocrit, temperature, and by low flow states.

Recognize that in adynamic system thefactors that impedeblood flow are knownas vascular impedance.

The term vascular resistance (R) applies when a steady (constant)flow is considered. Actually, when the pulsatile nature of arterialpressure and flow are combined with the elastic properties of thevessels, viscosity and the inertial nature of blood the mathematicalquantity that opposes blood flowis known as the vascularimpedance.

Define viscosity, itsrole in blood flow andidentify the factors

that determine bloodviscosity.

Viscosity is a property of fluidrelated to the internal friction ofadjacent fluid layers sliding past one another as well as the frictiongenerated between the fluid and the wall of the vessel. This

internal friction contributes to the resistance to flow. The viscosityof plasma is about 1.8-times the viscosity of water (termed relativeviscosity) at 37C and is related to the protein composition of theplasma. Whole blood has a relative viscosity of 3 to 4 dependingupon hematocrit,temperature, vessel sizeand flow rate (orshear rate).


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As hematocrit increases, there is a disproportionate increasein viscosity.

As temperature decreases, viscosity increases.

At very low flowstates in the microcirculation, as it occursduring circulatory shock, the blood viscosity can increasequite significantly. This occurs because at low flow statesthere are increased cell-to-cell and protein-to-cell adhesiveinteractions that can cause erythrocytes to adhere to oneanother and increase the blood viscosity.

Blood flow in minute vessels (less than 200 m in diameter;arterioles, capillaries and venules) exhibit far less viscouseffect than in larger vessels. This is called the "Fahraeus-Lindqvist effect".

Calculate the effect ofdoubling the radius of avessel in its flow (Q) whilethe pressure gradient is

constant.

Since Q=P x 1___ or Q =P x r4

l/r4 l

Assuming viscosity () and length (l) are constantIf r before the doubling = 1Therefore, r after the doubling = 2

When P is constant the change in r will generate an increase of 24

or 16 timesthe original flow.


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Compare graphically the

relationship between flow,pressure and resistancefor a fluid flowing througha rigid tube, distensibletube, and a distensibletube with autoregulation.

In an ideal system the flow of a fluid through a rigid tube (1) can be

represented graphically as a straight line in which the slope is thereciprocal of the resistance (1/R). If the tube is distensible thegraphical relation is hyperbolic (2) since the distention of the tube,upon the increase in pressure, decreases the resistance (increasesthe slope). In a vessel exhibiting autoregulation (arrow), the graphwill demonstrate a plateau or constant flow in a particular range ofpressures demonstrating the effect of changes in resistance tocounterbalance the change in pressure in order to maintain aconstant flow.

Distinguish between afluid system in whichresistance units are inseries with one in whichresistance units are inparallel.

The vascular anatomy of the entire body or for an individual organis comprised of both in-seriesand in-parallelvascularcomponents. Blood leaves the heart through the aorta from which itis distributed to major organs by large arteries, each of whichoriginates from the aorta. Therefore, these major distributingarteries (e.g., carotid, brachial, superior mesenteric, renal, iliac) arein-parallelwith each other. This further means that the vascular

networks of most individual organs are in-parallelwith otherorgan networks.

If resistances are in series, flow is constant, total resistanceis the sum of the individual resistances and pressure dropat each resistance will be proportional to each resistancemagnitude. Therefore, total resistance in the series circuitis always greaterthan any of the individual resistances.

Autoregulation from 60 140 mm Hg. At low pressures, R will have to

decrease.

At high pressures, R will have toincrease

in order to maintain flow constant

Pressure

1

(2)

Pressure


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Changes in large artery resistance have little effect on totalresistance, while changes in small artery and arteriolar

resistances greatly affect total resistance. This is why smallarteries and arteriolesare the principle vessels regulatingorgan blood flow and systemic vascular resistance.

When resistances are in parallel, pressure drop acrosseach resistance unit is similar, but flow is inverselyproportional to the resistance magnitude and the total flowis the sum of the individual flows. The total resistance of anetwork of parallel vessels is lessthan the resistance ofthe vessel having the lowest resistance. Therefore, aparallel arrangement of vessels greatly reduces total

resistance to blood flow. A second principle is also foundin this relationship: when there are many parallel vessels,changing the resistance of a small number of these vesselswill have little effect on total resistancefor the segment.

Define Total PeripheralResistance (TPR),Peripheral ResistanceUnits (PRU) and makecomparisons between theresistances offered at

various circulatory beds.

Total peripheral resistance (TPR) also refers to the resistance toblood flow offered by all of the systemic vasculature (from aorta tovena cava), excluding the pulmonary vasculature. TPR can becalculated if cardiac output (CO), mean arterial pressure (MAP),and central venous pressure (CVP, pressure at the vena cava) areknown.

TPR = (MAP - CVP)/ CO

Because CVP is normally near 0 mm Hg, the calculation is oftensimplified to:

TPR = MAP/ CO

Since P is constant and

QT = Q1 + Q2 + Q3 + Q4

1 = 1 + 1 + 1

RT R1 R2 R3

IN PARALLELIN SERIES

Since Q is constant and

PT

= P1

+ P2

+ P3+ P

4+ P

5

RT = R1 + R2 + R3 + ...

Dividing byP yields:Dividing by Q yields:

+ ...

Since P is constant and

QT = Q1 + Q2 + Q3 + Q4

1 = 1 + 1 + 1

RT R1 R2 R3

IN PARALLELIN SERIES

Since Q is constant and

PT

= P1

+ P2

+ P3+ P

4+ P

5

RT = R1 + R2 + R3 + ...

Dividing byP yields:Dividing by Q yields:

+ ...


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Although mathematically, MAP and CO can be used to calculateTPR, physiologically, TPR and CO are normally the independent

variables and MAP is the dependent variable.

Since MAP is expressed in mm Hg and Cardiac Output in L/min aPRU is expressed in mm Hg /L / min.

The figure below shows the distribution of flows in the systemiccirculation. Therefore, individual resistance (in PRU) offered byeach tissue can be calculated.

Brain tissue resistance = 100 mm Hg/ 0.8 L/min or 125 PRU

Skin tissue resistance = 100 mm Hg/ 0.3 L/min or 333 PRU

Renal tissue resistance = 100 mm Hg/1.1 L/min or 90.9 PRU

Observe that since tissues are in paralleltotal systemic resistance(SVR or TPR) is lessthan any resistance in the circuit.

(TPR) = 100 mm Hg /5.5 l/min or 18 PRU


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Define vascularcompliance, and

identify its graphicalrepresentation.

In physical terms, the relationship between a change in volume (V)

and a change in pressure (P) is termed compliance (C), where

C = V / P

Compliance, is a measure of vessel ability to distendandaccumulate volume with a change in internal pressure. Compliance ishigh when a large volume can be accommodated with small pressurechanges, and is low when small volume changes result in largepressure differences.

In biological tissues, the relationship between V and P is not linear

Compliance is the slope ( V/ P) of the relationship between volume

and pressure and it decreasesat higher volumes and pressures(Observe slopes of the vessels volume-pressure relationship in thefigure below).

At low pressure, veins are 24 times more compliant than arteries. Inaddition, vascular compliance decreases with age.

The large venous compliance allows them to store blood volume tobe available to return to the heart and lungs when needed (i.e.

exercise). Venous compliance may be decreased by compressionby muscles and by increased (sympathetic) tone of the vessels.

The decrease in venous compliance by compression, or neuronaltone, increases venous pressurewhich is one of the forces neededfor the driving pressure for venous return or the flow from the veinsback to the heart.


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Describe how the aortic

wall converts theintermittent volume andpressure output of theheart into a relativelyconstant blood flow to theperiphery.

As blood is ejected into the aorta during systole, the walls of the aort

expand to accommodate the increase in blood volume because theaorta and its large branches contain a great proportion of elastin

fibers. When these vessels are stretched they in fact are storing amajor fraction of the energy imparted to the blood by thecontracting ventricle. During ventricular diastole they recoil tresume their original configuration. This recoil imparts kinetienergy to the blood maintaining its flow throughout diastole.

Define hydrostaticpressure and describethe effect of posture inthe distribution ofpressures along thecardiovascular system

Hydrostatic pressure is the pressure that a liquid exerts due to theacceleration of earth gravity (g) on its mass. Apart from gravity othe

factors affecting the hydrostatic pressure of a liquid are its density (and the height of the column (h) it forms in the vertical position withrespect to a reference level. These factors are summarized in theexpression:

P = g h

Therefore, since g (gravity factor) is a constant, this means thathydrostatic pressures can be expressed by specifying the liquidand the height of the column it forms, i.e. 45 mm Hg.

For the cardiovascular system, the reference level (zero) is at theheart level and the hydrostatic pressures will affect the system onlyin the erect (vertical) position.


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When an individual is in the vertical position hydrostatic pressures

sum to the hydraulic pressure generated by the contraction of theheart. Total pressure over the heart will be lower, while totalpressure below the heart will be higher. However, since both arterieand veins are subjected to the same influence, the arterio-venouspressure difference is not altered. Transmural pressure (pressureacross the walls), however, will be higher and will affect vessels withthin walls (i.e. veins).

Differentiate betweensystolic, diastolic, pulseand mean arterialpressures. Giveapproximate normalvalues.

All refer to the arterialpressure values generated along thecardiac cycle (time it takes for one heart contraction (systole) andrelaxation (diastole)). Systolic pressure(SP) is the highestvalue, diastolic pressure (DP)- the lowest. Pulse pressure- thedifference between both (SP-DP) and the mean arterial pressure(MAP) is the pressure-time integral of the pressure wave as itoccurs in one cycle and is the best single measure of effectivedriving pressure.


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The mean arterial pressure is less than the arithmetic averagebetween systolic and diastolic pressures because the lower half of

the curve (ocurring during diastole) has a greater area than theupper half. In other words, pressures during diastole (which is 2/3of the cardiac cycle at resting heart rates) weight more in thedetermination of the MAP. A rough approximation of MAP isobtained from the equation = D.P + 1/3( SP-DP).

Recall normal values ofaortic and pulmonaryarterial pressures.

Systolic/DiastolicAortic : 120 / 80 Mean = 80 + 1/3 (120-80) = 93.33 mm HgPulmonary: 25 / 10 Mean = 10 + 1/3 (25 10) = 15.00 mmHg

Identify the main factorsaffecting Mean ArterialPressure (MAP).

For the systemic circulation : based on the Poiseuille equation (Q=P/R), and assuming that the output pressure of the systemiccirculation is zero (pressure at the right atrium) and the inputpressure is the MAP. then

Q = (MAP 0)/RQ x R = MAP

Therefore, MAP can be modified by changes in Q, which is the outputof the left heart (cardiac ouput, C.O.) and R, which refers to the totalperipheral resistance (TPR) : MAP = CO X TPR

Identify the basis for thedetermination of bloodpressure throughsphigmomanometry.

This is an indirect measure of arterial pressure in which a peripheralartery (brachial artery) is occluded with a bag in which air is pumpedto increase its compressing power. Traditionally a mercury (Hg)column is attached to the air bag to follow its pressure. First, thepressure in the air bag is increased in order to occlude the vessel.Then, with the aid of a stethoscope, placed over the brachial artery,one can follow the appearance and disappearance of Korotkoff

Onecardiac

cycle

systole diastole


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sounds. The pressure in the air bag at the moment of theappearance of the sounds is taken as the systolic pressure. The

pressure in the air bag when sounds dissappear is taken as thediastolic pressure. The Korotkoff sounds generate due to theturbulence created during the partial occlusion of the vessel due tothe high velocity that acquires the blood flow in this situation (sincethe occluded arterial lumen has a smaller cross-sectional area).

Describe and explainchanges in the form ofthe arterial pressure

wave as it moves fromthe aorta to theperiphery.

The Arterial pressure curve in peripheral arteries demonstrates anincrease in pulse pressure due to increases in systolic pressure anddecreases in diastolic pressure. Mean arterial pressure declines

slightly. The changes are due to the sum of reflected waveswithincoming waves in distal arteries that are less elastic and wherethey divide and become narrower. These structural changes makespressure waves reflect back and affects incoming waveconfiguration.


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Define the factors that

affect the configuration ofthe arterial pressurecurve.

Heart rate, arterial compliance, stroke volume and periphera

resistance affect the arterial pressure curve configuration.

Increases in heart rate will shorten the cycle and diastolicpressure will be higher.

decreases in arterial compliance will increase systolic pressurebut will decrease diastolic pressure.

Increases in stroke volume will increase systolic pressure and ifother factors are constant the increase in systolic pressure willincrease the diastolic pressure.

Increases in peripheral resistance will decrease the amount ofblood that flows out to the periphery generating higher diastolic

pressure.

Identify the determinantsof pulse pressure and theeffect of age.

Pulse pressure depends on the stroke volumeand the arterialcompliance. Therefore, the magnitude of pulse pressure (Systolic Diastolic pressure) will reflect the magnitude of stroke volume andarterial distensibility. Increases in stroke volume and decreases in

arterial compliance will increasethe pulse pressure.

The decrease in arterialcompliance with age is detectedby increases in pulse pressure.

Higher ppressu


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Identify the pressure

gradient that normallyexists across the aorticvalve during systole andhow aortic stenosis orregurgitation affect itsmagnitude.

Under normal conditions, aortic pressure (AP) follows similar

changes as those observed in the left ventricle (LVP) during systole.In other words, normally, the pressure gradient across the aorticvalve is very small (a few mm Hg);

During severe aortic stenosis (narrowing of the valve opening)the pressure gradient can become quite high (>100 mmHg).Grayarea represents the pressure gradient generated by the aortic valvestenosis.

Aortic valve regurgitation, occurs when the aortic valve leaks backto the ventricle during diastole. Therefore, with this valve lesionthere is no significant pressure gradient across the valve duringsystole. However, aortic pressure will exhibit a large pulse pressuresince the diastolic pressure will decrease as blood returns back tothe ventricle through the leaky valve.

Systolic pressure in the aortawill be more elevatedthannormal because both the

stroke volume and the rate ofejection are increased. Thelowered aortic diastolic

pressureallows ventricularejection to begin earlier thanusual. In addition, bloodejection is more rapid becauseis done against a reduced afterload.


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Distinguish peripheral

from central venouspressure and identify thefactors that determinetheir values.

Peripheral venous pressure(PVP) is the pressure that exists in

large extrathoracic veins of the Systemic Circulation. Normal valuesfall around 5-7 mm Hg.Central venous pressure (CVP) refers to the pressure in theintrathoracic inferior and superior vena cava as they enter the rightatrium. This pressure is in fact the right atrial pressure, which isfound around 0-2 mm Hg.

CVP is influenced by a number of factors, including cardiac output,respiratory activity, contraction of skeletal muscles (particularly legsand abdomen; also referred as the muscle pump), sympatheticvasoconstrictor tone, and hydrostatic forces (i.e., gravity, see

below). All of these factors, however, ultimately affect CVP bychanging either venous blood volume or venous compliance.

PVP is highly dependent on right atrial pressure, since if the latterincreases as a result of ventricular failure or tricuspid valve disease)this will be eventually transmitted backward producing high PVP,capillary transudation and ascites (fluid accumulated in abdominalcavity) and edema (fluid accumulated in the interstitial space).

Identify the meaning and

the factors that affect thevenous return or vascularfunction curve

Venous return(VR) is the flow of blood back to the heart. Under

steady-state conditions, venous return must equal cardiac output(CO) when averaged over time because the cardiovascular system isessentially a closed loop.

The vascular function curvedescribes the inverse relationshipbetween venous pressure (or atrial presssure) and the venous return(or cardiac output). This relationship is a function of

Vascular compliance and blood volumeand

the peripheral resistanceprovided by the arterioles.

The X intercept of the curveis the mean systemic filling pressure(MSFP) or the pressure in the vascular system in the absence ofcardiac pumping. In other words, if the heart beating is stopped, bloodwill continue to flow from the arteries into the veins until the pressure isthe same throughout the cardiovascular system. This pressure in thiscondition is a function of the complianceof the vascular system andthe total blood volume.


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Once the heart starts pumping again, venous pressure decreases,with the magnitude of the decrease being greater, the greater the CO.

There is a limit(a plateau) to how high venous return can go becauseof the fact that the large veins entering the thoracic cavity will collapseat low central venous pressure (or right atrial pressure) (i.e. look atsub-atmospheric pressures in the X-axis).

Increases in blood volume and venoconstriction (decreasedvenous compliance) shifts the curve upwardand to the right.

Decreases in blood volume and venodilatation (increased venouscompliance) shifts the curve downwardand to the left.

Arteriolar constriction will shift the curve downward(changes in

slope) without changing MSFP. Arteriolar dilation (vasodilation) will shift curve upward(changes in

slope) without changing MSFP.

Identify the effect ofrespiration (or thethoraco-abdominal pump)in venous return.

Pressures in the right atrium and thoracic vena cava depend ontheintrapleural pressure (Ppl). During inspiration, the chest wallexpands and the diaphragm descends. This causes Ppl to becomemore negative causing expansion of the lungs, atrial and vena cava.The expansion decreases the pressures within the vessels. As rightatrial pressure falls during inspiration, the pressure gradient forvenous return increases. During expiration the opposite occurs.


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Identify the meaning ofvenous pooling and itseffect on the cardiacoutput and bloodpressure.

Refers to the translocation of blood from the upper parts of the body tothe legs upon standing,In the erect position, about half of the central blood volumein the ches(about 500 ml) gravitates toward the lower extremities. Pooling of bloodin dilated veins reduces the central venous pressure.

The hemodynamic effects of such venous distension resemble those

caused by the hemorrhage of an equivalent volume of blood from thebody. This event may lead to such a reduction in cardiac output (byabout 2 L/min) that arterial pressure falls and susceptible persons mayfaint.

supine erect supine


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Identify the meaning ofthe muscle pump andits effect in venous return.

Is an auxiliary mechanism that increases venous return to the hearThis pump acts by compression of the veins within the skeletamusculature then the muscles contract. The blood is squeezed outoward the heart because the venous valves prevent retrograde flow.

Muscle contraction therefore reinforces the normal flow and reduces thvolume of blood (and the pressure) in the peripheral veins of a standinperson.

The pressure in the veins of the foot, which during quiet standing

corresponds to 90 mm Hg, falls to 20-30 mm Hg in the veins emptied bymuscle contraction. The decrease in venous pressure reduces thcapillary filtration pressure, so that there is less danger of edema.

With varicose veins, thevalves do not functionproperly, allowing blood toremain in the vein. Pooling of

blood in a vein causes it toenlarge.

This process usually occursin the veins of the legs,although it may occurelsewhere.


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Identify the relationship

between Tension andPressure as expressed Inthe Laplaces Law.

Under normal conditions, the vascular vessel diameter i

maintained because of the relationship that exists betweentransmural pressureand tension.

TRANSMURAL PRESSURE is the pressure that distends avessel. Is calculated from the difference between the internapressure (Pi) and the pressure outside the vessel (Po). Since imost parts of the body the external pressure exerted by thesurrounding tissues is notvery high or it is equal tothe atmospheric pressure

(which is referred as zero),the transmural pressurecan be considered to bethe SAME as the internalpressure or a force actingoutward.

TENSIONon the other hand is the force per unit length tangentiato the vessel wall. This force opposes the transmural pressurand tends to pull apart a theoretical longitudinal slit on the vesseThus, it tends to CLOSE the vessel.

As applied to a cylindrical structure, the Laplace equation statethat tension at the wall of a vessel, required to maintain a giveradius is proportional to the product of the transmural pressure anthe radius.

T = PTMr

The Laplace equation is used to explain various physiological anpathological events:

Capillaries (with small radius) withstand highinternal pressures withoutbursting.

Aneurysms (bulging of diseased arteries wall) arspots prone to rupture.

A dilated heart (with a large radius) exerts greatetension.


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STUDY QUESTIONS:

CVS OVERVIEW

1. Which characteristic of the human circulatory system permits the unidirectional

flow of blood?

2. Describe the relationship between the cross-sectional area and blood flow in the

vasculature?

3. Where would you expect a greater velocity of blood flow, at the aorta, or at the

arterioles and why?

4. What physical characteristic of large arteries, such as the aorta, maintains blood

pressure at a high level in between the contraction periods of the heart?

5. Identify the segment of the vasculature where the greater extent of blood volume

is found.

6. What generatesblood flow through the circulatory system?

7. Mention major differences between pulmonary and systemic circulation.

8. Describe (quantitatively) the pressure profile at the systemic circulation.

9. Define vascular compliance.

10. What is the structural basis for the following observation? Veins are more

compliant than arteries.

11. What it is meant by distributingvessels?

12. Why arterioles are the segment of the circulation that offers the greatest

resistanceto blood flow?

13. Identify vascular components which providepost-capillary resistance.

14. Which segment of the circulation will determine the extent of surface area available

for exchangepurposes? Explain.

15. Why blood flow at the exchange areas is not pulsatile as in the great arteries?

16. Differentiate between a paralleland an in seriesarrangement of resistances.

17. Why the parallel arrangement is the most favorable arrangement for the

circulatory network between tissues?


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HEMODYNAMICS

1. What relationship is expressed in the Poiseuilleequation?2. Which is the effect of viscosityand lengthon the magnitude of blood flow?3. Calculate the total resistancein the following circuit:

4. How does the flowthrough a vessel change when:radius doubles

- radius decreases in 50%- radius increases in 25%

5. The mean pressure in the artery supplying a given organ is 100 mm Hg. Thepressure in the vein draining it is 10 mm Hg and blood flow through the organ is 10ml/s. What is the vascular resistanceacross the organ, in PRU units?

6. Express the following pressures in cm of H2O.25 mm Hg ________________

90 cm blood ______________

dblood= 1.05 g/l dH2O = 1 g/l dHg= 13.6 g/l

7. How do you obtain the complianceof a vessel from a pressure-volume graph?

8. Calculate the compliance of a vessel which originally had an internal pressure of 75mm Hg and a volume of 100 ml, but after the addition of 10 ml of extra volume, itspressure increases to 100 mm Hg.

9. Why the radius of a vessel affects strongly the magnitude of the vascularresistance?

10. What is the Reynolds number?

11. How an increase in hematocritaffects the vascular resistance?12. Would an increase in blood flow velocity always produces turbulent flow?13. Where is the zero reference level for pressure measurements for the human

body?


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14. Why venous return curve plateaus when right atrial pressure falls belowatmospheric pressure?

15. Whenis venous return greater, during inspiration or with expiration?

16. How the muscle venous pumphelps to determine venous return to the heart?

17. Differentiate between mean systemic filling pressure and central venouspressure.

18. What events generate a decrease in mean systemic filling pressure?

19. What determines the slopeof the inverse relationship between venous return andatrial pressure?

20. What event might produce a downward rotationof the venous return curve?

ARTERIAL & VENOUS PRESSURES

1. What is the origin and the location of the incisura in the aortic pressure wavecurve?

2. Calculate the pulse pressureand mean arterial pressurefrom the following data:Ps = 200 mm Hg, Pd= 100 mm Hg.

3. How the descending limb of the aortic pressure wave would change if theperipheral resistance decreases?

4. Where would you expect a higher pressure wave velocity, at the femoral artery orat the abdominal aorta, and why?

5. Explain the pressure dropoccurring from arteries to arterioles.

6. Compared to the ascending aorta pressure values, would you expect to detect, ahigher or lower systolic pressure at the iliac artery?

7. What normally prevents venous poolingin the standing position?

8. What structural characteristic permits veins to function as blood reservoirs?

9. Which structural characteristic permits arteries to work as windkessel vessels?

10. Why arterial elasticityis important for circulatory function?


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Cardiovascular Physiology 2008,HEMODYNAMICS ARTERIAL & VENOUS PRESSURES

11. Arrange the following pairs of vessels in ascending order of pressure drop.FROM TOarteriole - muscular venulesuperior cava vein - right atriumaorta - femoral arteryskeletal muscle artery - skeletal muscle arterioles

aorta - cava veinbrachial vein - right atrium

hemodynamic & av pressures 2008

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