Cardiac Circulation

5/19/2018 Cardiac Circulation

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Copyright 2011 by Saunders, an imprint of Elsevier Inc.

Overview of the Circulation; Biophysics

of Pressure, Flow, and Resistance

UNIT IV

Chapter 14:


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Overall Objectives: Chapter 14

Physical characteristics of the circulation:

distribution of blood volumetotal cross sectional area

velocity

blood pressure

Determinants of blood flow

Define and calculate blood flow, resistance, and pressure

Define and calculate conductance

Know Poiseulle

s law


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Systemic circuitRight HeartPulmonary circuit

Pulmonary circuitLeft HeartSystemic circuit

Cardiovascular system consists of two pumps (right/left

ventricles) and two circuits (pulmonary/systemic)connected

in series

when circuits are connected in series, flow must be

equal in two circuits

Cardiac output is output of either left or right ventricle

because of series system they are equal

Cardiac Output is ~5000ml/min (mean ~100ml/sec)

CO=SV*HR= 70ml*70bpm = 4900ml/min = ~82ml/sec

Organization:


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.

Components of

Circulation

Blood flows in

a circuit at rate

of 5 L/min

Transporting nutrients,

hormones to, and wastefrom, the tissues

Systemic Circulation:

84% blood volume

64% venous

13% arteries

7% arterioles/capillaries

Pulmonary Circulation:

16% blood volume


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.

Components of

Circulation

Transports blood to

tissues under high pressure(100mmHg)& high velocity


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.

Components of

Circulation

Control site for BF

major resistancesite of

the circulation

Control conduits

(strong muscular walls)


7/65Copyright 2011 by Saunders, an imprint of Elsevier Inc.

.

Components of

Circulation

Major site water &

solute exchange

between blood & tissues

Containcapillary pores

permeable water/ solutes

~ 0.3-1mm in length

blood is inside for 1-2 sec

for diffusion but

not plasma leakage



.

Components of

Circulation

returns blood to heart

under low pressure

serves asblood reservoir

Largest cross-sectional area



.

Components of

Circulation

site of oxygenand

carbon dioxide exchange

Pulmonary Circulation:16% blood volume

9% lung:

7% heart



Capillaries have the largest total

cross-sectional area of the circulation

cm

Aorta 2.5

Small Arterioles 20

Arterioles 40Capillaries 2500

Venules 250

Small Veins 80

Venae Cavae 8

CSA of capillaries

is 1000 timeshigher than aorta

cross sectional area veins/capillaries > arteries

same volume blood must pass through each segment

of circulation each minute

Systemic circuit is branching circuit: large single vessel (aorta) branching

extensively into smaller vessels until capillaries are reached

M j it bl d



.

Majority blood

volume in veins

Total blood volume

in humans is about

5 Liters

CO &HR two circuits are equal so

SV are sameLargest blood volume in systemic

veins and

2ndlargest in pulmonary system

Systemic veins/Pulmonary

vessels have high compliance

Systemic circuit has

higher resistanceand lower

compliance



Velocity (V) blood flow (Q) is inversely proportional to vascular

cross-sectional area (A) V= Q/A

velocity is inversely related to total cross-sectional area

of all vessels in a particular segment

Velocity is greatest in Aorta(~33cm/sec) and decreases to a

minimum in capillaries (0.3mm/sec) and then increases from

venules to right atrium

low velocity (1-3 sec) capillaries (typical length 0.3-1mm) allows

for nutritional flow(exchange dissolved substances between

plasma and tissues)

Velocities of Blood



Velocity of blood flow is

greatest in the aorta

Velocityof Blood Flow = Blood Flow = QCross sectional area A

Aorta >Arterioles > Small veins

>Capillaries

Velocity of BF : speed blood flows in circulation (mm/sec)



force exerted by blood against any unit vessel wall

force required to push Hg against gravity to cm height

blood flows from high pressure to low pressure therefore

the sequence of vessels in any system will also be a

sequence of pressures (highest to lowest)

Pressures are higher in systemic circuit (peripheral circuit)

lower pressures mean work of RV is lower lower capillary pressure protects against development of

pulmonary edema

Pressures



small pressure drop in major arteries (low resistance segment)

small pressure drop occurs in major veins (low-resistancesegment)

largest drop in pressure is across arterioles

(highest resistance segment)

local arteriolar dilation:

arteriolar resistance

flow and pressure downstream

local arteriolar constriction:

arteriolar resistanceflow and pressure downstream

Pressure dissipates proportional to

Resistance in that segment.



Blood Pressure Profilein the Circulatory System

Systemic Pulmonary

Capil

laries

Pre

ssure

(m

mHg)

0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

Venules

Smallviens

Large

viens

Pulmonaryarteries

Capillaries

Pulmonaryviens

High pressures in arterial tree

Low pressures in the venous side of circulation

Largest pressure drop across arteriolar-capillary junction



Blood Pressure

Blood pressure: force exerted against unit area vessel wall

measured in millimeters of mercury (mmHg).pressure of 100 mmHg means force sufficient to push a column of

mercury 100mm high

low pressures are sometimes reported in units of cm of water.

Blood Pressure Monitoring:

discovered by Poiseuille in 1846 (~167 years ago)

Standard Measure: mmHg (occasionally water)

1mmHg = 1.36cm water

Mercury Manometer:

inertia Hg means cannot respond pressures > 1cycle/2-3 sec

Electronic Pressure Transducers: (invasive) record < 500cycles/sec



Pressure is pulsatile 120/80 mmHg

mean ~100mmHg actually ~93mmHg @ Aorta gradually decreasing to 0mmHg

at Right Atrium)

highest resistant segments are Arterioles

lower resistant segments are major arteries & veins

Aortic pressure mean ~100 mmHg

(Systolic ~120mmHg Diastolic ~80mmHg)

Vena cavae ~ 0 mmHg

Capillaries ~ 35 mmHg (arteriolar ends)

~ 15 mmHg (venous ends)

Capillary pressures mean ~ 17 mmHg (varies in tissues)

Mean Systemic Blood Pressure



Pulmonary Pressure is pulsatile 25/8 mmHg

Pulmonary artery mean ~ 16mmHg (versus 100mmHg systemic)Systolic ~ 25mmHgDiastolic ~ 8mmHg

Pulmonary capillary mean ~7mmHg (versus 17mmHg systemic)

Mean Pulmonary Pressure



Blood Flow Rate (Q) is controlled to tissue needs

tissue micro-vessels monitor O, waste and act directly onlocal blood vessels (dilating/constricting) to precisely control

local BF nervous control

hormone control

Cardiac Output is controlled primarily by sum of all tissue

blood flows

heart acts as automaton responding to venous return

Arterial blood pressure is controlled independently of either

local BF control orCardiac Output control

Basic Principles:



Variations in Tissue Blood Flow

Brain 14 700 50Heart 4 200 70Bronchi 2 100 25Kidneys 22 1100 360Liver 27 1350 95

Portal (21) (1050)Arterial (6) (300)

Muscle (inactive state) 15 750 4Bone 5 250 3Skin (cool weather) 6 300 3Thyroid gland 1 50 160Adrenal glands 0.5 25 300Other tissues 3.5 175 1.3

Total 100.0 5000 ---

Per cent ml/min

ml/min/

100 gm



Blood flow is the amount blood passing a given point in

circulation in a given period of time (ml/min orL/min) adult person at rest total circulation ~5000ml/min

Flowmeters: Direct measurements of flow (ml/min)

non-invasive (electromagnetic) invasive (intravascular-ultrasonic Doppler)

Total blood flow through lungs (pulmonary blood flow)per

minute is equal to systemic blood flow (normal blood volume

is ~5 L) per minute or Cardiac Output

Blood Flow is directly proportional to Pressure Differences

and inversely proportional to Resistance (Q=P/R)

BLOOD FLOW

Relationship between Pressure



Relationship between Pressure,Flow, and Resistance

Q= P/R

Flow (Q) through a blood vessel is determined by:

1) Pressure difference (

P or P1-P2) between two ends ofvessel

2) Resistance (R) of vessel

Pressure at

arterial end

Pressure at

venous end

P1 P2

Ohms Law: Flow = P1P2/R = P/R = Q



determines blood flow (Q) via vascular resistance

Flow (Q) = Pressure difference = P

Resistance R

Resistance (R) = Pressure difference = P

Flow Q

Pressure Difference (P) = Flow * Resistance = Q * R

Ohms Law:

R l ti hi b t


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Relationship betweenPressure, Flow, and Resistance

Q= P/R

Flow (Q) through a blood vessel is determined by:1) pressure difference (P or P1-P2) between two ends

of vessel

2) Resistance (R) of vessel

Pressure at

arterial end

Pressure at

venous end

P1 P2


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Characteristics of Blood Flow

blood usually flows in streamlines with each layer of blood

remaining same distance from wall, this type of flow iscalled laminar flow

when laminar flow occurs, velocity of blood in center of vessel is

greater than that toward outer edge creating a parabolic profile

(central portion/layer is faster) essentially noiseless

Blood Vessel

Laminar flow

occurs throughout normal CV system (excluding flow in heart) in small vessels, essentially all blood is near to wall soparabolic velocity of

laminar flow does not occur


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Laminar vs Turbulent Blood Flow

Turbulent flowdisorderly rather than

streamlined

Increases Resistance

Causes of turbulent BF:

high velocitiessharp turns in the circulation

rough surfaces in the circulation

rapid narrowing of blood vessels

Laminar flow is silent, whereas Turbulent flow tend to cause murmurs

murmursor bruitsare important in diagnosing vessel stenosis,

vessel shunts, and cardiac valvular lesions (heart murmur)

Reynolds Number:

Tendency for turbulent flow>2000 = turbulent flow


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Tendency for turbulent flow (Reynolds Number)

increases directly proportional to: Velocity of blood (V)

Diameter of blood vessel (D)

Density of blood ()

Inversely proportional to Viscosity of blood ()

Re = Velocity (cm/sec) * Diameter (cm) * Density (normally~1)

Viscosity (normally 1/30 poise)

Re = V * D * / increases with high velocities and pulsations

increases Resistance

plays a role in murmurs and bruits

Reynolds Number


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Aortic Root andPulmonary Artery,

Reynolds Number is high (5000-12,000 in systole) due tohigh velocity, pulsatile nature and sudden changes in

vessel diameter or large vessel diameter

in small vessels Reynolds Number is low (little turbulence)

Reynolds number in circulation 200-400 means turbulent

flow will occur at some branches of vessels but will die out

along smooth portions of vessels

Re > 2000 suggests increased flow turbulence even in

straight smooth vessels

Reynold

s Number : Clinical Note


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Aortic Aneurysm Atherosclerosis

Effect of Wall Stress on Blood Vessels

Turbulent flow increases wall stress

Athersclerosis: reduces Re at which turbulence begins to develop.

Thrombi:more likely to develop in turbulent flow than in laminar flow.

Relationship between


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Q= P/R

Flow(Q) through a blood vessel is determined by:

1) pressure difference (P or P1-P2)between two ends

of vessel

2) Resistance (R) of the vessel

Pressure at

arterial endPressure at

venous end

P1 P2



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Flow = upstream pressure minus downstream pressure

(pressure gradient) divided by Resistance (mmHg/ml/min)Q = P1 - P2

R

critical value that determines flow is the pressure gradient

P1 accompanied by an identical P2will not cause achange in flow

P2 (caused by stenosis)pressure gradientBF

Flow to organ (kidney) calculated as mean renal arterial

pressure minus renal venous pressure divided by resistance

of all vessels in renal circuit


Pressure, Flow, and Resistance continued


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Determinants of Blood Flow

FLOW = arterial - venous pressure (P) = Qresistance (R)

FLOW = 100 - 0 mmHg

.1 mmHg/ml/minFLOW = 100 - 20 mmHg

.1 mmHg/ml/minFLOW = 1000 ml/min FLOW = 800 ml/min

100 mmHg

0 mmHg 20 mmHg

100 mmHg

R = .1mmHg/ml/min R = .1mmHg/ml/min

A B



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Q=

P/R

Flow (Q) through a blood vessel is determined by:

1) pressure difference (P or P1-P2) between two ends ofvessel

2) Resistance (R) of the vessel

Pressure at

arterial endPressure at

venous end

P1 P2


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impediment to blood flow in vessel (mmHg/ml/min)

Calculated (not measured) from Blood Flow &BP difference if P between two points is 1mmHg & BF is 1ml/sec then

resistance =1 peripheral resistance unit(PRU)(mmHg/ml/sec)occasionally a CGS (cm, gm, sec) unit is used to express resistance

(dyne sec/cm)

Systemic Resistance Total Peripheral Resistance TPR

ArterialVenous Pressure (100mmHg-0mmHg) = 1 PRU

Cardiac Output (100ml/sec)BF through circulatory system equal to CO (~100ml/sec)

pressure systemic arteries to veins (~100mmHg)

total peripheral resistance 100/100 = 1 PRU

RESISTANCE

Total Peripheral Resistance TPR


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in conditions where blood vessels throughout body are

strongly constricted PRU ~ 4 if greatly dilated PRU~ 0.2

Most important factors to determine Resistance are:

Radius of the vessel blood Viscosity

Pulmonary Circulation Resistance

PRU = mean pulmonary pressure (16-2 mmHg) = 0.14

Cardiac Output (100ml/sec)

Total Peripheral Resistance TPR

(Systemic Circulation)


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Resistance = P1 -P2 Units of Resistance = mmHg = pressure

Q mL/min volume/timeResistance of vessel is determined by three major variables: R L/r4

1.Vessel Radius (r):

most important determining resistance is radius of vesselResistance of vessel is inversely proportional to radius4 rate of BFradius4

if radius is by resistance 16X

if radius doublesresistance to 1/16thof original2/3 of Total Peripheral Resistance is Arteriolar Resistance in

small arterioles (small changes in vessel diameter due to nervoussignals or local tissue chemical signals cause significant change in blood

pressure)

Determinants of Resistance

D t i t f R i t


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2. Blood Viscosity (): R L/r4

property of fluid that measures fluids internal resistance to flow greater viscosity, greater resistance

prime determinant of blood viscosity is hematocrit

percentage whole blood is RBCs (normally ~38 women & ~42men)

standard unit is poise(blood is ~1/30 poise)

varies with temperature(important in hypothermia)

varies with anemia, physical activity, altitude Viscosity of blood increases as Hematocrit increases

elevated in Polycythemia: increased blood viscosity > 10

normal blood viscosity is equal to three times water viscosity (primarildue to RBCs exerting frictional drag against adjacent cells and wall of blood vessel)

Normal whole blood viscosity = 3

Determinants of ResistanceContinued

Hematocrit and Viscosity Q = Pr4


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Hematocrit and Viscosityeffects on Blood Flow

Hematocrit Vascular ResistanceBlood Viscosity

Q = Pr8l

Determinants of Resistance


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3. Vessel Length: R L/r4

greater the length, greater resistance if length doubles resistance doubles

if length decreases by resistance decreases by

vessel length is usually constant

(not physiologic factor in regulation of resistance, pressure or flow)

Important conclusion: determination of capillary pressure is

Resistance of arterioles

arteriolar resistance (dilation)capillary pressure

arteriolar resistance (constriction)capillary pressure

Determinants of ResistanceContinued

Parallel and Serial Resistance Sites


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in the Circulation

Serial

Resistance

flow equal at all

points in a seriessystem

if flow changes, it

changes equally atall points

total is always

greater than

individualresistances

adding resistor in

seriesresistance

Series TPR =

sum resistance

each vessel



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in the Circulation continued

Parallel arrangementallows independent

control of flow

between tissues

Parallel TPR =

sum of inverseresistance each

vessel

Total resistance

is less than

resistance of

any single

blood vessel

2. Parallel TPR = sum of the inverse of resistance


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1/RTotal= 1/R1 + 1/R2 + 1/R3 + 1/R4

Total resistance < resistance of any single blood vessel connecting parallel resistors: low-resistance system parallel blood vessels: easier for blood to flow through circuit

because each is another parallel pathway (conductance)

if BP is kept constant, altering resistance (therefore BF) in oneparallel circuit will not change flow in remaining parallel

circuits flows can be independently regulated by changing resistance

Parallel vessels are found in: (greatest to least resistance) coronary,cerebral, renal, pulmonary

2. Parallel TPR sum of the inverse of resistance

of each vessel

Therefore: amputation of limb/surgical removal of kidney removes a parallel circuit and

reduces total vascular conductance (increasing total peripheral vascular resistance)

Obesity(adds parallel circuits) which decreases TPR and increases CO to maintain BP

Conductance


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Conductance

Conductance: measure blood flow through a vessel for a

given pressure difference units: ml/min per mmHg

increases proportion to radius to 4thpower

(Poiseuilles Law)

Conductance = 1____

Resistance

Poiseuilles Lawrepresents relationship of flow, pressure and resistance

Flow = pi * pressure difference * radius to 4thpower = P r4

8 * viscosity * length of vessel 8l

Effect of Vessel Diameter


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on Blood Flow

Q = Pr4

8l

Conductanceis sensitive to change in diameter of vessel

increases proportional tofourth power of radius

Blood Flow Autoregulation


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ability of tissue to maintain normal BF during changes in

arterial BP (70-175mmHg) calledblood flow auto-regulation otherwise increases arterial BP causes proportional increases BF

through various tissues

arterial BP increase force blood through vessels andinitiates compensatory increases in vascular resistance arterial BP BF Vascular Resistance

Arterial BP Vascular Resistance

tissues such as kidneyare able to maintain BF independent

of systemic arterial BP 70-175 mmHg Autoregulation is also seen in mesentery, skeletal muscle, brain,

liver and myocardium

Blood Flow Autoregulation

Mechanisms of blood flow


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Nervous:

Sympatheticvasoconstriction resistance

Sympatheticvasoconstriction resistance

(dilation)

Hormonal:

NE, Angiotensin II, Vasopressin, Endothelin

vasoconstrictionresistance

Mechanisms of blood flow

autoregulation include:


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No autoregulation

changes in arterial BP have important effects on BFart BPforce BF distending elastic vessels

(smooth muscle stress- relaxation)vascular resistance

BP BF dilates Vessels Resistance BP

art BPresistance as elastic vessels (smooth muscle stress-

relaxation) gradually collapse due to loss of distending ability

(critical closing pressure)

Sympathetics: can alter passive pressure-flow

sympathetic (stimulate): vasoconstrictionBF

sympathetic (inhibition): vasodilationBF

Passive Vascular Beds:


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CV system is two circuits & two pumps connected in series

Systemic pressure decreases slightly through arteries,

decreases markedly through arterioles, and then

decreases only slightly more through the major veins

loss of pressure is determined by regional resistance

cross-sectional area increases from minimum in aorta to a

maximum in capillaries.

Velocity blood is inversely related to regions cross-

sectional area

Summary


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main blood reservoir is systemic veins

of factors affecting a vessels resistance, radiusis most

important

radius arterioles determines total peripheral resistance

mean arterial BPis determined only by circulating blood

volume (cardiac output CO) and resistance(R) in arterioles

CV system is a laminar flow system

factors promoting turbulence include decreased fluid

viscosity, large-diameter tubes, increased fluid velocity and

vessel branching


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structures connected in series produce high resistance,and flow is dependent and equal at all points

systemic organs are connected in parallel, which permits

independent regulation of flow


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1. A 50-year-old woman has a renal blood flow of 1,000

mL/min and hematocrit of 50. Her arterial pressure is 120

mmHg and her renal venous pressure is 20 mmHg. She also

has a plasma colloid osmotic pressure of 25 mmHg.

Which of the following is the total renal vascular resistance (in

mmHg/mL/min) in this woman?

A. 0.10

B. 0.20C. 0.50

D. 1.00

E. 1.50


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1. A 50-year-old woman has a renal blood flow of 1,000

mL/min and hematocrit of 50. Her arterial pressure is 120

mmHg and her renal venous pressure is 20 mmHg. She also

has a plasma colloid osmotic pressure of 25 mmHg.

Which of the following is the total renal vascular resistance (in

mmHg/mL/min) in this woman?

A. 0.10

B. 0.20C. 0.50

D. 1.00

E. 1.50

Q= P/R

R= P/Q = 120-20 mmHg/1000mL/min= 0.10 mmHg/mL/min


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2. Under control conditions, flow through a blood vessel is 10

mL/min under a pressure gradient of 100 mmHg. An increasein which of the following would occur in response to a

twofold increase in the diameter of the vessel and the

pressure gradient was maintained at 100 mmHg?

A. Vascular resistance

B. Hematocrit

C. Vascular conductanceD. Blood viscosity

E. Plasma oncotic pressure


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2. Under control conditions, flow through a blood vessel is 10 mL/min

under a pressure gradient of 100 mmHg. An increase in which of thefollowing would occur in response to a twofold increase in the diameter

of the vessel and the pressure gradient was maintained at 100 mmHg?

A. Vascular resistance

B. Hematocrit

C. Vascular conductance

D. Blood viscosity

E. Plasma oncotic pressure

Conductanceis sensitive to

change in diameter of vessel

Increases proportional tofourth

power of radius

Q = Pr4

8l


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3. A decrease in the diameter of an arteriole would most

likely result in which of the following sets of changes inthe microcirculation?

Conductance Blood Flow Resistance

A. B. C. D. E. F.


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3. A decrease in the diameter of an arteriole would most

likely result in which of the following sets of changes in themicrocirculation?

Conductance Blood Flow Resistance

A.

B. C. D. E.

Q=P/R R=P/Q

Conductance = 1/R


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Vessel Blood Flow

(mL/min)

Pressure Gradient

(mmHg)

A. 1,000 100

B. 1,200 60

C. 1,400 20

D. 1,600 80

E. 1,800 40

4. Which vessel has the highest vascular resistance?


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Vessel Blood Flow

(mL/min)

Pressure Gradient

(mmHg)

A. 1,000 100

B. 1,200 60

C. 1,400 20

D. 1,600 80

E. 1,800 40

4. Which vessel has the highest vascular resistance?

0.1

0.05

0.014

0.05

0.02

R=P/Q


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5. Which one the following would tend to cause an increase

in blood flow?

A. An increase in hematocrit

B. A decrease in arterial pressure

C. A twofold increase in arteriole diameter

D. A twofold increase in blood viscosity


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5. Which one the following would tend to cause an increase

in blood flow?

A. An increase in hematocrit

B. A decrease in arterial pressure

C. A twofold increase in arteriole diameter

D. A twofold increase in blood viscosity

Poiseuille

Q = Pr4

8l


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Vessel Pressure

Gradient

Radius Viscosity

A. 100 1 10

B. 50 2 5

C. 25 4 2D. 10 6 1

6. The table below depicts the pressure gradient, radius,

and viscosity in various vessels of the same length. Which

vessel has the greatest flow?


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Vessel Pressure

Gradient

Radius Viscosity

A. 100 1 10

B. 50 2 5

C. 25 4 2

D. 10 6 1

6. The table below depicts the pressure gradient, radius,

and viscosity in various vessels of the same length.

Which vessel has the greatest flow?

Poiseuille

Q= P r/8 l = pi * P1-P2 * radius/ 8*viscosity*length

= *10*6/8*1*1


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7. If the blood flow through the kidney is 1,200 mL/min and

the renal artery pressure is 100 mmHg and the renal venouspressure is 0 mmHg, which of the following is the

conductance (in mL/min/mmHg) of the renal vessel?

A. 2

B. 4

C. 8

D. 12

E. 16


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7. If the blood flow through the kidney is 1,200 mL/min and

the renal artery pressure is 100 mmHg and renal venouspressure is 0 mmHg, which of the following is the

conductance (in mL/min/mmHg) of the renal vessel?

A. 2

B. 4

C. 8

D. 12

E. 16

Q=P/RR=P/Q

=100-0/1200

=1/12

Conductance=1/R

=1/1/12

Cardiac Circulation

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