AHS CVS Lecture 5.pdf

THE CARDIOVASCULAR SYSTEM LECTURE 5:

HAEMODYNAMICS

Eamonn O’Connor Allied Health Science

Lecture Outline 1

Blood flow   Pressure gradients   Resistance   Central venous pressure  

AHS Physiology - Cardiovascular System 11-12

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Path of Blood Flow Through the CVS 2

 

Remember:   CVS

= closed system   Flow through systemic and pulmonary circuits are in series

ventricle  aorta  systemic circuit  vena cavae  right atrium  right ventricle  pulmonary artery pulmonary circuit  pulmonary veins  left atrium  left ventricle

  Left


Figure 13.2

Parallel Flow in the CVS 3

 

Remember:   Flow

within systemic (and pulmonary) circuits is in parallel   Parallel flow allows independent regulation of blood flow to organs


Figure 13.3

2

Physical Laws Governing Blood Flow 4

Pressure Gradients in the Cardiovascular System   Resistance in the Cardiovascular System  


Blood Flow 5

Circulatory system = closed system   Pressure = force exerted by blood  

  Flow  

occurs from high pressure to low pressure

Heart creates pressure gradient for bulk flow of blood   A

gradient must exist throughout circulatory system to maintain blood flow

Flow = ΔP/R = pressure gradient/resistance AHS Physiology - Cardiovascular System 11-12

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Pressure gradient 6

 

Pressures throughout the vasculature are not constant

Figure 14.3 AHS Physiology - Cardiovascular System 11-12

Pressure Gradient Across Pulmonary Circuit 7

 

Pulmonary circuit:   Blood

vessels between the lungs & heart

Pressure gradient = pressure in pulmonary arteries minus pressure in pulmonary veins   Pulmonary arterial pressure = 15 mm Hg   Pulmonary venous pressure = 0 mm Hg   Pressure gradient = 15 – 0 = 15 mm Hg  


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Pressure Gradient Across Systemic Circuit 8

 

Pressure gradient   pressure

in aorta minus pressure in vena cava just before it empties into right atrium

 

Pressure in aorta = mean arterial pressure (MAP)   MAP

 

Pressure in venae cavae = central venous pressure   CVP

 

= 85 mm Hg = 0 mm Hg

Pressure gradient = MAP – CVP = 85 – 0 = 85 mmHg   So,

Pressure gradient = MAP AHS Physiology - Cardiovascular System 11-12

Pressure Gradients in Systemic Circuit 9


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Pressure Gradients in Systemic Circuit 10

 

MAP   Not

 

just average of systolic and diastolic pressure

Cardiac cycle = 0.8sec (72beats.min-1)   Systolic

= 0.3sec (approx. 1/3 of cardiac cycle)   Diastolic = 0.5sec (approx. 2/3 of cardiac cycle)  

MAP = (systolic/3) + (diastolic*2/3)   MAP

= (110/3) + (70*2/3)   MAP = 36.67 + 46.67 = 83.3mmHg (approx 85mmHg)  

MAP can also be calculated as:   MAP

= systolic + (pulse pressure/3)


Effect of Resistance on Flow 11

 

The lower the resistance, the greater the flow


6

Poiseuille’s Law 12

 

Poiseuille’s law   V

 

Already stated, Flow = ΔP/R   R

 

= (ΔPπr4/8*viscosity*length) = 8*length*viscosity/πr4

Factors affecting resistance to flow   Length

of vessel   Viscosity of fluid   Radius of vessel   Arterioles

 

(and small arteries) - can regulate radius

RADIUS IS THE MOST IMPORTANT FACTOR AHS Physiology - Cardiovascular System 11-12

Regulate Blood Flow by Regulating Radius 13

 

Regulation of radius of arterioles (and small arteries)   Vasoconstriction

radius (by contracting smooth muscle)  increase resistance  decrease blood flow

  decrease

  Vasodilation   increase

radius (by relaxing smooth muscle) decrease resistance increase blood flow


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Changes in Arteriole Radius 14

 

 

 

   

Radius dependent on contraction state of smooth muscle in arteriole wall At rest: arteriolar tone (partially contracted) Vasoconstriction: increased contraction (decreased radius) Vasodilation: decreased contraction (increased radius) FUNCTIONS:   Controlling   Regulating

BF to individual capillary beds MAP (next lecture)


Figure 14.10

Distribution of Blood Flow to Organs 15

 

Intrinsic Control   Regulation

of blood flow to organs based on need (eg to skeletal muscles during exercise)   Regulated by varying radius (and therefore resistance)  

Organ blood flow = MAP / organ resistance   i.e.

driving force for blood flow resistance to flow in that organ

 

Flow = ΔP/R   Flow

= (MAP (85) – CVP(0))/R   Flow = MAP/R AHS Physiology - Cardiovascular System 11-12

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16

Effects of Pressure Gradients and Resistance on Blood Flow to Organs

Flow varies due to differences in resistance

Blood flow changes when resistance changes

Figure 14.11b&c AHS Physiology - Cardiovascular System 11-12

Extrinsic Factors Influencing Vascular Tone 17

 

Extrinsic factors   Autonomic

nerves (Sympathetic NS)

  Constriction

  Hormones

(e.g. epinephrine induces vasoconstriction


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Intrinsic Factors Affecting Vascular Tone 18

 

Intrinsic Factors   Metabolism   Increased  

  Changes

in blood flow

  Reduced  

blood flow causes dilation

“Reactive Hyperaemia”

  Stretch   High  

metabolism decreases O2, causing dilation

“active hyperaemia”

of arteriolar smooth muscle

perfusion pressure leads to contraction

“Myogenic response”

  Locally

secreted chemical messengers

  Vasodilators:

NO, prostacyclin, adenosine Endothelin-1

  Vasoconstrictors:


Central Venous Pressure (CVP) 19

 

 

CVP: pressure in the large veins of the thoracic cavity that lead into the heart Pressure gradient between central veins and atria drives blood back to heart   Venous

 

 

pressure – atrial pressure = 5 - 10 mm Hg

A decrease in venous pressure decreases driving force for venous return Decrease in venous return  decreases end- diastolic volume  decreases stroke volume  decreases cardiac output  decreases blood flow to organs


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Factors Influencing CVP & Venous Return 20

   

Skeletal muscle pump Respiratory pump   Inspiration:

thoracic pressure down - abdominal pressure raises -- increases blood flow to the heart   Expiration: thoracic pressure raises - abdominal pressure falls. Valves prevent backward flow, so, blood driven towards the heart.  

 

Blood volume: decreased blood volume decreases CVP (bleeding, dehydration…) Venomotor tone (sympathetic nerves constrict veins) favors venous return AHS Physiology - Cardiovascular System 11-12

Factors Affecting CVP (and MAP) 21

Figure 14.23


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