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CVS > 4. Blood Vessels and Blood Flow > Flashcards

4. Blood Vessels and Blood Flow Flashcards

1
Q

Put the following in order in circulation: resistance, reservoir, elastic arteries, exchange

A 
(pump)
1) Elastic arteries
2) Resistance
3) Exchange
4) Reservoir
(pump)
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2
Q

Outline the structure of the systemic circulation

A
  • Blood leaves heart through large, thick-walled, elastic arteries (dampening vessels)
  • Smaller arteries and arterioles with extensive smooth muscle (regulates diameter/resistance)
  • Capillaries make up the largest cross-sectional area - exchange function
  • Veins are stretchy and compliant - reservoir for blood volume (highest blood volume out of all vessles)
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3
Q

What happens to the veins during exercise?

A
  • Venoconstriction
  • Decreased stored blood
  • More blood moves back to the heart
  • i.e. more venous return and more cardiac output
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4
Q

How is the fluid circuit similar to an electrical circuit?

A

• Electrical: V=IR
current x resistance
• Fluid: ΔP = QR
or Pressure difference = volumetric flow x resistance
(or mean arterial blood pressure = cardiac output x resistance)

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5
Q

What does resistance (peripheral vascular resistance) assume in ΔP = QR?

A
  • Steady flow
  • Rigid vessels
  • Negligible right atrial pressure
  • Therefore it is an approximation
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6
Q

How is blood flow regulated with reference to resistance and pressure?

A
  • Regulation of flow achieved by variation in resistance
  • Constant blood pressure using feedback mechanisms
  • Blood directed by specific contraction and relaxation of blood vessels
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7
Q

How does pressure change throughout circulation?

A
  • High pressure in large arteries, and falls across the circuit due to viscous (frictional) pressure losses
  • Small arteries and arterioles - most resistance
  • Pressure slightly rises in the right side of the heart
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8
Q

What 3 variables does the resistance of a tube to flow depend on?

A

1) Fluid viscosity (η) - not fixed but in most physiological conditions is constant
2) Length of tube (L) - fixed
3) Inner radius (r) - variable, main determinant of resistance

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9
Q

What is Poiseuille’s equation?

A

R = (8Lη) / (πr^(4))
• Emphasises the importance of arterial diameter as a determinant of resistance
• Small changes in vascular tine (vasoconstriction/dilation) can produce marked changes in flow
• 1/2 radius decreases the flow 16x

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10
Q

How does blood flow to the heart and brain change when exercising?

A

It doesn’t

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11
Q

What is laminar flow?

A
  • Blood flowing in stream lines
  • No interference
  • No sound
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12
Q

Why do the sounds of korotkoff disappear after further dropping of the cuff pressure?

A
  • Vessel is no longer occluded

* Blood starts to flow in a laminar fashion

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13
Q

What is turbulent flow and what can it result in?

A
  • Whirlpool like regions
  • No constant velocity
  • Can bring about pathophysiological changes
  • Can change the shear stress on the vessels - damage the lining - atheroma
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14
Q

What is the shear rate?

A
  • The velocity gradient that is established
  • Difference between the highest velocity of blood in the middle of the lumen and the lowest velocity blood that adheres to the walls
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15
Q

What is shear stress?

A
  • Sheer rate x viscosity
  • Disturbs endothelial function - important for laminar flow and production of various transmitter substances which gives rise to vessel dilation and constriction
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16
Q

What happens when shear stress is high or low?

A
  • High/laminar - promotes endothelial cell survival, quiescence, cell alignment and secretion of substances that promote vasodilation and anticoagulation
  • Low, (disturbed or changed/turbulent) - promotes endothelial proliferation - bearing on vasoconstriction, coagulation, platelet aggregation and atheroma formation
17
Q

How do you take blood pressure and explain the steps?

A

1) Cuff around the upper arm, and increase the pressure until cuff pressure > arterial pressure
2) Place stethoscope distal to the cuff - initially no sound as blood flow is occluded
3) Arterial pressure (just) > cuff pressure - blood starts to squirt through the occlusion - turbulent flow - light tapping sound (sounds of korotkoff) - SBP
4) Continue to reduce pressure - no occlusion - laminar flow - no sound - DBP

(difference between SBP and DBP = pulse pressure)
(mean blood pressure = DBP + 1/3 pulse pressure)

18
Q

Why do ventricular and aortic pressure differ?

A
  • Aortic valve closes - ventricular pressure falls rapidly, aortic pressure falls slowly (diastole)
  • Explained by the elasticity of the aorta - buffers changes in pressure - doesn’t drop to zero like the ventricular pressure
19
Q

How does the elasticity of the aorta play a part in causing the dichrotic notch?

A
  • Blood enters the aorta faster than it leaves
  • About 40% of the stroke volume is stored by the elastic arteries
  • Aortic valve closes - ejection of blood stops but recoil due to elasticity produces dichrotic notch
20
Q

What is the Windkessel effect?

A
  • The dampening effect of the aorta
  • Elastic - buffer
  • Blood flow becomes smoother and constant
21
Q

What happens to the Windkessel effect and pulse pressure when arterial compliance decreases (with age)?

A
  • Damping effect of the Windkessel is reduced

* Pulse pressure increases

22
Q

What is transmural pressure?

A

Pressure inside the vessle (difference in pressure between 2 sides of the wall)

23
Q

How can Transmural pressure be described by Laplace’s relationship?

A
  • Transmural pressure causes a tension force (T) in the wall, so
  • T = P x r
24
Q

How do you calculate circumferential stress?

A
  • circumferential stress = tension force / wall thickness
  • σ = T / h
  • σ = (P x r) / h
25
Q

What does a maintained high circumferential stress cause?

A

Vessel distension (dilation)

26
Q

What is the relationship between the transmural pressure vessel volume called?

A
  • Compliance

* Dependent on vessel elasticity

27
Q

How do aneurysms form?

A
  • Vessel walls can leak over a prolonged period of time
  • Balloon like distension forms
  • Result of Laplace’s Law
  • If they form, for the same internal pressure, the inward force exerted by the muscular wall must increase
  • If the muscle fibre is weakened and compliance isn’t great, force cannot be produced - aneurysm expands further
  • Laplace’s Law also applies to diverticuli formation in the gut wall
28
Q

What are the compliance properties of the vein?

A
  • Small changes in venous pressure distends veins and increases the volume
  • For the same pressure, veins can therefore hold a much larger volume of blood (than arteries)
  • Nervous supply => smooth muscle => contraction => decrease venous volume => increase venous pressure
29
Q

What is postural hypotension?

A
  • Standing up quickly
  • Gravity makes blood pool in the legs - venous volume/capacitance
  • Reduced venous return to the heart
  • Cardiac output falls
  • Less blood to brain

• Venous constriction increase blood return and cardiac output

30
Q

Why don’t we faint when standing and what happens when these mechanism fail?

A
  • Activation of sympathetic nervous system
  • Stiffens and constricts veins
  • Arteries constrict - increase total peripheral resistance and maintain BP
  • Slight increase in heart rate and an increase in the force of contraction - more blood return to the brain
  • Failure => fainting (syncope) or hypovolaemia (decreased blood volume, salt depletion - thirst)
31
Q

What is the skeletal pump?

A
  • The contraction of the muscles squeezing blood back through the veins to the heart
  • Decreases venous capacitance
  • Helps with standing for long periods of time without fainting
32
Q

What is the respiratory pump?

A
  • Breathing in => chest expands & intrathoracic pressure decreases
  • Blood allowed back into the right atrium
  • Increased venous return
  • Helps with standing for long periods of time without fainting
33
Q

What happens to people with incompetent venous valves?

A
  • Could lead to varicose veins
  • Often seen in older people
  • Standing for a long period of time - prolonged elevation of venous pressure - oedema in feet

CVS (21 decks)

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