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Cardiovascular System > Week 5 - Flow Through Tubes > Flashcards

Week 5 - Flow Through Tubes Flashcards

0
Q

What drives flow through blood vessels?

A

A gradient of pressure

  • Proportional to the pressure difference between the ends of a vessel
  • So the higher the pressure difference, the greater the flow
  • Determined by the resistance of the vessel
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1
Q

Define flow

A

The volume of fluid passing a given point per unit time

- Flow = mean velocity x cross-sectional area

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2
Q

What determines the resistance of a vessel?

A
  • The nature of the fluid

- The vessel itself

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3
Q

Define velocity

A

The rate of movement of fluid particles along the tube

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4
Q

What is the relationship between flow and velocity?

A
  • Flow must be the same at all points along a vessel
  • Velocity can vary along the length if the radius of the tube changes
  • At a given flow, velocity is inversely proportional to cross-sectional area
  • So vessels with a small cross-sectional area have a high velocity
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5
Q

Describe laminar flow

A
  • There is a gradient of velocity from the middle to the edge of the vessel
  • Velocity is highest in the centre
  • Fluid is stationary at the edge
  • The fluid moves in concentric layers
  • The middle layers move faster than the edge layers, so the fluid layers must slide over each other
  • In most blood vessels, the flow is laminar
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6
Q

Describe turbulent flow

A
  • As the mean velocity increases, flow becomes turbulent (e.g, if a vessel nips narrowed)
  • The velocity gradient breaks down as layers of fluid try to move over each other faster than physics will allow
  • The fluid tumbles over, greatly increasing flow resistance
  • It generates sound
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7
Q

Define viscosity

A

The extent to which fluid layers resist sliding over each other

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8
Q

What is the relationship between viscosity and velocity?

A
  • The higher the viscosity the slower the central layers will flow
  • So the lower the average velocity
  • Viscosity determines the slope of the gradient of velocity
  • At a constant gradient, the wider the tube the faster the middle layers move, so the mean velocity is proportional to the cross-sectional area of the tube
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9
Q

What is the relationship between mean velocity and viscosity?

A

Mean velocity is inversely proportional to viscosity

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10
Q

What is the relationship between mean velocity and cross-sectional area?

A

Mean velocity is directly proportional to cross-sectional area

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11
Q

What is flow resistance?

A

Pressure = flow x resistance

  • Resistances increases as viscosity increases
  • It decreases with the 4th power of the radius (velocity increases as the square of radius, flow = velocity x surface area (increases as radius squared) so flow = r^2 x r^2)
  • Hence a very small change in the tube’s radius makes a very big change to resistance
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12
Q

Describe the relationship between pressure, resistance and flow

A

If flow is fixed
- The higher the resistance, the greater the pressure change from 1 end of the vessel to the other
If pressure is fixed
- The higher the resistance, the lower the flow

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13
Q

What is the effect of combining flow resistance in series?

A

Resistances add together

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14
Q

What is the effect of combining flow resistance in parallel?

A

The effective resistance is lower

  • As there is more than 1 path for the current to flow down
  • For 2 vessels in parallel: the resistance of 1 of the vessels is half of the original, as the blood has 2 parts
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15
Q

How does the flow differ over the whole circulation?

A

It doesn’t differ: it is the same at all points

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16
Q

What is the resistance and hence pressure drop in arteries?

A

Low: so the pressure drop over arteries is small

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17
Q

What is the resistance and hence pressure drop in arterioles?

A

High: so the pressure drop over arterioles to large

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18
Q

What is the resistance and hence pressure drop in capillaries?

A

Individually they are resistance

  • But there are many connected in parallel so the overall resistance is low
  • The pressure drop over capillaries is small
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19
Q

What is the resistance and hence pressure drop in venules and veins?

A

Low: so pressure drop is low

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20
Q

Why is the pressure within arteries high?

A

Because of the high resistance of the arterioles

  • For a given total flow, the higher the resistance of the arterioles, the higher the arterial pressure
  • If the heart pumps more blood and the resistance of arterioles remains the same, the arterial pressure will rise
21
Q

How does the distensibility of blood vessels affect flow and pressure?

A
  • The pressure within the vessel generates a transmural pressure between the inside and outside
  • This tends to stretch the tube
  • As the vessel stretches resistance falls (so the higher the pressure in a vessel, the easier it is for blood to flow through it)
  • As the pressure within a distensible vessel falls, the walls eventually collapse (blood flow ceases before the driving pressure falls to 0)
22
Q

How is the property of capacitance produced?

A

As vessels widen with increasing pressure more blood transiently flows in than out
- This allows distensible vessels to ‘store’ blood; they have capacitance

23
Q

Which are the most distensible vessels?

A

Veins

24
Q

Why does blood flow faster than plasma?

A

Blood cells congregate in the middle of the flow, pushing plasma to the edges so they travel much faster than plasma

25
Q

What is systolic arterial pressure?

A

The peak pressure achieved in systole (typically 120 mmHg)

26
Q

What is diastolic arterial pressure?

A

The minimum pressure reached in diastole (typically 80 mmHg)

27
Q

What factors affect systolic pressure?

A
  • How hard the heart pumps
  • The total peripheral resistance
  • Stretchiness (compliance) of the arteries (they become less stretchy with age, hence why blood pressure is higher in old age)
28
Q

What factors affect diastolic pressure?

A
  • Systolic pressure

- Total peripheral resistance

29
Q

What is pulse pressure?

A

The difference between systolic and diastolic pressure (typically 40 mmHg)

30
Q

How do you calculate the mean arterial pressure?

A

Diastolic pressure + 1/3rd pulse pressure

= 80 + 40/3 = 93.3 mmHg

31
Q

How do you calculate cardiac output?

A

Stroke volume x heart rate

- ~5l/min in a healthy adult

32
Q

What is the ‘total peripheral resistance)?

A

The sum of the resistance of all of the peripheral vasculature in the systemic circulation

33
Q

What would happen if arteries had rigid walls?

A
  • Pressure would rise enough in systole to force the whole stroke volume through the total peripheral resistance
  • It would then fall to 0 in diastole
34
Q

Why does arterial pressure have to be high?

A

In order to drive the cardiac output through the resistance of the arterioles

35
Q

How does the elasticity of arteries affect blood flow?

A
  • In systole, arteries stretch
  • So more blood flows in than out
  • Pressure does not rise so much
  • The arteries recoil in diastole, and flow continues through the arterioles
36
Q

What is the pulse wave?

A
  • Contraction of the ventricles generates a pulse wave
  • This propagates along the arteries faster than blood
  • It can be felt at a variety of locations where arteries come close to the surface and can be pushed against a reasonably hard surface
37
Q

What is the dicrotic notch?

A
  • The slight dip seen in the pulse wave
  • It is due to pressure in the left ventricle falling below aortic pressure and the subsequent backflow of blood closing the aortic valve
38
Q

What is the dicrotic wave?

A
  • The slight rise seen in the pulse wave directly after the dicrotic wave
  • It is due to the recoil of blood off the closed aortic valve
39
Q

What is the role of arterioles?

A

They act as resistance vessels

  • They control flow to tissues by variable flow restriction
  • Their walls contain much smooth muscle
  • The state of smooth muscle contraction determines lumen diameter and therefore flow resistance
40
Q

Define vasoconstriction

A

The decrease in flow due to an increase in smooth muscle contraction

41
Q

Define vasodilatation

A

The increase in flow due to a decrease in smooth muscle contraction

42
Q

What is the vasomotor tone?

A

Continuous contraction of smooth muscle

  • Muscles do not actively relax
  • So except under maximum flow conditions there must always be some vasoconstriction
43
Q

What factors affect vasomotor tone?

A
  • Mostly produced by the sympathetic branch of the autonomic nervous system
  • This tone is antagonised by vasodilator factors
  • Actual resistance is determined by a balance between the 2
44
Q

How do vasodilator metabolites affect vasomotor activity?

A

Metabolically active tissues produce vasodilator metabolites (e.g. H+, K+, adenosine)

  • These act to relax vascular smooth muscle
  • Their effect depends on the balance between the rate at which they are produced and the rate at which the blood flow washes them away
  • They lower resistance and increase blood flow
45
Q

What is reactive hyperaemia?

A
  • If the circulation to an organ or limb is cut off for a minute or 2, then restored, a large amount of blood enters after a period of no blood flow
  • The organ/limb has continued metabolising and producing vasodilator during the period of no blood flow
  • There metabolites accumulate
  • So arterioles dilate maximally
  • When flow is returned, resistance is very low and so flow is very high
  • But high flow washes away the metabolites, so the smooth muscle constricts again
46
Q

What is the effect of a high metabolic rate?

A

There is high blood flow to the region

- This is due to production of metabolites, causing vasodilatation

47
Q

What is autoregulation?

A

At most levels of metabolic activity, most organs can automatically take the blood flow they need

  • As long as the pressure in the arteries supplying them is kept within a certain range
  • The resistance of the arterioles is altered
48
Q

What is central venous pressure?

A

The pressure in the great veins supplying the heart (during diastole)

49
Q

What is venous return?

A

The rate of flow of blood back to the heart limits cardiac output

Cardiovascular System (11 decks)

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