Week 5 - Flow Through Tubes Flashcards
What drives flow through blood vessels?
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
Define flow
The volume of fluid passing a given point per unit time
- Flow = mean velocity x cross-sectional area
What determines the resistance of a vessel?
- The nature of the fluid
- The vessel itself
Define velocity
The rate of movement of fluid particles along the tube
What is the relationship between flow and velocity?
- 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
Describe laminar flow
- 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
Describe turbulent flow
- 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
Define viscosity
The extent to which fluid layers resist sliding over each other
What is the relationship between viscosity and velocity?
- 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
What is the relationship between mean velocity and viscosity?
Mean velocity is inversely proportional to viscosity
What is the relationship between mean velocity and cross-sectional area?
Mean velocity is directly proportional to cross-sectional area
What is flow resistance?
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
Describe the relationship between pressure, resistance and flow
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
What is the effect of combining flow resistance in series?
Resistances add together
What is the effect of combining flow resistance in parallel?
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
How does the flow differ over the whole circulation?
It doesn’t differ: it is the same at all points
What is the resistance and hence pressure drop in arteries?
Low: so the pressure drop over arteries is small
What is the resistance and hence pressure drop in arterioles?
High: so the pressure drop over arterioles to large
What is the resistance and hence pressure drop in capillaries?
Individually they are resistance
- But there are many connected in parallel so the overall resistance is low
- The pressure drop over capillaries is small
What is the resistance and hence pressure drop in venules and veins?
Low: so pressure drop is low
Why is the pressure within arteries high?
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
How does the distensibility of blood vessels affect flow and pressure?
- 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)
How is the property of capacitance produced?
As vessels widen with increasing pressure more blood transiently flows in than out
- This allows distensible vessels to ‘store’ blood; they have capacitance
Which are the most distensible vessels?
Veins
Why does blood flow faster than plasma?
Blood cells congregate in the middle of the flow, pushing plasma to the edges so they travel much faster than plasma
What is systolic arterial pressure?
The peak pressure achieved in systole (typically 120 mmHg)
What is diastolic arterial pressure?
The minimum pressure reached in diastole (typically 80 mmHg)
What factors affect systolic pressure?
- 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)
What factors affect diastolic pressure?
- Systolic pressure
- Total peripheral resistance
What is pulse pressure?
The difference between systolic and diastolic pressure (typically 40 mmHg)
How do you calculate the mean arterial pressure?
Diastolic pressure + 1/3rd pulse pressure
= 80 + 40/3 = 93.3 mmHg
How do you calculate cardiac output?
Stroke volume x heart rate
- ~5l/min in a healthy adult
What is the ‘total peripheral resistance)?
The sum of the resistance of all of the peripheral vasculature in the systemic circulation
What would happen if arteries had rigid walls?
- 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
Why does arterial pressure have to be high?
In order to drive the cardiac output through the resistance of the arterioles
How does the elasticity of arteries affect blood flow?
- 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
What is the pulse wave?
- 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
What is the dicrotic notch?
- 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
What is the dicrotic wave?
- 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
What is the role of arterioles?
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
Define vasoconstriction
The decrease in flow due to an increase in smooth muscle contraction
Define vasodilatation
The increase in flow due to a decrease in smooth muscle contraction
What is the vasomotor tone?
Continuous contraction of smooth muscle
- Muscles do not actively relax
- So except under maximum flow conditions there must always be some vasoconstriction
What factors affect vasomotor tone?
- 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
How do vasodilator metabolites affect vasomotor activity?
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
What is reactive hyperaemia?
- 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
What is the effect of a high metabolic rate?
There is high blood flow to the region
- This is due to production of metabolites, causing vasodilatation
What is autoregulation?
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
What is central venous pressure?
The pressure in the great veins supplying the heart (during diastole)
What is venous return?
The rate of flow of blood back to the heart limits cardiac output
