CVS S5 - Blood Flow Flashcards
Define the terms ‘flow’ and ‘velocity’ with respects to fluid moving through tubes
What drives flow of blood?
Flow:
- Volume of liquid passing a given point per unit time
Velocity:
- Rate of movement of fluid particles along the tube
Flow driven by and proportional to pressure difference over the vessel, higher pressure difference = faster flow
What is meant by ‘laminar flow’?
When/where does this flow pattern occur?
Laminar flow is a gradient of velocity from the middle to the edge of the vessel
The velocity of particles moving through the centre of a tube have the highest velocity and those near the edge are stationary
The flow in most blood vessels under normal conditions is laminar
What is meant by ‘turbulent flow’ in regards to blood vessels?
As mean velocity in a vessel increases flow becomes turbulent
Normal velocity gradient (laminar flow) breaks down as layers of fluid try to move over each other faster than physics will allow
The fluid tumbles over and becomes turbulent, resulting in greatly increased flow resistance (and the generation of sound)
What is meant by ‘viscosity’?
How does this affect flow in a vessel?
The extent to which layers of fluid resist sliding over one another
Higher viscosity results in slower flowing central layers, the difference between layers velocity is smaller
Lower viscosity results in a faster flowing central layers relative to edge layers, so the difference in velocity is larger
Velocity is inversely proportional to viscosity
How does changing tube diameter affect velocity?
Mean velocity is proportional to the cross sectional area of the vessel (when flow is not fixed)
At a constant velocity gradient the wider the vessel is the greater the velocity of the central layers, hence there will be a greater mean velocity
Narrower vessels will therefore have a lower mean velocity than a wider vessel if we assume equal viscosity
How does resistance affect flow?
Higher resistance will result in a lower flow within the vessel and vice versa
Note: Comparable to flow of electricity (V= IR)
What factors increase flow resistance in a vessel?
Increases in viscosity with increase resistance:
- ‘Thicker’ blood harder to push around vessels
Resistance decreases with the 4th power of the radius:
- Very small change in radius makes a big difference to resistance
- i.e. It is much more difficult to push blood through smaller vessels than larger ones
If you were to magically raise the resistance of a vessel while keeping either:
Pressure OR Flow
As constants then what would be the effect?
Flow kept constant:
- Raised resistance will result in a greater pressure change from one end of the vessel to the other
Pressure kept constant:
- Raised resistance will result in decreased flow
What effect does combining vessels in parallel or in series make?
(As if they were resistors in an electrical circuit)
Series:
- Total resistance is the sum of the resistance of both vessels
Parallel:
- Total resistance is half the average resistance of the two vessels
How does flow vary throughout the circulation at a given moment?
How might velocity vary across the circulation at fixed flow?
Flow doesn’t vary throughout a vessel, it’s the same at all points
However, flow can vary between vessels
Velocity varies dependent on the radius of the tube, can therefore vary throughout circulation as tube radius changes
Velocity is inversely proportional to cross sectional area (When flow Is FIXED)
How does the relative resistances of the different vessel types affect the pressure in these vessels?
Arteries:
- Low resistance, therefore there is a low pressure drop over the artery
Arterioles:
- High resistance, therefore there is a large pressure drop over the arteriole
- Overall pressure in the arteries is high due to high resistance of the arterioles (difficult to push blood into them, therefore pressure in arteries increases)
Capillaries:
- Individually high resistance, but many in parallel means overall resistance is low, therefore a low pressure drop over the capillaries
Veins and venules:
- Low resistance, therefore there is a low pressure drop over them
If arteriolar resistance is fixed and cardiac output is increased or decreased what is the effect on arteries?
Rise in cardiac output:
- Raise in mean arterial pressure
Fall in cardiac output:
- Fall in mean arterial pressure
How does the distensibility of vessels affect flow of blood through them?
Pressure within the vessel generate a transmural pressure across the wall, stretching the vessel
As the vessel stretches the diameter of the lumen increases, therefore resistance falls and flow increases
If pressure falls the walls may eventually collapse if pressure becomes too low, blood flow will cease before driving pressure falls to zero
What is ‘capacitance’ of a blood vessel?
What vessel type has the greatest capacitance?
Distensible vessels will widen with increasing pressure and hence more blood transiently flows in than out
This allows the distensible vessel to ‘store’ blood, this is capacitance
Veins are the most distensible vessel, with 67% of blood in them at rest, therefore they have the greatest capacitance
What condition might result in turbulent flow?
Atherosclerosis (narrowing of the vessels)
Why do blood cells travel faster than the plasma?
Blood cells congregate in the centre of flow, which travels faster than the surrounding plasma
How does the relationship between blood velocity and cross sectional area of a tube change when flow is fixed or variable?
Explain
When fixed:
- Inversely proportional
When variable:
- Proportional
If flow is fixed then velocity must lower in larger vessels so the same volume of blood passes a given point per unit time
When flow is variable, velocity is determined by cross-sectional area and viscosity
Arrange Poiseulle’s law to give pressure gradient in a vessel
What is one assumption we make?
Pressure change
=
Flow x 8 x viscosity x length of vessel
DIVIDED BY
Pi x radius^4
We assume laminar flow
What must arterial pressure be hight enough to do?
Drive the cardiac output through the resistance of the arterioles (total peripheral resistance)
What is ‘pulsatile flow’?
Heart ejects blood intermittently
In systole blood flows into arteries
In diastole it does not
What is systolic pressure?
What factors affect it?
Maximum arterial pressure (occurs in systole)
Typically 120mmHg
Affected by:
- How hard the heart pumps
- Total peripheral resistance
- Compliance of the arteries (stretchiness)
What is diastolic pressure?
What factors affect it?
Minimum arterial pressure (occurs in diastole)
Typically 80mmHg
Affected by:
- Systolic pressure
- Total peripheral resistance
What is pulse pressure?
The difference between systolic and diastolic pressure
Typically 40mmHg
What is the average pressure in the CVS?
Diastolic pressure + 1/3 systolic pressure (as systole is 1/3 the time of diastole)
Assuming arteries had rigid walls, what would the pressure differences in systole and diastole be?
Why is this pattern not the seen?
Would rise to high enough to force blood through the total peripheral resistance in systole then drop to zero in diastole
In systole arteries stretch and more blood flow in than out, so pressure doesn’t rise so much (systolic pressure)
Arteries then recoil in diastole and blood flow continues (diastolic pressure)
What is vasomotor tone?
What is the effect of changes in vasomotor tone?
Tonic contraction of smooth muscle in the vasculature
Increase (vasoconstriction) raises TPR
Decreases (vasodilation) lowers TPR
What is ‘total peripheral resistance’?
The sum of resistances in all the peripheral vasculature of the systemic circulation
What are the two major factors affecting vasomotor tone?
Give some specific examples
Sympathetic output to smooth muscle of the vasculature (adrenoceptors, A1 agonist, B2 has some antagonist effects in specific locations)
Local vasodilatory metabolites (H+, K+, Adenosine)
Describe the pulse wave
Contraction of the ventricles generate a pulse wave (pulsatile flow) where pressure rises in systole and falls in diastole
Contains a dicrotic notch/wave where the aortic valve closes
Can be felt as the pulse on the skin where arteries can be pushed against a reasonable hard surface
(you should be able to draw this)
What is the Dicrotic notch and wave?
Dicrotic notch:
- Slight dip in the pulse wave when blood flows back into the left ventricle after pressure in the LV drops to below arterial pressure (this closes the aortic alve)
Dicrotic wave:
- Slight rise seen in the pulse wave due to recoil of blood off the closed aortic valve
What is reactive hyperaemia and why does it occur?
If the circulation to a part of the body is cut off (e.g. the arm) for a minute or two when blood flow is restored there is an enormous increase in flow for a short time
This is due to build up of vasodilatory metabolites (not being washed away) causing maximal dilation of arteries
When flow is returned, resistance is very low so flow is very high
But flow will wash away the metabolites so smooth muscle will constrict shortly after
If there is a sustained increase metabolic activity in a tissue, how is correct blood flow established?
If it the metabolic activity falls back to baseline, what happens?
Increased metabolic activity results in an increase in vasodilatory metabolites which reduce resistance and increase flow, as vasodilatory metabolites are washed away, more are produced (if sustained)
This increase in flow is proportional to metabolic need
When metabolic activity falls so does production of vasodilatory metabolites, the increased blood flow washes them away and vasoconstriction occurs, lowering blood flow back to required levels
Describe autoregulation
Hint: Not exact mechanism, general principles
At most levels of metabolic activity, most organs can automatically take the blood flow needed provided blood pressure in supplying arteries is kept within a certain range
It does this through increasing or decreasing vasodilatory metabolite levels in proportion with metabolism
What’s the relationship between total peripheral resistance and blood flow?
They’re inversely proportional
What is central venous pressure?
Pressure in the great veins
Determined by:
- Total volume
- Return of blood from the body
- Output from the heart
- Gravity and muscle pumping
What is ‘venous return’?
The rate of flow into the heart from the veins, this limits cardiac output
