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)
