Flow Through Tubes (5) Flashcards
What is pressure’s relation to flow? And why?
- Flow is proportional to pressure
- Blood flow is driven through blood vessels by pressure gradient
What is meant by ‘flow’?
- Volume of fluid passing a given point per unit of time.
The resistance of a vessel is determined by what?
- The nature of the fluid and vessel
What is meant by velocity of blood flow?
- Rate of movement of fluid particles along the tube
What relation is there between flow velocity and cross sectional area?
- Flow is inversely proportional to cross sectional area
- (small cross sectional area = high velocity)
What is laminar flow and in which blood vessels is it found?
- Uni-directional flow of blood
- Most blood vessels
What part of the blood vessel has the fastest/slowest flow velocity?
- Fastest in centre
- Slowest at peripheries
Why does turbulent flow occur?
- Mean velocity is greater than critical value
What’s the result of turbulent flow?
- Velocity gradient breaks down
- Fluid tumbles over
- Flow resistance’s greatly increased
What is viscosity?
- The extent to which fluid layers resist sliding over one another.
How is viscosity spread in laminar flow?
- Fluid moves in concentric layers
- Middle layers move faster than those at the edge
- Fluid layers move over one another.
What does a high viscosity do out the average velocity?
- Decreases it
What effect does the radius have on the velocity is everything else is kept constant?
- Greater the radius the smaller the velocity
What does Poiseulles law state?
- Flow is proportional to :
- pressure change x r^4 / (viscosity x length)
State the equation relating flow, pressure and resistance.
- Pressure = flow x resistance
What would happen to resistance if viscosity and r^4 increase?
- Increased viscosity: increased resistance
- Increased r^4: decreased resistance
How do resistances act in series (of blood vessels)
- Add up
How do resistances act when in parallel?
- Split between blood vessels.
Fill in the gaps: Arteries are \_\_\_ resistance Arterioles are \_\_\_ resistance Pressure drop over arterioles is \_\_\_\_ Pressure drop over arteries is \_\_\_\_
- 1) Low
- 2) High
- 3) Large
- 4) Small
Individual capillaries have a high resistance so why is their flow velocity so small?
- Individually high but when in parallel with many others the resistance drops in individual capillaries
- Pressure drop is small
- Lower resistance, pressure drops so flow must be lower too
What is the resistance like in venules and veins?
What is their pressure drop like too?
- Low resistance
- Low pressure drop
Why is the pressure within arteries so high?
- Due to the high resistance of arterioles.
- Arterioles need a high pressure drop so a higher initial pressure level.
What is transmural pressure and what does it do to blood vessels?
- Transmural: pressure between inside and outside of vessel
- Stretches the tube
As the walls of a vessel stretch what happens to the resistance?
- Decreases
- (Increase in r^4)
If the pressure within a distensible vessel decreases too low, what may occur?
- Walls of vessel collapse
What do distensible vessels have the ability to do?
- Store blood
- Have a capacitance
