Week 5 - Flow through Tubes Flashcards
What is flow directly proportional to?
The difference in pressure between the two ends of a vessel - i.e. greater pressure difference = greater flow
Define velocity of a fluid through a tube
What is it expressed as?
The rate of movement of fluid particles along the tube
Distance per unit time
How do flow and cross sectional area relate?
Flow = velocity x (cumulative) cross-sectional area
Capillaries are tiny vessels. Why then do capillaries have the slowest flow?
Velocity = flow/cumulative cross sectional area
Although capillaries are individually tiny, they have a very large cumulative area, meaning a slow velocity.
What is laminar flow?
In laminar flow, there is a velocity gradient within the vessel - highest in the middle, stationary at edges.
Why/how does turbulent flow occur?
Occurs when velocity within the vessel is really high. Velocity gradient breaks down as the fluid “tumbles over” itself.
Is resistance high or low in turbulent flow?
High
What is the name given to the sound that turbulent flow makes?
A bruit
What does velocity depend on?
Viscosity
Radius of the tube
Explain viscosity, referring to laminar flow.
In laminar flow, the various layers moving at different speeds must slide over one another.
Viscosity is the extent to which the layers resist sliding over one another.
What does high viscosity mean?
High resistance to sliding
Lower velocity gradient, as the middle layers are slower, and hence more similar to the outer layers. Slower velocity = reduced flow.
How do radius and flow relate?
As cross-sectional area is radius*2, and cross-sectional area is inversely proportional to flow, radius therefore influences flow.
Describe where the equation “pressure = flow x resistance” comes from.
Flow of fluid down a tube is just like flow of electricity down a wire - follows Ohm’s law.
Therefore voltage = current x resistance becomes the above equation.
What factors affect resistance to flow?
- Viscosity - resistance up with viscosity
- Radius - resistance up with 4th power of radius
What is poiseulle’s law?
Resistance decreases with the 4th power of the radius
What happens to resistance if the vessels are in series?
Resistance adds - hence larger
What happens to the resistance if vessels are in parallel?
Resistance is lower, as there is more than one path for the blood to flow along.
R1 x R2/R1 + R2
Does flow change across the circulation?
No - same across whole circulation
Which vessels are low resistance?
Arteries, veins and venules
Capillaries, as a network
Which vessels are high resistance?
Arterioles
Is the pressure drop across high resistance vessels big or small?
Large
What happens to flow if a vessel becomes narrowed?
Becomes turbulent
Why do vessels stretch?
Pressure difference between the inside and outside of the vessel - transmural pressure
What is the name given to the stretching of a vessel wall?
Compliance
What happens to the resistance of a vessel as it gets wider?
How does this affect flow?
Lowers dramatically
Increases
What happens if the pressure within a distensible vessel drops too low?
The walls collapse in and blood flow ceases
How can vessels have capacitance?
As vessels widen due to distensibility, transiently more blood flows in than out - means they “store blood”
Define flow, and give its units (general)
The volume of fluid passing a given point per unit time
Expressed as volume per unit time
