~Flow Flashcards
What defines flow and what defines velocity?
Flow: volume of fluid passing a given point per unit of time
Velocity: Rate of movement
What determines the flow for a given pressure gradient?
The amount of resistance; determined by…
- The nature of the fluid (viscosity)
- The vessel (length and cross-sectional area)
What is the relationship between velocity and cross-sectional area at a constant flow?
Apply this principle to capillaries
Velocity is inversely proportional to the cross-sectional area
Although capillaries individually have a small cross-sectional area and therefore a high velocity as a whole they have a large cross-sectional area which lowers their velocity to match the aorta’s (5L/min) `
What is laminar flow?
The velocity gradient from the middle to the edges of the vessel; velocity is highest in the centre (higher % of RBCs) and fluid is stationary at the edges
What generates turbulent flow? What can be heard over an area with turbulent flow?
As the mean velocity increases (can be due to a higher resistance), the gradient of velocities in laminar flow breaks down. As particles stumble over each other there is backwards and forwards movement - which generates a turbulent flow.
Can hear a “bruit” (Brewy)
What does mean velocity depend on?
The viscosity of the fluid and the radius of the tube
What defines viscosity?
That being said, what is the relationship between viscosity and velocity?
The extent to which fluid layers resist sliding over each other
Low viscosity: Low resistance, central layers flow faster
High viscosity: High resistance, central layers flow slower
Therefore, mean velocity is inversely proportional to viscosity
Now, with constant pressure what is the relationship between mean velocity and cross-sectional area
Mean velocity is proportional to the cross-sectional area
Name the factors that make up flow
- The pressure difference aka pulse pressure: systolic – diastolic pressure (typically 40 mmHg)
- Cross-sectional area
- 1/length
- mean velocity
What does Poiseuille’s law state?
That resistance increases as viscosity and length increases, and resistance will decrease with the 4th power of the radius (in the formula)
How does resistance change when blood vessels are connected? Explain how this works with capillaries
Resistance increases when vessels are in series and decreases when they are put in parallel.
Individual resistance in capillaries is high, but since each capillary makes an alternative route in parallel the overall resistance is much lower
What occurs when a fixed flow meets an area with higher resistance? Give an example of when this happens
A pressure drop occurs from one end of the vessel to the other, e.g; arterioles have greater pressure drops
What is the relationship between resistance and pressure drop?
Resistance and pressure drop are proportional
Describe the resistance and pressure in the following vessels;
a) arteries
b) arterioles
c) individual capillaries
d) venules and veins
a) Low resistance and high pressure, small pressure drop
b) High resistance and pressure drop is high
c) Highly resistant individually but since they are connected in parallel the overall resistance is low, the pressure drop is small
d) venules and veins have low resistance and low pressure drops
Why do arteries need high pressure?
To drive the CO and overcome the arterioles’ resistance
