Session 5 Flashcards
What is Flow Resistance?
The flow for a given pressure gradient is determined by the resistance of the vessel.
If the resistance is high at a given pressure gradient, flow is slow.
Resistance is determined by the nature of the fluid and the vessel.
What is Flow and what is it driven by?
Flow: the volume of blood passing a given point per unit time.
Flow of blood through blood vessels is driven by a gradient of hydrostatic pressure.
Flow is proportional to the pressure difference between the ends of a vessel (fluid moves from high pressure end to low pressure end)
The higher the pressure difference, the greater the flow
What is Velocity?
The rate of movement of fluid particles along the tube (distance per unit time)
Flow must be the same at all points along a vessel.
Velocity can vary along the length if the radius (size) of tube changes.
At a given flow, velocity is inversely proportional to cross sectional area of the tube; the bigger the cross-sectional area, the slower the velocity (at a given flow)
NOTE: Vessels with a large overall cross sectional area have a low velocity e.g. Capillaries. Each capillary is individually small but there are millions of them.
What is Laminar flow?
In most blood vessels, flow is laminar
No sound is generated
There is a gradient of velocity from the middle to the edge of the vessel.
Velocity is highest in the centre.
Fluid is stationary at the edge (close to the walls)
Describe Turbulent flow
As the mean velocity increases, flow eventually becomes turbulent (once the velocity increases past threshold).
The velocity gradient breaks down.
Fluid tumbles over - no constant regular change in velocity.
Flow resistance is greatly increased.
Turbulent flow generates sound.
Describe the flow in a vessel with constant pressure driving flow (supplied with an infinite amount of fluid at a constant pressure)
The flow will be determined by the mean velocity - linear movement along the tube.
The mean velocity depends upon: the viscosity of the fluid and the radius of the tube.
What is viscosity?
(Stickiness of the fluid)
In laminar flow the fluid moves in concentric layers like layers of an onion- the middle layers move faster than the layers at the edge. So fluid layers must slide over one another.
The extent to which fluid layers resist sliding over one another is known as viscosity.
The higher the viscosity, the slower the central layers will flow and the lower the average velocity.
The lower the viscosity, the faster the velocity.
Describe the effects of radius on velocity
Viscosity determines the slope of the gradient of velocity At a constant gradient, the wider the tube, the faster the middle layers move.
Mean velocity is proportional to the cross sectional area of the tube.
So mean velocity is
Inversely proportional to viscosity
Directly proportional to cross sectional area (r2)
BUT flow is the product of mean velocity and cross sectional area (flow is not fixed)
What does Poiseulles Law mean?
Flow resistance obeys Ohms Law (V =IR)
Pressure = Flow x Resistance
Rearranging Poiseulles Law:
Delta Pressure = (flow x 8x viscosity x length) / (pi x radius(2) )
Therefore Resistance = (8 x viscosity x length) / (pi x radius(2) )
Resistance increases as viscosity increases
Resistance decreases with the fourth power of radius
The thicker the blood the harder it is to push blood through blood vessels
The smaller the blood vessels, the harder it is push blood through blood vessels .
The flow rate at a given pressure gradient is directly proportional to the fourth power of the radius of the tube.
What happens if the blood vessels are connected together?
If vessels are connected together in series, add the resistances together.
If vessels are connected together in parallel, the single equivalent resistance is lower than the any of the single resistances (because there are alternative pathways for blood flow)
What happens if flow is fixed?
The higher the resistance the greater the pressure changes from one end of the vessel to the other.
If the pressure is fixed, describe flow and resistance
The higher the resistance, the lower the flow
What are the three variables?
Pressure
Flow
Resistance
Describe flow over the whole systemic circulation
Flow is same at all points
Discuss pressure and resistance in arteries
Arteries are large tubes and have low resistance; large radius = large cross sectional area –> easy for blood to flow through
At a constant flow, pressure drop over arteries (from end to end) is small
Discuss pressure and resistance in arterioles
Arterioles are small with a thick smooth muscle layer and have high resistance
Pressure drop over arterioles (from end to end) is large.
Why do arteries have a high hydrostatic pressure within them?
They are connected to arterioles which have high resistance so high pressure is needed to drive flow through arterioles.
For a given total flow, the higher the resistance of the arterioles, the higher the arterial pressure.
If the heart pumps more blood and the resistance of arterioles remains the same, the arterial pressure will rise.
Discuss pressure and resistance in capillaries
Individual capillaries have high resistance but there are so many of them connected in parallel that the overall (combined) resistance is low - pressure drop over capillaries is small.
Describe the pressure across the systemic circulation (in mmHg)
Arteries: 100mmHg (low resistance)
Arterioles: 100mmHg (high resistance)
Capillaries: 35mmHg (high resistance individually but low effective resistance overall)
Venules: 10mmHg (low resistance)
Veins: 8mmHg (low resistance)
Back to the heart from veins: 3 mmHg
From heart to arteries: 100mmHg
Discuss pressure and resistance in venules and veins
Large vessels with low resistance
Pressure drop over venules and veins is low
Low pressure within veins
Describe how flow may become turbulent in some vessels
E.g. In the aorta, if the resistance increases to an extent, aorta flow may become turbulent.
Flow is also turbulent if a vessel is narrowed so flow is restricted e.g.. In atherosclerosis.
Sound generated could be ‘bruit’ - swooshing sound