CVS - Flow Through Tubes Flashcards
What must happen to blood pressure to allow it to keep flowing?
The pressure must fall all the way from the aorta to vena Cavae
What is blood flow driven by?
What is flow proportional to?
Gradient of pressure
Proportional to the pressure difference between the ends of the vessel
Higher the pressure going to low pressure = increased flow
What is flow resistance?
What is it determined by?
Flow for a given pressure is determined by the resistance of the vessel
Nature of fluid and the vessel
Define flow
Define velocity
The volume of fluid passing a given point per unit time
The rate of movement of fluid particles along the tube
At given flow rate, In vessels with small cross sectional area e.g. Aorta what would the velocity of blood flow be?
High velocity
What is meant by laminar?
There is a gradient of velocity from middle to the edge of the vessel
Velocity is highest in the centre and fluid is stationary at the edges.
What is meant by turbulent?
What happens to flow resistance?
As mean velocity increases, flow eventually becomes turbulent.
Velocity gradient breaks down as layers of fluid try to move over each other faster than physics will allow
Fluid tumbles over –> greatly increasing flow resistance
What type of flow occurs in most blood vessels?
Laminar flow
In a vessel with constant pressure driving flow, how is flow determined?
What does that depend upon?
Mean velocity
Dependent upon - viscosity of the fluid, radius of the tube
What is viscosity?
What would happen to the flow in high viscosity?
What would the average velocity be?
The extent to which fluid layers resist sliding over one another
Central layers flow slower, lower average velocity
What does viscosity determine?
What is the effect of the radius of the tube on flow rate?
What is mean velocity proportional to?
Slope of the gradient of velocity
Wider the tube, the faster the middle laters will move
Cross sectional area of the tube.
What may cause hyperviscosity?
Abnormally high plasma proteins
Abnormally high RBC or WBC count
Underlying condition must be treated otherwise it re-occurs
What may cause cardiac murmurs in a severely anaemic patient?
High blood flow velocities
Decreased viscosity of the blood caused by low RBC
What is the relationship between pressure, resistance and flow?
What factors affect flow resistance?
Pressure = flow X resistance
Resistance increases as viscosity increase
Resistance decreases to the fourth power of an increase in radius
If pressure is fixed, what will happen to resistance and flow?
If flow is fixed, what is the effect of high resistance?
Higher resistance, lower the flow
Higher resistance, the greater the pressure change from one end of the vessel to the other
What happens to flow at all points in circulation?
What resistance are arteries?
Arterioles?
Veins?
Flow is the same
Low resistance
High resistance
Low resistance
Why is the pressure within the arteries high?
Due to the arterioles high resistance.
It is difficult to push blood into them, therefore pressure increases
When does flow become turbulent?
Flow velocity is high
Low viscosity
Lumen of vessel is irregular
If vessel irregularly narrowed ee.g atherosclerosis
Given each capillary is very small and has high resistance why is their collective resistance low?
Many capillaries are connected together in parallel which gives them low resistance
What is the transmural pressure?
What happens when pressure increases?
The pressure difference between the inside and outside of the tube
Pressure increase –> walls stretch –> resistance falls
What is capacitance?
Distensible vessels being able to store blood
What is the function of the: Heart Arteries Arterioles Capillaries Veins?
Heart - cyclic muscular pump, enable circulation
Arteries - gross conduction and distribution of blood supply
Arterioles - local distribution of fine control to defined tissue volume
Capillaries - micro diffusion and filtration
Veins - collection, return and capacitance
Define compliance
Define capacitance
Ability to distend and increase volume due to pressure increase
Effectively same as compliance - measure of relative volume increase per unit increase in pressure. (reservoir)
Define pressure
Measure of mechanical energy gradient in blood that drives its flow around different parts of the system
