Haemodynamics:pressure, flow and resistance Flashcards
What is darcy’s law ?
States that flow is linearly proportional to the pressure difference between 2 points
Flow= pressure gradient/resistance
A larger pressure difference means greater flow
What is the driving force of blood in the systemic circulation ?
The difference in the aortic pressure and venous pressure which is about 90mmHg approx the same as MAP
How is the velocity of blood flow determined ?
Flow/total cross sectional area
- as total cross sectional area increases the velocity of flow decreases - this is why flow is slowest in the capillaries
What factors affect the resistance to blood flow ?
VESSEL RADIUS- smaller radius increases resistance
FLUID VISCOSITY- a highly viscous blood increases resistance
VESSEL LENGTH- an increased length increases resistance
When can blood viscosity change ?
Anaemia - reduces viscosity
Certain cancers - increase viscosity by increasing the proliferation of blood cells
What is poiseuille’s law ?
Resistance to steady flow along a straight cylindrical tube is proportional to tube length and fluid velocity and inversely proportional to the radius raised to the
A small change in vessel radius causes…
Large change in resistance because the vessel radius is raised to the 4th power
What is Bernoulli’s theory ?
Flow between points A and B is proportional to the difference in the fluids mechanical energy between A and B
If the MAP in the aorta is 95 and the MAP in the foot is 180 then darcy’s law would State that blood would flow from the foot to the aorta but this doesn’t happen, why ?
Because the MAP of the aorta is combined with the gravitational potential energy in the aorta which makes the pressure larger than the pressure in the foot
What happens to the total resistance if you add series’s unit ?
Increases due to the sum of all the individual resistances
What happens to the total resistance if you added parallel unit ?
Decreases
Because your increasing the ease of flow which is the hydraulic conductance and this is the opposite of resistance
What are the 3 different ways blood flows through our circulation ?
Laminar flow
Turbulent flow - occurs in ventricles and sometimes I ascending aorta, can occur in arteries of leg of an unhealthy person
Single file flow- occurs in capillaries
Explain laminar flow
Streamline flow
1st layer next to blood vessel wall has no velocity due to cohesive forces with the wall
2nd later flows faster and then the 3rd flows even faster
Therefore max velocity is reached in the centre so blood flows in shell shape
Explain turbulent flow
Increases in pressure will increase flow up to a point until it breaks down
Point is reached when flow increases only to a proportion of square root of pressure
Some pressure energy is dissipated as heat - Reynolds number
What encourages turbulent flow ?
High fluid velocity
Large tube diameter
High fluid density -increases momentum of flow and causes distortions
What discourages turbulent flow ?
High fluid viscosity - dampens distortions
Explain single file flow
Rbcs flow like this in the capillaries and deform into a parachute like configuration
Efficiency of this flow is dependent critically on the deformity of the rbcs
What is compliance ?
Measure of distensibility
With a specific pressure change you get a specific volume change
What is transmural pressure ?
Pressure inside - pressure outside
How do you calculate compliance ?
Change in volume/ transmural pressure
Why does your pulse pressure increase as you get older ?
Because vessel compliance decreases
Do veins of arteries have a higher compliance ?
Veins - can accommodate a larger increase in blood volume in response to a small increase in BP
What is the law of Laplace ?
States that the tension in the blood vessel walls is necessary to withstand the transmural pressure and is influenced by both radius and thickness of the wall
Why do large arteries need to have thick walls ?
Because they have large transmural pressures and radius’ so wall needs to be thick to compensate for that
How can the walls of capillaries be thin ?
Because they have a low transmural pressure and small radius
