Haemodynamics:pressure, flow and resistance

Question 0

What is darcy’s law ?

Answer 0

States that flow is linearly proportional to the pressure difference between 2 points
Flow= pressure gradient/resistance
A larger pressure difference means greater flow

Question 1

What is the driving force of blood in the systemic circulation ?

Answer 1

The difference in the aortic pressure and venous pressure which is about 90mmHg approx the same as MAP

Question 2

How is the velocity of blood flow determined ?

Answer 2

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

Question 3

What factors affect the resistance to blood flow ?

Answer 3

VESSEL RADIUS- smaller radius increases resistance
FLUID VISCOSITY- a highly viscous blood increases resistance
VESSEL LENGTH- an increased length increases resistance

Question 4

When can blood viscosity change ?

Answer 4

Anaemia - reduces viscosity

Certain cancers - increase viscosity by increasing the proliferation of blood cells

Question 5

What is poiseuille’s law ?

Answer 5

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

Question 6

A small change in vessel radius causes…

Answer 6

Large change in resistance because the vessel radius is raised to the 4th power

Question 7

What is Bernoulli’s theory ?

Answer 7

Flow between points A and B is proportional to the difference in the fluids mechanical energy between A and B

Question 8

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 ?

Answer 8

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

Question 9

What happens to the total resistance if you add series’s unit ?

Answer 9

Increases due to the sum of all the individual resistances

Question 10

What happens to the total resistance if you added parallel unit ?

Answer 10

Decreases

Because your increasing the ease of flow which is the hydraulic conductance and this is the opposite of resistance

Question 11

What are the 3 different ways blood flows through our circulation ?

Answer 11

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

Question 12

Explain laminar flow

Answer 12

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

Question 13

Explain turbulent flow

Answer 13

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

Question 14

What encourages turbulent flow ?

Answer 14

High fluid velocity
Large tube diameter
High fluid density -increases momentum of flow and causes distortions

Question 15

What discourages turbulent flow ?

Answer 15

High fluid viscosity - dampens distortions

Question 16

Explain single file flow

Answer 16

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

Question 17

What is compliance ?

Answer 17

Measure of distensibility

With a specific pressure change you get a specific volume change

Question 18

What is transmural pressure ?

Answer 18

Pressure inside - pressure outside

Question 19

How do you calculate compliance ?

Answer 19

Change in volume/ transmural pressure

Question 20

Why does your pulse pressure increase as you get older ?

Answer 20

Because vessel compliance decreases

Question 21

Do veins of arteries have a higher compliance ?

Answer 21

Veins - can accommodate a larger increase in blood volume in response to a small increase in BP

Question 22

What is the law of Laplace ?

Answer 22

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

Question 23

Why do large arteries need to have thick walls ?

Answer 23

Because they have large transmural pressures and radius’ so wall needs to be thick to compensate for that

Question 24

How can the walls of capillaries be thin ?

Answer 24

Because they have a low transmural pressure and small radius