Haemodynamics

Question 1

blood flow is analogous to give comparison

Answer 1

electrical current
V = IR 
v = Voltage 
I = current 
R = resistance
/_\P = QR
/_\ P = Pressure gradient
Q = flow
R = Resistance

Question 2

Ohm’s law

Answer 2

V =IR
V =voltage
I = current
R = Resistance

Question 3

Darcy’s law

Answer 3

perfusion or flow = pressure gradient / resistance

Q = /_\P/R

Question 4

how can Darcy’s law be applied to the whole CVS system

Answer 4

flow = pressure gradient / resistance
– CO = MAP ÷ TPR
– cardiac output (CO)
– mean arterial pressure (MAP)
– total peripheral resistance (TPR)

Question 5

what do we mean by mean arterial pressure

Answer 5

the average pressure across the cardiac cycle

Question 6

where is the pressure generated

Answer 6

in heart during systole

Question 7

cardiac output determines

Answer 7

systolic pressure

increased CO = increased systolic pressure

Question 8

pressure of aorta

Answer 8

~100mmHg

Question 9

pressure of capillaries

Answer 9

~25mmHg

Question 10

pressure of great veins

Answer 10

~2 mmHg

Question 11

vascular resistance influenced by

Answer 11

viscosity - thicker =faster
length = shorter = faster
vessel radius = wider = faster

Question 12

how is poiseuille’s law applied to resistance to flow

Answer 12

R = ηL / r^4

– η is viscosity, L is length and r is vessel radius

Question 13

vessel radius on resistance

Answer 13

tiny change to radius can have big impact

inversely proportional to power of 4

Question 14

site of most vascular resistance

Answer 14

arterioles

Question 15

why are resistance greater in small arteries than large arteries

Answer 15

• Large elastic conductance arteries have limited capacity to vary diameter
– unlike small muscular arteries and arterioles
• Thus resistance is largely generated in these small arteries and arterioles

Question 16

small changes in diameter of blood vessels lead to

Answer 16

big changes in flow
blood will go where resistance is less - control perfusion to allow blood to go to tissue where nutrients is most in demand

Question 17

what is viscosity proportional

Answer 17

proportional to resistance

inversely proportional to flow

Question 18

why is blood thicker than water

Answer 18

2.5 -3 times more viscous than water

plasma proteins and red blood cells

Question 19

haematocrit - HCT

Answer 19

the proportion of blood volume occupied by red cells

Question 20

haematocrit of women

Answer 20

37-47%

Question 21

haematocrit for men

Answer 21

40-54% - more viscous than women

Question 22

how will haematocrit affect viscosity

Answer 22

anaemia - lower HCT - less viscous

polycythaemia - higher HCT increase viscosity

Question 23

why slight decrease in haematocrit in capillaries and smaller vessels

Answer 23

Capillaries and smaller vessels (<300 μm) tend to have (slightly) lower
haematocrit due to axial accumulation
rbc cells in centre of flow - branches get more plasma than rbx

Question 24

do we excpet radius or viscosity to have a greater impact on resistance and therefore flow

Answer 24

– r
4 has much more effect on flow than 
– it is just slightly faster than it would expected

Question 25

blood flow is

Answer 25

laminar

Question 26

laminar speed

Answer 26

speed of blood slows as we move closer to vessel wall - silent smooth and continuous highest velocity in the centre

Question 27

if blood flow is laminar what factor must be important

Answer 27

size

Question 28

how is flow changed in small arteries

Answer 28

Since small arteries have a large area to volume, most blood is near the wall and there is a large effect on flow – here increased viscosity slows flow

Question 29

how is flow changed in larger arteries

Answer 29

In larger arteries, there are more layers and so increased velocities – this diminishes the effect of viscosity

Question 30

consequence of speed

Answer 30

increased chance of turbulence

Question 31

where does non laminar flow tend to occur

Answer 31

in ascending aorta and around branch points - narrowed points where velocity increases

Question 32

basis of measuring turbulent bloodflow

Answer 32

he basis of Korotkoff sounds in measuring BP – the basis of bruit in atheroma – and also bronchial breathing

Question 33

transmural pressure

Answer 33

• The transmural pressure is the pressure that either distends or collapses blood vessels

Question 34

transmural pressure formula

Answer 34

pressure pushing in - pressure pushing out

Question 35

• If the pressure outside (Po) exceeds the pressure inside (Pi) what happens

Answer 35

• If the pressure outside (Po) exceeds the pressure inside (Pi) then the vessel will collapse

Question 36

what is transmural pressure related to

Answer 36

the wall tension and the radius | – The Law of LaPlace

Question 37

law of laplace transmural pressure

Answer 37

wall tension divided by radius | – PTM = T ÷ r or rearranged to T = PTM x r

Question 38

wall tension

Answer 38

how “thick” the vessel wall needs to be

Question 39

wall tension needed for vessels with small radius

Answer 39

less wall tension is required to balance | out the distending pressure

Question 40

wall tension needed for larger radius

Answer 40

more wall tension is required to | balance the distending pressure

Question 41

which vessels are more distensible veins or arteries

Answer 41

veins x 8

Question 42

compliance

Answer 42

the change in volume for a given change in pressure and so relates to a vessels distensibility – C = ΔV ÷ ΔP so can be rearranged to ΔV = C x ΔP

Question 43

what vessels are more compliant

Answer 43

veins x 20 more compliant than arteries