Haemodynamics

Question 1

Darcy’s law

Answer 1

• hydraulic equivalent of Ohm’s law
• flow is linearly proportional to pressure difference
• flow is inversely proportional to resistance

Q = ΔP/R

Q - flow in the steady state
ΔP - P1 - P2 pressure difference between two points
R - resistance

Question 2

perfusion pressure

Answer 2

ΔP or Pa - Pv

Question 3

vascular resistance

Answer 3

resistance of the circulation

Question 4

apply Darcy’s law to circulation

Answer 4

blood flow = perfusion pressure/vascular resistance

Question 5

what is perfusion pressure usually equivalent to? hence what could be another equation for cardiac output?

Answer 5

• perfusion pressure = Pa - Pv
• arterial blood pressure since venous blood pressure is only a few mmHg
• cardiac output is proportional to arterial blood pressure but inversely proportional to vascular resistance

cardiac output = blood pressure/vascular resistance

Question 6

which equations demonstrate the main determinants of blood pressure?

Answer 6

blood pressure = cardiac output x vascular resistance

cardiac output = stroke volume x heart rate

• arterial blood pressure determined by cardiac output and vascular resistance
• cardiac output determined by stroke volume and heart rate
• this in turn goes back to affecting arterial blood pressure (due to change in cardiac output from SV and HR)

Question 7

laminar flow

Answer 7

—>
——->
—>

• blood closest to the vessel wall has the slowest velocity due to the greatest amount of friction
• blood in the centre has the fastest velocity due to the least amount of friction

Question 8

turbulent flow

Answer 8

blood flows back and forth disorderly within the vessel

Question 9

components which influence the resistance to laminar flow

Answer 9

• radius of the tube
• length of the tube
• viscosity of the fluid

Question 10

Poiseuille’s Law

Answer 10

R ∝ (Lη)/r⁴

R - resistance
L - length of vessel
η - viscosity of blood
r - radius of vessel

note that radius has a significantly greater effect on resistance than the other factors (to the power of 4) so it is more efficient to increase radius than decrease length of vessel to decrease resistance

Question 11

how do capillaries have such little resistance despite their small radius? why is some resistance still necessary?

Answer 11

• very short (most less than 1mm)
• many in parallel (capillary beds)
• small cross-sectional area but many of them to distribute the volume of blood across
• little resistance is still necessary to slow blood flow and allow time for diffusion of substances

Question 12

what determines blood viscosity?

Answer 12

haematocrit - percentage ratio of erythrocytes to plasma

Question 13

what causes changes in haematocrit? what effect does each have?

Answer 13

• high altitudes (increase haematocrit)
• anaemia (decreases haematocrit)
• haemorrhage (decrease haematocrit)

Question 14

Fåhræus-Lindqvist effect

Answer 14

viscosity of a fluid changed with diameter of the tube it travels through

Question 15

what happens to blood viscosity when diameter of the vessel decreases? why?

Answer 15

• blood viscosity decreases, only if the vessel’s diameter is between 10-300 micrometres
• erythrocytes move to the centre of the vessel, leaving plasma around the walls (plasma cell-free layer)
• plasma cell-free layer (in contact with walls) viscosity is lower than that of the whole blood so reduces resistance within a capillary
• maintains arterial pressure

Question 16

vasomotion

Answer 16

term encompassing vasoconstriction and vasodilation

Question 17

what are vasoconstrictor nerves always doing?

Answer 17

tonically active imposing a constant squeezing “tone” on vessels which maintains TPR and therefore arterial blood pressure

Question 18

what do sympathetic vasoconstrictor nerves innervate? which has the greatest innervation?

Answer 18

• arterioles
• venules
• arterioles are more innervated than venules

Question 19

result of constriction of the arterioles vs venules

Answer 19

arteriole constriction - increases TPR, increases arterial blood pressure

venule constriction - increases venous return and/or speed of venous return

Question 20

what stimulates vasodilation?

Answer 20

inhibition of sympathetic tone

Question 21

which layer of the blood vessels has a role in vasomotion? how was this discovered?

Answer 21

• tunica intima (endothelium)
• vasodilator arterial response to ACh changed to a vasoconstrictor response if the endothelial lining was rubbed away

Question 22

how does ACh stimulate vasodilation?

Answer 22

• ACh stimulates endothelium to produce NO (nitric oxide)
• ACh stimulates an influx of Ca2+ (use knowledge from smooth muscles)
• increase [intracellular Ca2+-calmodulin complex]
• increases rate of NO synthesis by NO synthase (membrane enzyme)

Question 23

how is NO produced by the endothelium?

Answer 23

• cleaved from arginine (amino acid)
• by membrane enzyme NO synthase

Question 24

what regulates the activity of NO synthase?

Answer 24

[intracellular Ca2+-calmodulin complex]

Question 25

what device measures blood pressure?

Answer 25

sphygmomanometer

Question 26

how does a sphygmomanometer work?

Answer 26

- cuff pumped up to a pressure greater than systolic blood pressure to prevent blood flow - cuff is released until a "whoosh" of blood is heard (Korotkoff sound) - this pressure is measured (systolic blood pressure) - sound disappears when the pressure of the cuff is no longer compressing the artery - this pressure is measured (diastolic blood pressure)

Question 27

average blood pressure for humans (similar in horses)

Answer 27

120/80mmHg

Question 28

mean arterial blood pressure calculation

Answer 28

MAP = D + 1/3(S-D) D - diastolic blood pressure S - systolic blood pressure (S-D) - pulse pressure

Question 29

what causes arterial stiffness?

Answer 29

- ageing - disease (e.g. fatty streaks in blood vessels which decrease radius of blood vessel and increase resistance according to Poiseuille's Law) - vessels deposit calcium and collagen, increasing rigidity

Question 30

arterial stiffness

Answer 30

a measure of blood vessel rigidity

Question 31

what are the arterial pressure waveforms? what can this be used to diagnose?

Answer 31

direct waveform - increase in arterial pressure when the heart pumps blood away from the heart reflected waveform - arterioles reflect the blood backwards towards the heart - this creates a "traffic jam" - the faster the blood reaches the arteriole resistance, the faster the reflected waveform occurs - creates an augmentation index (peak in the middle of the arterial pressure curve) - taller augmentation index suggests a more stiff than an elastic artery (more resistance, greater and faster reflected waveform)