Lecture 3: haemodynamics Flashcards
What is Darcy’s law?
Flow = Pressure difference/resistance
Q = ΔP/R
What is perfusion pressure?
Difference in pressure between the arteries that supply a region and the veins that drain it.
ΔP = PA - Pv
Why is arterial blood pressure considerred to be almost equal to perfusion pressure?
Pressure in venous system is very low
What is the relationship between CO, BP and vascular resistance?
CO = BP/VR
What is meant by laminar flow?
For any Newtonian fluid the velocity profile is parabolic as the stream directly next to the vessel wall has a higher resistance than the liquid at the centre and so travels at a slower velocity
How does the velocity profile of water compare to that of blood?
More blunt in blood
What influences resistance to laminar flow?
- Friction in vessel wall
- radius of the tube
- length of the tube
- viscosity of the fluid
What is Poiseuille’s law?
R α (Ln)/r4
Resistance α (length of vessel x viscosity of blood) / radius4
How does resistance change as the length of a vessel is increased?
It increases
How does resistance change as blood viscosity increases?
It increases
How does resistance change as the radius of a vessel increases?
It decreases dramiatically
Why do capillaries have a low resistance despite their small radius?
They are short (<1mm)
They are connected in parallel
How is blood viscosity determined?
The ratio of red blood cells to plasma, calculated by the haematocrit (a percentage)
What factors affect blood viscosity?
Haematocrit increases at high altitude and decreases with anaemia
What does the Fahræus-Lindqvist effect describe?
An effect where the viscosity of a fluid changes with the diameter of the tube it travels through
What happens to viscosity as tube diameter decreases?
For what range of diameter is this valid?
Viscosity decreases
10-300µm
Why does the Fahræus-Lindqvist occur?
Plasma cell-free layer forms as RBCs move to centre of vessel and plasma is left around vessel walls. The cell-free layer has a lower viscosity and so lowers the resistance to blood flow
What is vasomotion?
Changes in the diameter of a vessel
Increase = vasodilatation
Decrease = vasoconstriction
What factors affect vasomotion?
Nerves
Hormones
Local factors
What are the physical characteristics of sympathetic nerves?
Short pre-ganglionic fibre and long post-ganglionic fibre
What is the neurotransmitter released from the pre-ganglionic fibre of a sympathetic nerve?
Acetylcholine
What is the neurotransmitter released from the post-ganglionic fibre of a sympathetic nerve?
Noradrenaline
What is the main regulator of total peripheral resistance?
The sympathetic nervous system
What is the neurotransmitter released to cause vasoconstriction?
Noradrenaline
Which type of vessel(s) have vasoconstrictor nerves?
Arterioles and venules
More abundant in arterioles than venules
What is the result of increased vasoconstriction of arterioles?
Increased TPR
Increased APB
What is the result of increased constriction of venules?
What is the most efficient method of vasodilatation?
Inhibition of sympathetic tone
What is the effect of adrenaline on vasomotion?
In most peripheral systems it causes vasoconstriction
May cause vasodilatation in skeletal muscle
What did Furchgott and Zawadski discover in 1980?
A vasodilator response to acetylcholine changed to a vasoconstrictor response if the endothelial lining was rubbed away
Name a local factor which has an effect on vasomotion
How does this affect blood vessels?
Nitric Oxide
Acetylcholine stimulates to emdothelium to synthesise NO which promotes vasodilatation
How is nitric oxide formed in blood vessels?
- Acetylcholine stimulates endothelium
- Cleavage of arginine by an endothelial membrane enzyme called No synthase
- Enzyme activity regulated by Ca2+- calmodulin complex so that ACh increases rate of No synthesis by promoting Ca2+ entry into cells
How is blood pressure measured?
Sphymomanometer
Inflatable cuff around brachial artery inflated higher than systolic pressure
Pressure gradually released
Systolic BP: pressure when korotkoff sounds start
Diastolic BP: pressure at which sounds stop
What is arterial stiffness?
A measure of the rigidity of blood vessels
What causes arterial stiffness to increase?
Ageing and disease leads to calcium and collagen deposition which increases rigidity
How can an increase pulsatility be calculated?
Increased systolic to diastolic ratio
Increased pulsatility index (PI), S:D/mean
What does an increased pulsatile index signify?
Increased vascular resistance
