L37: Control Of Blood Flow

Question 1

Flow through a blood vessel depends on

Answer 1

1. Pressure gradient
2. Resistance
   - viscosity
   - length
   - radius
   —> Passive: Passive stretch of wall due to transmural pressure (Pin always > Pout) —> Compliance of vessel
   —> Active: Vascular smooth muscle contraction
   —> most important (flow ∝ radius^4)
   —> controlled by small arteries and arterioles (resistance vessels)
   —> 2/3 of total peripheral resistance
   —> control blood flow to organ

Question 2

How to calculate compliance

Answer 2

Compliance = volume change / pressure

Question 3

Passive influence: Compliance of vessel wall depends on

Answer 3

1. Thickness (thick wall —> ↓ compliance)
2. Composition (elastin —> ↑ compliance)
3. Neurological influence (contracted —> ↓ compliance)
4. Pathology (inability to secrete factors to relax, atherosclerotic plaque —> ↓ compliance)
   - Atherosclerotic plaque
   —> ↓ radius (by obstructing lumen)
   —> ↓ compliance

Question 4

***Active influence: Vascular smooth muscle contraction

Answer 4

Extrinsic: (modify blood flow —> appropriate distribution —> adapt to different situation)

1. Neural (local distribution): Sympathetic (ALL arteriole), Parasympathetic (face and genitalia)
2. Hormonal (global response on most vessels): Adrenaline, Angiotensin, ADH (ALL constrict)

Intrinsic: (match supply to need —> autoregulation / negative feedback —> non-linear pressure-flow relationship)
Autoregulation: P ↓ —> flow ↓ —> vasoconstriction —> flow ↑

3. Myogenic (↑ transmural pressure —> ↑ radius —> stretch of vascular smooth muscle —> depolarisation and myogenic contraction —> ↓ radius —> return to original size)
4. Metabolic (blood flow matched to metabolic demands by local metabolite e.g. Lactic acid, Carbon dioxide, Adenosine)

• Metabolic autoregulation (↓ flow —> ↓ metabolite removal + O2 supply —> ↑ [interstitial metabolite] —> vasodilation —> ↑ flow)
• Active hyperaemia (exercise —> ↑ [interstitial metabolite] —> vasodilation —> ↑ flow)
• Reactive hyperaemia (flow is interrupted —> ↑ [interstitial metabolite] —> vasodilation —> high flow until excess metabolites are washed away)

Question 5

Sympathetic NE nerve vs Cholinergic nerve

Answer 5

Distribution:
ALL vascular smooth muscle
few blood vessel

Neurotransmitter:
NE
ACh

Receptors:
α1 receptor
Muscarinic receptor

Effect:
Increase in sympathetic tone —> Vasoconstriction (withdrawal of sympathetic tone —> ↑ compliance —> dilate passively) (Pin always > Pout)
Increase in cholinergic tone —> relaxation —> ↑ compliance —> dilate passively