2019B02 Describe the normal regulation of cerebral blood flow and outline the physiological
factors which may alter it. DO NOT discuss the effect of medications or pathology.

 

List:

·         Intro

·         Cerebral blood flow dynamics

·         Regulation of cerebral vascular resistance

·         Regulation of arterial pressure

·         Anatomical factors

 

Intro: the brain

·         Highly metabolically active

o   CMRO2 46mL/min = 3.3mL/min/100g

·         Well perfused

o   CBF 15% cardiac output = 750mL/min = 58mL/min/100g

·         Minimal anaerobic capacity

o   Interruption -> loss of consciousness, head injury, ischaemia

 

Cerebral blood flow dynamics:

Ohm’s law

Cerebral blood flow (CBF) = (mAP – ICP or CVP) / cerebral vascular resistance

·   Starling resistor: whichever of ICP or CVP is higher

Hence factors ↓CBF:

·   ↓ mAP

·   ↑ ICP

·   ↑ CVP

·   ↑ CVR

Poiseuille’s law

Resistance = (8 x length x viscosity) / (π x radius4) – assuming laminar flow

Hence factors ↑resistance:

·   ↓Radius (note power of 4, most important)

·   ↑Length (not under control)

·   ↑Viscosity

 

 

Regulation of cerebral vascular resistance:

Autoregulation

Myogenic autoreg:

·   Global CNS blood flow constant 58mL/min/100g

·   ↑flow -> ↑stretch -> reflex contraction -> ↓radius -> ↓flow

·   Effective for perfusion pressure 50-150mmHg

 

 

Metabolic autoreg:

·   Regional blood flow spinal cord metabolic rate (MR)

·   ↓MR -> ↓H+/K+/adenosine/lactate/pCO2 and ↑pO2 -> local vasoconstriction -> ↓radius -> ↓flow

Physiological variables

·   ↓PaO2 <50mmHg -> vasodilate (non-linear)-> ↑radius -> ↑CBF (see graph above)

·   ↑PaCO2 -> vasodilate (linear 20-80mmHg) (see graph above)

·   ↓Temperature: ↓metabolic rate -> ↓CBF via autoregulation (↓7% per 1°C)

Other

·   Neural (insignificant)

o SNS noradrenaline -> α1 adrenoceptor -> ↓radius

o PSNS ACh: minimal innervation

·   Hormonal: (insignificant)

o Adrenaline at α1: ↓radius

o Adrenaline at β2: ↑radius

·   Rheologic: e.g. ↑Hct -> ↑viscosity -> ↑vascular resistance

 

Regulation of arterial pressure:

Baroreceptor response

·   Stretch-activated mechanoreceptors in walls of aortic arch and carotid sinuses

·   ↓mAP -> ↓stretch -> ↓activation -> ↓inhibition of SNS ->

o  Vasoconstriction -> ↑SVR

o  Venoconstriction -> ↑preload

o  ↑HR, ↑contractility

-> ↑mAP

·   Important for maintaining CBF during posture change

Central ischaemic response

·   ↓↓CBF -> brainstem ischaemia -> ↑↑SNS activity

o  ↑BP

o  ↓HR (reflex)

 

Anatomical factors:

Arterial supply

Circle of Willis:

Supply design:

·   Chicane-like arteries supply the circle of Willis

·   Turbulent flow -> ↑pressure drop -> ↓effective arteriolar pressure

·   Prevents massive rise in cerebral perfusion pressure during SNS activation

Venous drainage

Dural venous sinuses:

Drainage design:

·   No valves: allows equilibration of venous pressure

·   Elastic and distensible: minimizes resistance to flow

 

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