2021B07 Describe the normal regulation of cerebral blood flow and outline physiological factors which may alter it. DO NOT discuss the effect of medications or pathology.



·        Intro

·        Flow dynamics

·        Regulation of vascular resistance

·        Regulation of arterial pressure

·        Anatomical factors


Intro: the brain


·     Highly active: CMRO2 46mL/min = 3.3mL/min/100g

·     Highly perfused: CBF 15% cardiac output = 750mL/min = 58mL/min/100g


·     Interruption to flow -> unconsciousness, head injury, ischaemia


Cerebral blood flow dynamics:

Ohm’s law

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


Hence factors ↓CBF:

·     ↓ mAP

·     ↑ ICP / ↑ CVP (Starling resistor – whichever is higher)

·     ↑ 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:



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

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

·     Effective for perfusion pressure 50-150mmHg

Chart, line chart

Description automatically generated



·     Regional blood flow cerebral metabolic rate (CMR)

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

Physiological variables


·     ↓PaO2 <50mmHg -> vasodilate -> ↑radius -> ↑CBF

·     Non-linear response

Carbon dioxide:

·     ↑PaCO2 -> vasodilate

·     Linear response 20-80mmHg


·     ↓Temp: ↓CMR -> ↓CBF via metabolic autoregulation

·     Near linear response: ↓7% per 1°C


Chart, line chart

Description automatically generatedChart, line chart

Description automatically generated


·     Neural: noradrenaline at α1 -> vasoconstriction (minor)

·     Hormonal: adrenaline -> mixed effects at α1, β1 (minor)

·     Rheologic: ↑Hct -> ↑viscosity -> CVR


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

CNS ischaemic response

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

o  ↑BP

o  ↓HR (reflex)


Anatomical factors:

Arterial supply

Circle of Willis:


Description automatically generated

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:


Description automatically generated

Drainage design:

·     No valves: allows equilibration of venous pressure

·     Elastic and distensible: minimizes resistance to flow



Feedback welcome at ketaminenightmares@gmail.com