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The Circle of Willis

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The Circle of Willis

Take home messages

  • Neurons die after five minutes of no blood flow
  • The Circle of Willis is a unique bilateral arterial anastomosis
  • 70% of ischaemic strokes are in the anterior circulation

For a structure that you never really see or do anything to or for as an anaesthetist, the Circle of Willis gets examined extraordinarily frequently.

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The main concern is being on the lookout for perioperative stroke

You need to be able to draw it, and to label it, and to answer some questions about it.

Key facts about the Circle of Willis

  • It is an arterial anastomosis
  • It receives around 700ml/min (50ml/100g/min of brain tissue) of blood
  • That's around 15% of the total cardiac output
  • 2/3rds of the supply arrives via the internal carotid arteries
  • 1/3rd via the vertebral arteries

The Circle of Willis primarily exists as an insurance policy against vascular occlusion. Loss of one artery should hopefully be compensated for by the anastomotic ring, and cerebral perfusion preserved.

Which arteries form the Circle of Willis?

  • Two internal carotid arteries (from the common carotids)
  • Two anterior cerebral arteries
  • One anterior communicating artery
  • Two posterior communicating arteries
  • Two posterior cerebral arteries (arising from the basilar artery)

The middle cerebral arteries, while very important, aren't technically part of the circle - they supply the lateral cerebral cortex

What does the anterior cerebral artery supply?

  • Superomedial parietal lobe
  • Medial frontal lobe
  • Corpus callosum
  • Prefrontal cortex
  • Primary motor cortex (mainly lower limbs)
  • Broca's area

What does the middle cerebral artery supply?

  • Lateral frontal lobe
  • Lateral parietal lobe
  • Lateral temporal lobe
  • Basal ganglia
  • Internal capsule
  • Motor and sensory areas to face and upper limbs

What does the posterior cerebral artery supply?

  • Occipital lobe
  • Medial temporal lobe
  • Thalamus
  • Part of the internal capsule

Where are arterial aneurysms most commonly found?

  • 30-40% at junction of anterior communicating and anterior cerebral arteries
  • 30% at junction of internal carotid and posterior communicating artery
  • 20-30% in middle cerebral artery
  • 10% in posterior circulation (basilar and superior and posterior inferior cerebellar arteries)

How much of the arterial flow does each artery receive?

  • ICA 36%
  • MCA 21%
  • Basilar 20%
  • Vertebral 15%
  • ACA 12%
  • PCA 8%
  • Ophthalmic 2%

This is probably more than you need for the exam, so don't stress about this bit.

What factors affect flow in a blood vessel?

  • Length of the vessel
  • Diameter of the vessel
  • Viscosity of blood
  • Pressure gradient

Resistance = (8 x length x viscosity)/ π x radius4

So you can see that changing the radius is the most effective way to alter flow.

What factors affect cerebral autoregulation?

  • CO2
  • PaO2
  • pH
  • Lactate
  • Potassium

What proportion of strokes occur in the anterior circulation?

  • 70%
  • 90% of these are in the MCA

Where is the only place in the body that an artery passes through a venous structure?

  • Cavernous sinus

A reminder of the important graphs

Read our full post on cerebral blood flow and autoregulation here

Cerebral Blood Flow and Intracranial Pressure
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