Special Circulations Quiz

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Which physiological response occurs in the cerebral circulation when arterial carbon dioxide levels rise?

A Sympathetic-mediated vasoconstriction of cerebral arteries
B Cerebral vasoconstriction to reduce intracranial pressure
C No change in vessel diameter due to rigid autoregulation
D Cerebral vasodilation to increase blood flow ✓
E Reduced oxygen extraction by neurones

The answer is D: Cerebral vasodilation to increase blood flow

Explanation

Why D is correct

See Cerebral Circulation. Option D is correct because carbon dioxide is one of the most powerful local controllers of blood flow in the brain. When the partial pressure of carbon dioxide in the blood (pCO2) rises, a state called hypercapnia, the small arteries and arterioles of the brain relax and widen, which is vasodilation. This increases cerebral blood flow, and the extra flow washes carbon dioxide away and returns the local chemical environment towards normal. The logic is sensible: a high carbon dioxide level usually means neurones are working hard or that blood supply is inadequate, so the brain asks for more flow. The reverse also holds, because a low carbon dioxide level (hypocapnia, for example from over-breathing) causes vasoconstriction and reduces blood flow. Other things that dilate brain vessels include a rise in potassium, a rise in adenosine (a by-product of working cells) and a fall in oxygen.

Trigger: A raised carbon dioxide level in the blood (hypercapnia).
Response: Cerebral arterioles widen (vasodilation), so cerebral blood flow increases.
Reverse: A low carbon dioxide level (hypocapnia) causes vasoconstriction and reduces flow.

Why the other options are incorrect

A. Sympathetic-mediated vasoconstriction of cerebral arteries: Brain blood flow is controlled mainly by local chemical signals such as carbon dioxide, not by sympathetic nerves. Even in Cushing's reflex (the response to raised pressure inside the skull), the sympathetic surge works by raising the systemic blood pressure, not by constricting the brain's vessels.
B. Cerebral vasoconstriction to reduce intracranial pressure: Vasoconstriction in the brain is the response to a low carbon dioxide level (hypocapnia), not a high one. In hypercapnia, constricting the vessels would reduce flow and leave carbon dioxide to build up further.
C. No change in vessel diameter due to rigid autoregulation: Cerebral vessels do not have a fixed diameter. Autoregulation is an active process in which the arteries change their calibre to keep flow steady when blood pressure changes, and carbon dioxide is a separate, powerful stimulus that changes that calibre.
E. Reduced oxygen extraction by neurones: Brain tissue already extracts a large share of the oxygen in the blood passing through it. A rise in carbon dioxide changes the diameter of the vessels, not how much oxygen the neurones extract.

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