Special Circulations Quiz

See Section. Option D is the expected response to hypercapnia, an increased carbon dioxide level in arterial blood. Carbon dioxide readily enters the fluid around the brain and contributes to a fall in pH, making that fluid more acidic. Cerebral arterioles, the small arteries controlling resistance to blood flow, relax and widen in response. Lower resistance allows more blood through the brain when perfusion pressure is maintained. This response can follow inadequate breathing and does not by itself prove that neurones are more active. Think of carbon dioxide as a signal to open the cerebral circulation, rather than as evidence of one particular cause.

Constriction (B) is the usual response to low, rather than high, arterial carbon dioxide and would reduce cerebral flow. Autoregulation (C) means adjusting vessel resistance to buffer changes in blood pressure; it requires vessels to change diameter, not remain rigid. Sympathetic vasoconstriction (A) is a neural effect and does not explain this local dilating response. Oxygen extraction (E) is the amount taken from blood by tissue: it may change as delivery changes, but it is not the primary vascular mechanism being tested.