Which of the following factors causes cerebral vasoconstriction?

Cerebral vasoconstriction is primarily influenced by the levels of carbon dioxide (CO2) in the blood. Hypocarbia, which refers to a state of reduced CO2 levels, leads to cerebral vasoconstriction because low levels of CO2 cause a decrease in hydrogen ion concentration, resulting in increased blood pH (alkalosis). This higher pH level triggers the smooth muscle in the cerebral blood vessels to constrict, thereby reducing blood flow to the brain.

In contrast, factors such as hypercarbia (elevated CO2), acidosis (increased hydrogen ions), or hypotension (low blood pressure) generally cause vasodilation, increasing cerebral blood flow. Hypercarbia leads to cerebral vasodilation due to the stimulatory effects of elevated CO2 levels, while acidosis can also promote vasodilation. Hypotension may reduce perfusion pressure but does not directly induce vasoconstriction like hypocarbia does. Thus, understanding the role of CO2 levels is crucial in differentiating the various vascular responses of the cerebral circulation.