Final answer:
Hyperventilation leads to a decrease in blood CO2 levels and causes the brain's blood vessels to constrict. The correct answer is d) Decrease; constrict.
Step-by-step explanation:
Hyperventilation causes a decrease in CO2 levels, which, in turn, causes the blood vessels of the brain to constrict. When CO2 levels fall significantly, the blood becomes more alkaline (increased pH), a condition known as respiratory alkalosis. This constriction response is opposite to what occurs when CO2 levels are high, which would typically lead to the dilation of blood vessels. The correct answer to the question is d) Decrease; constrict.