Answer
Chemoreceptors in the circulatory system detect this pH change, which leads to nerve signals that go to the brain and stimulate increased breathing. This, in turn, expels carbon dioxide, which had been lowering blood pH, raising the pH again.
Work Step by Step
The key here is to see 1) that the pH change is a chemical change, requiring a chemoreceptor, and 2) that breathing will remove the carbon dioxide which had lowered pH.