It is the decrease in pH that signals the brain to breathe faster and deeper, expelling the excess CO2 and resupplying the cells with O2.
ɪɾ ɪz ðə dˈiːkɹiːs ɪn pˈiː ˈeɪtʃ ðæt sˈɪɡnəlz ðə bɹˈeɪn tə bɹˈiːð fˈæstɚ ænd dˈiːpɚ, ɛkspˈɛlɪŋ ðɪ ɛksˈɛs sˌiːˈoʊ tˈuː ænd ɹˌiːsəplˈaɪɪŋ ðə sˈɛlz wɪð ˈoʊ tˈuː.