You go for a run and breathe heavily for an extended amount of time. How is this affecting your pH levels?

The level of pH decreases on running.

Explanation:

The body start increasing breathing rate as body oxygen requirement increases. The need to remove blood carbon dioxide increases. The hypothalamus and chemosensors (present in carotid arteries) provides the first signal for this outcome. The running causes rise in metabolic activity which causes increase in blood pCO2 and thereby reducing pH.