A man walking at a slow pace starts running. What change do you expect after he starts running, if he continues to breathe at the same rate?

Exercise causes consumption of more oxygen (decrease in pO₂) and production of more carbon dioxide (increase in pCO₂). Unless the breathing rate increases to eliminate excess pCO₂ from the tissues and supply more O₂ to the tissues, his blood pCO₂ increases. CO₂ combines with H₂O to form H₂CO₃ which dissociates into H⁺ and HCO₃^–. Generation of more H⁺ would decrease his blood pH.