During exercise, a person's respiratory rate increases due to

• A. A decrease in CO2 concentration in the blood
• B. An increase in O2 concentration in the blood
• C. A decrease in pH
• D. An increase in CO2 concentration in the blood

Answer: (D)

When you exercise, your cells produce more carbon dioxide as a waste product. This CO2 combines with water in the blood to form carbonic acid, which releases hydrogen ions and lowers blood pH. Chemoreceptors in the brainstem and in the carotid bodies detect the higher CO2 and the resulting drop in pH, signaling the respiratory center to increase the rate and depth of breathing. The goal is to expel CO2 and restore pH balance, so ventilation rises in response to the increased CO2 concentration. Oxygen levels don’t typically drive this response as strongly, since oxygen is usually still adequate during exercise; the primary trigger is the rise in CO2 (and the associated acidification) in the blood.