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

• A. an increase in O2 concentration in the blood
• B. a decrease in CO2 concentration in the blood
• C. an increase in CO2 concentration in the blood
• D. a rise in blood pH

Answer: (C)

During exercise, working muscles produce more CO2 as they metabolize fuel. This CO2 accumulates in the blood and lowers pH because CO2 forms carbonic acid. Chemoreceptors detect the higher CO2 and the accompanying acidosis, signaling the brain to raise the rate and depth of breathing to blow off CO2 and replenish O2. That increased CO2 concentration is the main trigger for speeding up respiration. An increase in O2 concentration wouldn’t drive ventilation higher, a decrease in CO2 would reduce the drive, and a rise in pH would dampen breathing.