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

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

Answer: (A)

Ventilation during exercise is driven mainly by the buildup of carbon dioxide in the blood. As muscles work harder, they produce more CO2, which raises its concentration in the blood and lowers the blood’s pH. The brain’s chemoreceptors detect this change and signal the respiratory centers to increase both the rate and depth of breathing. This helps blow off the excess CO2 and bring in more oxygen to meet the muscles’ higher needs. Oxygen levels in the lungs don’t typically drop enough to be the primary trigger, and changes in hemoglobin don’t by themselves drive the breathing rate. So the rise in CO2 is the main reason respiration increases during exercise.