The demand during exercise
When muscles contract more frequently during exercise, they require more energy. This energy comes from aerobic respiration, which uses oxygen and glucose and produces carbon dioxide and water. The rate of respiration increases, so the demand for oxygen increases and the rate of carbon dioxide production increases.
If the respiratory system does not supply oxygen fast enough, muscles may also carry out anaerobic respiration, producing lactic acid. This is less efficient and leads to an oxygen debt that must be repaid after exercise.
How the body responds
Both breathing rate (breaths per minute) and breathing depth (tidal volume: the volume of air per breath) increase during exercise. These changes increase the volume of air exchanged per minute (ventilation rate), bringing more oxygen into the alveoli and removing more carbon dioxide.
At the same time, heart rate increases to pump more blood through the lungs for gas exchange and to deliver oxygenated blood to muscles more rapidly. The combined increase in ventilation and heart rate ensures the concentration gradients at alveolar and tissue surfaces remain steep.
Interpreting exercise data
Data questions may show breathing rate or ventilation rate before, during and after exercise. Key points to identify: the resting rate before exercise; the increase during exercise and whether it plateaus; and the recovery period after exercise (how quickly breathing returns to resting rate).
A fitter person typically has a lower resting breathing rate, reaches a plateau more quickly during exercise, and recovers faster afterward. If asked to calculate ventilation rate: ventilation rate = breathing rate × tidal volume.