We breathe harder when we exercise because our muscles work harder and consume more oxygen to produce the energy they need. This increased demand for oxygen causes an increase in carbon dioxide production, which the body needs to expel. To meet these demands, our breathing rate and depth increase to bring more oxygen into the body and remove the excess carbon dioxide. This process supports the elevated rate of respiration in muscle cells during exercise. During exercise, carbon dioxide levels in the blood rise, creating a more acidic environment. Chemoreceptors in the brain detect this and send signals to increase breathing rate and depth. This results in more oxygen being absorbed and carbon dioxide being expelled, supporting continued muscular activity. Also, if oxygen supply is insufficient, muscles can switch to anaerobic respiration, producing lactic acid, which must later be removed by using oxygen ("repaying oxygen debt").
Additionally, consistent exercise improves lung capacity and efficiency, strengthening the diaphragm and other respiratory muscles, enabling deeper and more efficient breaths, which helps meet the body's increased oxygen requirements during exercise.
In summary, harder breathing during exercise is a physiological response to supply more oxygen to muscles and remove increased carbon dioxide, helping meet the higher energy demands of physical activity while maintaining the body's chemical balance.
