Answer: To meet the demand of more oxygen by the body.
Explanation: During the time of exercise the need for oxygen by the body increases. The minute ventilation of an individual will increase due to increase in the tidal volume and frequency of breathing.
The extra space is provided by the inspiratory reserve volume which acts as a reservoir of extra lung volume that can become inflated with air and as we need it.
So, Inspiratory reserve volume and expiratory reserve volume decreases during heavy exercise as there is a need of more inhalation of oxygen and more exhalation of carbon dioxide as tidal volume increases and cover the space.
