Muscle cells require more energy during exercise. Energy is made during respiration.
When exercising, cells will need more oxygen and produce more carbon dioxide as a result of increased respiration.
When blood reaches the lungs, a larger volume of air is needed to replace the oxygen used and remove the carbon dioxide produced by this extra respiration.
In order to supply more oxygen to the exercising cells, the body increases the rate and depth of breathing.
The time taken for the breathing rate to return to normal is known as the recovery time.
This can be used as a measure of fitness.