As a person exercises, the oxygen levels in his body begin to drop. hence, the person begins to breathe more rapidly in order to ensure an adequate supply of oxygen to his cells. this increase in his breathing rate is an attempt to restore:

This increase in his breathing rate is an attempt to restore Homeostasis. 
Homeostasis, any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that are optimal for survival. It is the stable state of an organism and of its internal environment, as the maintenance or regulation of the stable condition, or its equilibrium, or simply as the balance of bodily functions.