The main factor that controls the rate of breathing is..
A.) concentration of carbon dioxide.
If we think about it.. When we breath in oxygen, right away it enters your lungs and turns into carbon dioxide. Well since your lungs need to process oxygen and bring oxygen into the blood stream. Then as soon as the oxygen comes inside (inhale) the oxygen turns into carbon dioxide and you breath it out (exhale) to get a new oxygen load.
Makes sense? lol
Good Luck! :)
