"Breathing through the mouth tends to inflate only the upper lobes of the lungs, which are connected to sympathetic nerve fibers, the branch of the nervous system that activates the fight or flight response...When you switch to nose breathing, you inflate the entire lungs, including the lower lobes, which are connected to the parasympathetic branch of the nervous system, the branch that calms the body, slows the heart rate, relaxes and soothes. Through proper nose breathing, you employ both branches of the nervous system. At times the foot is on the brake, at times it is on the gas. The back-and-forth fluctuation is a balancing act that your body intrinsically knows how to do and that your mind appreciates." Peg Jordan "The Fitness Instinct."
I have carried out this practice in running and find it has made me a more efficient runner! Stop and notice how you are breathing, are you a nose or mouth breather! Check out some of the links below!