Airflow is designed to come in come in through the nose, where the air can be warmed, humidified, and filtered before it goes into the lungs. Pressure receptors in the nasal cavity sense airflow and send feedback to the brain that everything is OK. Your brain then allows your nervous system to operate the rational parasympathetic system, rather than the reative sytmpathetic system. Your parasympathatic nervous system aids in creating regular breathing patterns, normal heart rate, and rational thought. While this won't help if a lion is attacking, it is a more effective system to govern your daily thought processess and actions.
Physical performance can be affected as well. When we"stress" (mouth) breathe, our reactive sympathetic nervous system is in charge That can make us play "scared" or "tight." When I was a live-in intern at the Oympic Training Center the year prior to the Sydney Oympics, breathing excercises were part of every athlete's pre-competition ritual. On your next "recovery" run, bike session, or other workout, try only breathing in and out through your nose. Of course, with high-intensity work, you will most likely need to breathe in through your nose and out through your mouth. The harder you work, the greater the volume of C02 you have to expel quickly.
-Brett Klika-CSCS, NASM.ORG