And here's an excellent explanation!
"1. The lungs are a primary source of our energy level. They extract oxygen from the air we breathe primarily on the exhale. Because the nostrils are smaller then the mouth, air exhaled through the nose creates back pressure when one exhales. It slows the air escape so the lungs have more time to extract oxygen from them. When there is proper oxygen-carbon dioxide exchange, the blood will maintain a balanced pH. If carbon dioxide is lost too quickly, as in mouth breathing, oxygen absorption is decreased."
[link to www.breathing.com]