The nose filters dirt and pollen and warms the air before it enters the lungs. Mouth-breathing is too shallow and allows dust, pollen and other pollutants into the lungs. When someone is tense, breathing quickens, the body produces adrenaline and the heart pumps faster. As a result, breathing becomes quick and shallow, preventing adequate oxygen intake. To shift breathing back to a normal rate, relax the stomach muscles.
Many people fail to breathe properly while exercising.
Exercisers often breathe through their mouths or tighten their abdominal muscles, and don't take deep enough breaths.