Why do we often breathe in through the nose and out through the nose in yoga?
Breathing in through the nose helps to warm and filter out air...the nose has blood vessels that help to heat the air to body temperature and it is filtered by cilia which help to remove common irritants such as dust, pollen and bacterial organisms. When we breathe in through the mouth, it is less filtered.
Breathing out through the nose helps to keep the warmth inside of you. Place your hand in front of your mouth and nose and try breathing out through the mouth...and then try out through the nose. Which air is warmer? Typically the breath out through the mouth feels warmer because it is allowing heat to escape back out.
In addition, simply by paying attention to our breath and breathing slowly and deeply, we invite a relaxation response into our bodies.