The topic of breathing and pranayama (the practice that works to direct the movement of life force) is a fascinating one.
Exhaling through the mouth can be beneficial in that it allows for a greater volume of air to be released at once and may help your jaw to relax. We all do this naturally when we are exasperated, tired, or weary. Take a breath in, then breathe out with a soft, sighing sound: You will feel your shoulders release, and as your jaw releases, your tongue will relax down into the base of the mouth, creating a quieting effect on your mind.
However, in most instances, it’s preferable to breathe through your nose. There are several reasons for this.
The first reason is that the nose does much more than just let air in and out. There are texts that claim it performs more than 30 functions, such as containing the receptors for smell, filtering out dirt and pathogens, and moisturizing and warming incoming air.
The yogic viewpoint is less concerned with the mechanical functions of the nose and breath and more interested in the process of how our breathing affects the nervous system. The ancient texts describe a network of subtle channels, called nadis, the three most important of which originate at the base of the spine. The ida flows to the left nostril, the pingala flows to the right nostril, and thesushumna is the central channel and balance point of the other two.
The ancient yogis were able to map out thousands of these channels, not through dissection of the body, but through intense practice of introspection and awareness development of both the gross and subtle levels of the body-mind. Current research supports the yogic observations.
The reason that nose breathing is more effective in creating energy changes is that when you breathe in or out through your nose, you stimulate the olfactory nerve; this impulse is then passed on to the hypothalamus, which is connected to the pineal gland, which is associated with the third eye area—seat of the “sat guru,” inner wisdom. Some say the ida and pingala interlace their way up the sushumna and end somewhere in the sinus chambers; others say that they end in the “third eye.” When you breathe through your nose, you are helping to open and balance the sushumna and quiet and steady the mind.
For more helpful information go to the Yoga Journal