At our yoga classes I have heard something like: "Breathing via mouth is almost like eating through the nose".
Of course, there might be some situations (cardio-aerobic exercises) when it seems that there is not enough oxygen and one wants to use his mouth.
I’ve heard from people with some medical background (as well as yoga), that oxygen is transferred to the blood only primarily at the moment when you breathe out. This happens due to the fact that the pressure in the lungs is at maximum when one makes an exhalation. And when people take a lot of air into their lungs (e.g. using their mouth to “help”) that is no guarantee at all that you will get more oxygen to your muscles and other parts of your body that need it.
Thus, basically you need to prolong your breathing-out phase to get more oxygen to your body.
At our yoga classes we breathe only through the nose. And due to the fact that the practice is VERY intensive, we use special breathing type named Ujjayi breath (
http://en.wikipedia.org/wiki/Ujjayi_breath - a good and small article on it, where they state it is very similar to Tu-Na of Taoist Qi Gong practice, as well as some other details).
One of the physiological effects of this breathing is that the pressure at the lungs is several times higher than at normal breathing. And that is why it helps very much in oxygenizing your body at intensive practices that put high demand on you breathing system. This help to avoid mouth breathing.
I myself notice that during some of ILC basic exercises or at partner training I start unintentionally use this Ujjayi breath and it helps me doing everything else without using my mouth.
Apart from this breath there are some other pranayama exercises in yoga that can help make (with some regular practice) the muscles that are used while breathing stronger, increase the active volume of lungs, open substantially more alveoli. Some other ones can put some more blood from “depots” into circulation, which can also somewhat help oxygenize better. This might be especially beneficial for people with relatively small blood amounts, e.g. quite slim/underweight persons.
There are some other practices that minimize the overall consumption of oxygen (or I would say using it much more effectively) by the cells. This is primarily done by 2 types of practices – static muscle work (asansas for at least 2-3 minutes to make red muscles work) and pranayma with “non-breathing” exercises. Both of these practices do something like “chocking” effect on the cells. And since cells “do not like being suffocated by the “stupid” body owner”, they substantially increase amount of mitochondria (cell energy generators) - around 10 times after a correct regular practice for around 3 years. However, these practices of static asanas and pranayma with breath delays MUST go one after the other. Asanas re-build the cells of muscles initially. Pranayama does this for the whole body – brain, hearth, liver, kidneys, etc. And these internal organs are VERY much oxygen dependant/demanding. Starting regular strong pranayama practices before muscles use little oxygen can and in many cases documented in the long history of yoga does give huge damage to these important organs of ones body. Thus, these things are done one by one, 3 years for one, 3 years for the other.
These practices are some of the strongest things that yoga can do to a person from the physiological point of view. However, these things are dangerous and without a competent in both medicine and yoga people I would not recommend this to anybody.
I’m personally in the middle of the “asanas” 3 years, have done 1.5 years of these activities. I’m very lucky to do my classes with people that are dozens of years in both yoga and contemporary medical sciences. They were able to successfully combine the 2 things – huge experience of many thousands years and the contemporary approaches, methods, etc. Plus they have been doing themselves/teaching these practices for some time already.
I’m also more than sure that very many Qi Gong practices have also some very good and positive effect on human bodies from the physiological point of view (an mental of course as well). I simply do not know people who can/are interested in putting all these Taoist practices at their shelves from the contemporary scientific point of view. Anyway, some day somebody will, because there is a strong indication and historical evidence of substantial positive influence of them on humans.