The blood, when properly exposed to the air in the lungs, not only has its impurities consumed, and parts with its noxious carbonic acid gas, but it also takes up and absorbs a certain quantity of oxygen which it carries to all parts of the body, where it is needed in order that Nature may perform her processes properly. When the oxygen comes in contact with the blood, it unites with the hemoglobin of the blood and is carried to every cell, tissue, muscle and organ, which it invigorates and strengthens, replacing the worn out cells and tissue by new materials which Nature converts to her use. Arterial blood, properly exposed to the air, contains about 25 per cent of free oxygen. Not only is every part vitalized by the oxygen, but the act of digestion depends materially upon a certain amount of oxygenation of the food, and this can be accomplished only by the oxygen in the blood coming in contact with the food and producing a certain form of combustion. It is therefore necessary that a proper supply of oxygen be taken through the lungs. This accounts for the fact that weak lungs and poor digestion are so often found together. To grasp the full significance of this statement, one must remember that the entire body receives nourishment from the food assimilated, and that imperfect assimilation always means an imperfectly nourished body. Even the lungs themselves depend upon the same source for nourishment, and if through imperfect breathing the assimilation becomes imperfect, and the lungs in turn become weakened, they are rendered still less able to perform their work properly, and so in turn the body becomes further weakened. Every particle of food and drink must be oxygenated before it can yield us the proper nourishment, and before the waste products of the system can be reduced to the proper condition to be eliminated from the system. Lack of sufficient oxygen means Imperfect nutrition, Imperfect elimination and imperfect health. Verily, "breath is life." The combustion arising from the change in the waste products generates heat and equalizes the temperature of the body. Good breathers are not apt to "take cold," and they generally have plenty of good warm blood which enables them to resist the changes in the outer temperature. In addition to the above-mentioned important processes the act of breathing gives exercise to the internal organs and muscles, which feature is generally overlooked by the Western writers on the subject, but which the Yogis fully appreciate.