Yogi pranayama is not physical breathing alone. Nor is prana to be confused with air. It is a far more subtle substance. Without going too deeply into Yogi metaphysical concepts, let us try to arrive at an understanding of it in terms acceptable to us Western materialists.
A century before the scientists Priestley and Lavoisier conducted their famous experiments isolating oxygen as the substance which made air all-important to life, a British chemist named Maynow ran a series of tests, revolutionary for his times, which convinced him that life was supported "not by air alone but by a more active and subtle part of it." He went so far as to suggest that the lungs were the organ which separated this substance from the atmosphere and passed it into the blood. This substance, or rather this hidden property in the atmosphere, he called spiritus igneoaereus, and excitedly presented a paper on it to his fellow-members in the Royal Society. But because in mid-seventeenth century the accepted medical belief was that the purpose of breathing was "to cool the heart," Maynow's learned colleagues held him up to ridicule. His theory was most effectively buried.
The Hindu theory of prana goes much further than Maynow's intuitive reasoning. We might paraphrase it by saying that oxygen itself is the overall stream within which flows a far more subtle force about which we Occidentals know all too little; nor have we ever paid much attention to it inasmuch as no instruments have yet been invented with which to measure and define it. (No need here to dwell on how skeptical we in this part of the world tend to be of whatever cannot be explained away in purely materialistic terms!) Yet whether we like it or not this force seems to exist just the same--a latent vital power ready to be harnessed for our benefit if we so desire. In fact, the Hindus believe that all physical and mental manifestations are dependent upon prana. They call it the breath of life. Another definition of it is Absolute, or Cosmic, energy, and it may help your concept of it if you think of pranayama as the means for filling the physical body with this cosmic energy.
The Yogis say that prana circulates through the human body via a network of special channels called nadis, roughly equivalent to our network of nerves and blood vessels. The nadis, in turn, are governed by seven chakras, or wheels, which roughly are the astral counterparts of our anatomic plexuses (see pages 53-54). The three main nadis are called Ida, Pingala and Shushamna. Shushamna corresponds to the spinal cord, while Ida and Pingala are represented as two inter-circling snakes on either side of it and may be identified with the sympathetic nervous system.