From universalyoga.com
The breathing process is the main one among all the factors of the vital provision of a human being. A man can live without food for a couple of months, without water — for a couple of days, but without breathing a man cannot survive for even ten minutes.

The process of breathing is closely related to the state of consciousness, and its characteristics determine the quality of the psychic-energy structure of a man. It can be said that the rhythms of the breath and the consciousness are similar. Therefore, in order to achieve the principal goal of Yoga, which is the stabilization of consciousness, it is absolutely necessary to completely master the breath.

Through conscious control of breathing, one can directly affect the state of consciousness, and can cause specific effects such as stopping or accelerating the work of the heart, brain, blood, nervous, lymphatic and endocrine systems, and restoration of the organism and the accumulation of the life-power also.

When considering the process of breath, it is essential to understand the wave-like process with which all laws of the wave theory operate. The investigation of various breathing characteristics, rhythms and accompanying processes forms a complete breathing system (to the extent the wave theory is complete). This system consist of all types of breathing rhythms and exercises can. And includes, not only rhythms and exercises discovered empirically and practiced by different world schools, but also that rhythms and exercises which have been overlooked and have been unknown until now.

In the process of breathing , the internal volume (V) of the lungs changes with the passing of the time (T). And different characteristics of the breath are described by such characteristics as the depth (amplitude) (A), frequency (B), correlation of the time of the inhalation (C) and of the exhalation (D), volume of the lungs after the full inhalation (E) and after the full exhalation (F).

The «normal» unconscious breath of a man is controlled automatically by a subconscious program. Its pattern is set by the brain’s breathing center, and is usually close to the harmonic sine curve (Fig. 30). The duration of the inhalation and the exhalation in such breath is approximately equal, and there are no delays between the inhalation and the exhalation.

The practice of Pranayama in Yoga implies conscious control of the life-power flow during the process of breathing by controlling the parameters of the breath. In this case any type of breathing, where different characteristics are controlled consciously, is called Sahita Pranayama. And any automatic breath is called Kevala Pranayama and, in addition to «normal» breathing, it can comprise any type of Pranayama with different automatic breathing characteristics, reduced to automatism which do not require conscious control.