Body and soul, through breathing, are intimately associated with prana, the potential vital energy. This is inherent in all life-substances. It is well known that when we breathe in, we inhale oxygen, which is essential for life. Oxygen is delivered to each cell of the body to generate vital energy. Thus ultimately, breath is the only efficient means of providing prana, which is essential for generating vital energy. It is continuous and constant.
Fresh air, inhaled in the lungs, contains oxygen, which enters the blood stream and is delivered to the cells in the active tissue. Cells use the oxygen to produce energy. Thus breathing is a source of vital energy. The animation of the entire organism, that is each and every activity of life, needs energy. Functioning of sense organs, brain, intellect, speech and bodily movements are all powered by energy. Flow of bio-energy is the source of power for all activities. Flow of bio-energy is capable of animating sense organs, brain and body, which would remain inanimate otherwise.
Controlled and conscious breathing, as in svasa-preksa (perception of breath), is the source of still greater power. The deeper and slower the breathing is, the greater is the production and availability of energy. The soul, the conscious element of the psychic existence, possesses infinite vitality and power.
Pranayama - scientific complete breathing - starts with fuller utilization of the vital capacity of the lungs by slow, silent and deep breathing.
Regular practice of Pranayama (regulated breathing) is an efficient means of improving the quality of respiration. It is the process of reinforcing and disciplining the generation of vital energy - prana. Pranayama ensures complete evacuation of the lungs by a slow, calm and complete exhalation; maximum intake of fresh air by inhalation and full exchange of gases in the lungs by holding the breath. It also ensures full utilization of vital capacity.
All the centres for generating vital energy are controlled by our brain. Stream of vital energy can flow in two separate directions: one would be an external route and the other an internal one. When the flow is in the external route, it animates our vital organs. Our normal strength is produced in this way and there is nothing extra-ordinary about it. The stream of vitality activates all centres and our normal life is carried on without difficulty. But when we change the direction of the stream of vital energy, that is, allow it to flow by internal route instead of the external one, different and extraordinary powers are generated.