Breath of Life
Prana - "breath"
ayama - "extension"
The control of breath has 4 different stages:-
(1) In Breath
(2) Holding (full) Breath (after in breath)
(3) Out Breath
(4) Holding (empty) Breath (after out breath)
The numerous Pranayama techniques alters the :-
Proportions (1-4 of above)
The In Breath creates a negative pressure (vaccum) in the Lung and extracts the carbon dioxide out of the lung tissue (aveoli).
It has a detoxifying effect on the system and leads us to an inactive state.

The Out Breath creates a Positive pressure in the lung and pushes the oxygen into the lung tissue.
It has a nurturing effect on the body and leads us to an activite state.

By altering the breath by concious control you can see how it affects the way we feel.
This is why Pranayama practice must be taken seriously.