तस्मिन् सति श्वासप्रश्वास्योर्गतिविच्छेदः प्राणायामः|
The word Pranayama formed by two word Prana + Ayama. Prana means life or age and Ayama means length.
The purpose of the Pranayama is to take Pran to all parts and organs of the body and to make it healthy.
In details Prana means a subtle life force which provides energy to different organs including mind and also controls many vital life processes e.g. circulation, respiration etc. Ayama signifies the voluntary effort to control and direct the Prana.
Breathing is one of the vital activities managed by Prana on a gross level. This is only Pranic activity available to us, which can be regulated voluntarily.
Breathing system is also linked with the nervous system hence Pranayama take best advantage of this situation, considering that the mind could be controlled effectively with the voluntarily regulation over breathing.