Yoga Sutras: II.49 Pranayama is the regulation of the incoming and outgoing flow of breath with retention. It is to be practiced only after perfection in asana is attained. – (Iyengar translation).
Pranayama is not merely the control of breath. Breath is the force that helps us access prana. Through control of breath we can access and control prana. Prana works in the the subtle or energetic body, as opposed to the physical, or musculo-skeletal.
Prana is the life energy in all of us. In Yogic and Ayurvedic physiology prana pervades our systems and is blocked when there is damage or illness. Or vice versa, when prana is blocked, damage and disease occur.
Pranayama should not be practiced less than a half hour before or after asana. The practice profoundly affects the chemistry and state of the systems in the physical and emotional layers of our self. Health of the nervous system, the organs and the mind are enhanced through pranayama and the meditative state it may bring about.