"Asana is meditation on the body, pranayama is meditation on the breath and subtle energy currents within us, and then we work with the mind directly, with the ultimate aim of transcending body and mind and experiencing the higher Self."
Pranayama, the formal practice of controlling the breath, lies at the heart of yoga. It has a mysterious power to soothe and revitalize a tired body, a flagging spirit, or a wild mind. The ancient sages taught that prana, the vital force circulating through us, can be cultivated and channeled through a panoply of breathing exercises. In the process, the mind is calmed, rejuvenated, and uplifted. Pranayama serves as an important bridge between the outward, active practices of yoga--like asana--and the internal, surrendering practices that lead us into deeper states of meditation.
Taken from Yoga Journal - Prescriptions for Pranayama