In the human body, prana is supposed to be of the greatest importance. Next important is apana. The word vayu in Sanskrit means wind, or the air. It is supposed to be the life principle or source of chetana (life activity). Without it the various functions of the living organism cannot go on.
Ten vayus are supposed to be responsible for ten different functions going on in the human body. The vayus are said to flow along different nadis which are channels and are said to be located at different place in the human body.
Apana Vayu is mentioned in Yajuveda (VII:27), where an ancient rishi express the desire that the vayus may flow brightly in the body, ensuring efficiency in the activities of life.
In Amritanada Upanishad (verse 34), the location of the Apana Vayu is said to the guda (anus). It is said to flow in the area of the anus, generative organ, thighs and knees (Trisikhibrahmana Upanishad, verse 80).
It is believed to bring about, and control the activities of excretion of urine, semen, etc.
Apanam means going downwards. Hence apanavayu is the name given to the vital air which moves in a downward direction in the body. It is also that air which enters the body through the nose and goes down, coming out through the anus.
In Hathayogapradipika (III: 65-69), the practice of mula bandha and kumbhaka is said to bring about unification of Prana and Apana which leads to the arousal of the power of Kundalini.