The Prana and the Apana airs are interposed within the Samana and the Udana airs. The heat generated by their coalition causes the growth of the body.
समानोदानयोर्मध्ये प्राणापानौ समाहितौ। समर्थितस्त्वधिष्ठानं सम्यक् पचति पावकः॥