Maitri (Maitrāyaṇīya) Upaniṣad 2.7

2.7

atha yo'yamūrdhvamutkrāmatītyeṣa vāva sa prāṇo'tha yoyamāvañcaṃ saṃkrāmatveṣa vāva so'pāno'tha yoyaṃ sthaviṣṭhamannadhātumapāne sthāpayatyaṇiṣṭhaṃ cāṅge'ṅge samaṃ nayatyeṣa vāva sa samāno'tha yo'yaṃ pītāśitamudgirati nigiratīti caiṣa vāva sa udāno'tha yenaitāḥ śirā anuvyāptā eṣa vāva sa vyānaḥ ||

Now, that breath which passes up—that, assuredly, is the Prāṇa breath. Now, that which passes down—that, assuredly, is the Apāna breath. Now, that, verily, by which these two are supported—that, assuredly, is the Vyāna breath. Now, that which conducts into the Apāna breath [what is] the coarsest element of food and distributes (_sam-ā-nayati_) in each limb [what is] the most subtile—that, assuredly, is named the Samāna breath. It is a higher form of the Vyāna breath, and between them is the production of the Udāna breath. Now, that which 'belches forth and swallows down what has been drunk and eaten'—that, assuredly, is the Udāna breath.