Praśna Upaniṣad ("Six Questions") 89.2

89.2

suptatsu śrotrādiṣu karaṇeṣu etasminpure navadvāre dehe prāṇāgnayaḥ prāṇā eva pañca vāyavo 'gnaya ivāgnayo jāgrati /
agnisāmānyaṃ hi āha - gārhapatyo ha vā eṣo 'pānaḥ /
kathamityāha - yasmādgārhapatyādagneragnihotrakāla itaro 'gniḥ āhavanīyaḥ praṇīyate praṇayanāt praṇīyate 'smāditi praṇayano gārhapatyo 'gniḥ /
tathā suptasyāpānavṛtteḥ praṇīyata iva prāṇo mukhanāsikābhyāṃ saṃcaratyata āhavanīyasthānīyaḥ prāṇaḥ /
vyānastu hṛdayād dakṣiṇasuṣiradvāreṇa nirgamāddakṣiṇadiksambandhādanvāhāryapacano dakṣiṇāgniḥ //3//

When the organs, such as the ear, sleep etasmin pure, in this city, of the body, which is possessed of nine gates; prāṇāgnayaḥ, the five divisions of vital function counting from Prāṇa, which are comparable to fires; jāgrati, keep awake. The resemblance with fire is being stated: Eṣaḥ apānaḥ vai gārhapatyaḥ, this Apāna is really (the sacrificial fire called) Gārhapatya. How that can be so is being stated: Since the other fire, called Āhavanīya, is praṇīyate, taken (extracted); gārhapatyāt praṇayanāt, from the Gārhapatya fire, standing as the source, from which (Āhavanīya fire) is extracted at the time of the Agnihotra sacrifice; therefore from the derivative sense of "that from which something is taken away", Gārhapatya fire is the praṇayana, the source of extraction. Similarly, for a man in sleep, Prāṇa seems to be moving through the mouth and nostrils, having been extracted from Apāna. Therefore Prāṇa is comparable to Āhavanīya. As for vyānaḥ, Vyāna, since it moves out from the heart through the dakṣiṇa, right, orifice, and is thus associated with the dakṣiṇa, southern direction, therefore it is (the fire called) Dakṣiṇāgni, known other-wise as Anvāhāryapacana.