Shat.Upa 04.03

प्राणाग्नय एवैतस्मिन् पुरे जाग्रति। गार्हपत्यो ह वा एषोऽपानो व्यानोऽन्वाहार्यपचनो यद्गार्हपत्यात् प्रणीयते। प्रणयनादाहवनीयः प्राणः। यदुच्छ्वासनिःश्वासावेतावाहुती समं नयति स समानो। मनो ह वाव यजमानः इष्टं फलमेवोदानः। स एनं यजमानमहरहर्ब्रह्म गमयति ॥३॥

In this body, the fires of 'prāṇa' i.e. life-breath are always active. The household fire represents the downward and diffusive breaths (apānaḥ), which are offerings led from the household fire. The sacrificial fire symbolizes the life-breath (prāṇa). The inhalation and exhalation are offerings that are equally led by the equalizing breath (samāna). Indeed, the mind is the sacrificer, and the upward breath (udānaḥ) is the desired fruit. Day by day, this leads the sacrificer to the ultimate reality, Brahman.