praśnopaniṣad - tritīyaḥ praśnaḥ

Third Question: Then Kausalya, the son of Asvala, asked Him: "O Lord, from where is this Prana (Life-Force) born? How does it come into this body? How does it distribute itself and establish itself? By what means does it depart? How does it support the external, and how does it support the internal?"

He replied to him: "You ask transcendental questions, for you are most devoted to Brahman; therefore, I shall speak to you."

"This Prana is born of the Atman (Self). As a shadow is cast by a person, so this Prana is extended from the Atman; it enters this body by the activity of the mind."

"Just as an emperor commands his officials, saying, 'Rule over these villages and those villages,' so this Prana directs the other vital forces, each to their own specific place."

"The Apana dwells in the organs of excretion and generation; the Prana itself resides in the eyes, ears, mouth, and nose; while the Samana dwells in the center. It is the Samana that distributes the food offered in the fire of digestion equally, and from this, the seven flames (the sense organs) arise."

"The Atman resides in the heart. Here, there are one hundred and one principal nerves, and from each of these, there branch out a hundred more, and each of those branches out into seventy-two thousand sub-branches; in all these, the Vyana circulates."

"Then, through one of these (the Sushumna), the Udana leads the soul upward to the higher worlds through righteous deeds, to lower worlds through sinful deeds, and to the world of men through both."

"The sun is indeed the external Prana, rising to assist the Prana in the eye. The deity that is in the earth supports the Apana in man; the space between is the Samana, and the air is the Vyana."

"The Udana is indeed the essence of light (Tejas); therefore, when one's inner light (fire of life) is extinguished, the person reaches rebirth, with the senses merging into the mind."

"Whatever state of consciousness one is in at the time of death, with that he approaches the Prana; the Prana, joined with that light (the Udana) and united with the Atman, leads him to the world he has fashioned by his own intentions."

He who understands Prana in this manner—that it originates from the Self—does not lose his progeny and becomes immortal. Regarding this, there is the following verse.

Having understood the origin, the arrival, the abode, the all-pervading nature, and the fivefold distribution of Prana, as well as its presence within the individual self, one attains immortality; yes, one attains immortality.