Sri Aurobindo translates Prashna Upanishad, Third Question, Verses 6-7: “The Spirit in the heart abideth, and in the heart there are one hundred and one nerves, and each nerve hath a hundred branch-nerves and each branch-nerve hath seventy-two thousand sub-branch-nerves: through these the breath pervasor moveth. Of these many there is one by which the upper breath departeth that by virtue taketh to the heaven of virtue, by sin to the hell of sin, and by mingled sin and righteousness back to the world of men restoreth.”
These verses make it clear that the term “breath” is used in an entirely different sense than the normal usage in the English language. Prana, while it is translated generally as “breath” is the life-force of the universe and when it acts in the human body, it is not solely as the operation of breathing, but as all the energetic activities of the body, that it functions. The breath pervasor (vyana), circulates the subtle energy of life throughout the body, not just as blood but as energetic force. The Rishis understood a subtle energetic body which had various centers through which energy was received and channeled, which are called chakras, as well as the channels through which these energies are distributed, called nadis. Western anatomy and physiology have not recognised these channels, but serious practitioners of yoga and meditation are able to not only recognise, but open, and utilise these energies in their spiritual development.
The Rishis also held that the Atman was “seated in the heart”. This, again, is not the physical heart, but the subtle heart center where all the nerves/channels come together. The Atman, the Spirit embodied, is the controller of the life and its actions.
The channel that controls the departure of the life from the body is called the sushumna, and this channel extends from the base of the spine through the top of the head. The movement of the energy at the time of death is governed by the focus of the individual and expressed through the prana known as udana. For those focused on virtuous life and actions, the departure from the body takes them to a heaven of virtue. Those who focus on dissipation and ‘sin’ go to a ‘hell’ of suffering. And those who have a mixed focus wind up reborn in the human world. It must be noted here that “sin” and “virtue” as concepts do not mean the same thing as we might assume when using the English terms. We can generally understand that life-focus that is primarily clear, untroubled and open-hearted will lead to the heavens of virtue; that which is dark and troubled, unsettled and disturbed, greedy, lustful and seeking sensual enjoyment, will lead to a place of suffering. Most human beings, having a mixture of these varying energetic directions, wind up reborn to work through the unresolved issues.
Sri Aurobindo, The Upanishads, Prashna Upanishad, pp.297-315