This life (prana) is born from Atman.
As in the case of a person there is this shadow extended, so it is in this case. By the action of the mind it comes into this body.
As an overlord commands his overseers, saying: "Superintend such and such villages," even so this life (prana) controls the other life-breaths one by one.
The out-breath (apana) is in the organs of excretion and generation. The life-breath (prana) as such establishes itself in the eye and ear, together with the mouth and nose. While in the middle is the equalizing breath (samana), for it is this that equalizes whatever has been offered as food. From this arise the seven flames.
In the heart, truly, is the self (atman). Here there are those hundred and one arteries. To each one of these belong a hundred smaller arteries. To each of these belong seventy-two thousand branching arteries. Within them moves the diffused breath (vyana).
Now, rising upward through one of these [arteries], the up-breath (udana) leads in consequence of good work to the good world; in consequence of evil, to the evil world; in consequence of both, to the world of men.
The sun (Aditya), verily, rises externally as life; for it is that which helps the life-breath in the eye. The divinity which is in the earth supports a person's out-breath (apana). What is between, namely space (akasa), is the equalizing breath (samana). The wind (vayu) is the diffused breath (vyana).
Heat (tejas), verily, is the up-breath (udana). Therefore one whose heat has ceased goes to rebirth, with his senses sunk in mind (manas).
Whatever is one's thinking, therewith he enters into life (prana). His life joined with his heat, together with the self (atman), leads to whatever world has been fashioned [in thought]. -- Prasna-Upanishad, III 3-10 (based on R. E. Hume's translation)
The function and the character of the pranas in the human body are reckoned as ten and even twelve in esotericism, yet they also are spoken of as being seven, for the same reasons that the planetary chain is usually stated as consisting of seven globes instead of the full number twelve. However, we use the term prana as a generalizing word to signify the aggregate of psycho-vital-astral fluids which the pranas really are. We may otherwise call them the vital essences.
Even in mediaeval Europe -- which of course drew its ideas from ancient Greek and Roman writings -- pretty much the same conception of the human body, as being an entity infilled with vital spirits and with humors, prevailed until a relatively recent time, when these were rejected by medical science, which laughed at the superstitions of our forefathers. Nevertheless, these vital spirits and humors corresponded, however imperfectly, to the pranic fluids of ancient Hindu teaching -- considered to be both ethereal essences and physical humors. From early mediaeval times up to the recent present, medicine consistently taught that normal physical health in the human body was maintained when these vital spirits and humors were operating in equilibrium, and that disease and even death were products of their malfunctioning. The archaic ages were unanimous in their agreement on these points.
Exoteric Hindu writings usually give their number as five: (1) Prana (a Sanskrit compound: pra, forth; an, to breathe, this verbal root being found in the terms for all the pranas), 'a breathing forth,' and hence the vital essence which controls the respiration, particularly the outbreathing, the inbreathing or reflex action of the lungs is considered to be an automatic adjustment of the function. Its organ or seat is the lungs. (2) Vyana, 'a breathing around or apart,' the vital psycho-astral-physical fluid governing the circulations, whether of the blood or the nerves, and therefore its organs are the veins and arteries on the one hand, and the nerves as the higher aspects of the general circulatory function on the other hand, (3) Samana, 'a breathing together or around,' the breath or essence which has to do with controlling the digestive function as well as the assimilation and distribution of fluids; its organs are the stomach, the bowels, etc. (4) Apana, 'a breathing down or away,' signifying a throwing off, governing the organs of excretion. (5) Udana, 'a breathing upwards or above,' the vital essence which causes upward circulatory movement. Its locus is in the navel with corresponding sympathetic loci in the heart and the spinal column; it controls the movement of the vital essence from the lower organs upwards into the skull.
There are two higher 'pranas': the organ of one is located in the heart and the other in the head. Likewise, there are five other secret 'pranas,' which pertain not so much to the body as to the circulatory 'breathings' or movements of atmic spirit and buddhi-manas in and throughout the human constitution.
All the different pranas of the akasic vital stream really make up the completely imbodied man, because they are the vital fields, or what are sometimes spoken of as the nervous fluids, in and through which the finer spiritual, intellectual, and psychical essences work and manifest themselves. When all the pranas are properly balanced, and no one or more of them is either over-stimulated or sub-active, then the man is healthy throughout his entire constitution. This is why any attempt to meddle with these pranic currents -- by yoga or psychic practice -- brings about a change in the human constitution, which practice when conducted through ignorant experimentation, as is almost always the case, will invariably result in disease and very likely in subsequent death, or else in psychical and mental disturbance.
The various pranas are not merely vital winds, as the term is commonly translated, but are streams or flows of psycho-astral substance which work in the body as substantial energies. They are all formed of excessively minute particles or atomic units or entities, which indeed are the same as the life-atoms.
In the last analysis, a man's body is built out of these pranic streams of atomic particles. Furthermore, all the pranas which manifest themselves in the human body are the psycho-astral-physical expression of corresponding and causative currents of vitality in the auric egg. Indeed, they are the vital energic form which the auric egg takes on the physical plane; and the auras, which these pranas exude, producing something like a vapor or mist around the body, are their psychomagnetic atmosphere. In other words, the pranas are the vehicle of expression for all the higher attributes and qualities of the human constitution.
The pranas find their respective fields of action in the auric egg, from which they manifest in the physical body, which is the most material concretion of the grosser aspects of the auric egg. Corresponding to the various physical organs, including the different nervous ganglia or plexuses, there are equivalently active centers or foci or ganglia in the auric egg; and indeed, these latter are the originants or auric causes which produce their effects as corresponding centers or organs in the physical body.
Thus it is that the physical body receives the seven or ten pranas from the auric egg which, in its turn, receives them from the monadic centers in the human constitution -- ranging from the atman down to the physical body. Due to the unceasing activity of the forces or energies at work in man, these forces flow forth from the different monadic foci of his constitution as streams of vitality, i.e. currents of life-atoms, into the various layers of the auric egg. These streams of vital force actually compose the auric egg, with its compounded vital fluids and their characteristic auric qualities or swabhavas; and thence from the various layers of the auric egg these pranic auras are reflected into the different organs or centers or chakras of the physical body.
Thus, then, the complete man during incarnation, when viewed as an objective entity, presents a most marvelous picture of interacting and continuously coruscating streams of pranic vitality, which in the higher ranges are like currents of flowing light, and in their lower ranges are like streams of quasi-material vitality. (Frequent mention has been made in theosophical literature of the 'nervous fluids' of the physical body. The fact is there are as many nervous fluids in man's physical frame as there are pranas, these being but another name for the seven or ten pranas working in and through the nervous system. It is the pranas which cooperate in producing the general flow of nervous energy or force or nervous vitality.) What we call magnetism and electricity, each being the alter ego of the other, are but pranic or vital psychomagnetic flows of life. In the manifested cosmos, they are two aspects of the vital activity of our solar hierarch, interblending and combining with the vital magnetism and electricity of our planetary chain, and again with the magnetism and electricity of our globe earth -- these cosmic forces representing in the solar system what the different pranas are in the human constitution.
Hence, man on earth, and equivalently other beings on other planets, is surrounded not only by all the pranas of the solar system and of the planetary chains, but likewise by the twelve cosmic magnetisms or electricities flowing into the solar system from the zodiacal constellations which surround it. Bearing this in mind, and remembering which planets are ruled by which houses of the zodiac -- modern Occidental astrologers incorrectly say that the planets rule the signs -- the student may correlate the swabhavas of the different pranas of man not only to the swabhavas of the planets but likewise to the pranic swabhavas of the zodiacal houses or constellations.
During the lifetime of a man, all these pranas are more or less at work in his constitution. (In one sense, the only difference between a mahatma and an average man is that the mahatma centers his consciousness in his higher pranas, leaving the other pranas to do their quasi-automatic labors in the lower parts of the constitution.) That is why man during incarnation is like a pillar of dazzling light, of which the top portion seems to vanish into the colorless glory of infinity, while the intermediate and lower parts grow progressively more concrete and more pronounced in color until, when the body is reached, the pranas become gross and heavy and furnish the combined swabhava of the imbodied animal monad.
When a man dies, these pranas are successively indrawn by regular stages from the bottom upwards until the human ego undergoes the second death in the kama-loka, sinks into its dreaming or swapna condition, and enters the devachan in the bosom of the spiritual monad. The pranas, which have been able to rise thus far, then re-enter the monads that originally gave them birth when the ego had previously descended from its devachan into incarnation. This is what is meant by the statement that the pranas return to their respective sources in nature.
Finally, it may be said that even the loftiest activities of the human being, such as consciousness, intellection, intuition, etc., are merely different ways of describing the swabhavas of the divine and spiritual pranic forces pouring forth from the monads in the human constitution which are on its higher planes. The significance of this is that all of nature is but imbodied life, otherwise imbodied consciousness, thought, intelligence. It is the highest which produce the lower; so that the vital flows or fluids on the manifested planes, and therefore in and through the physical body, are but the expression of the higher vitality manifesting itself on the lower and lowest planes.