Mbh 3.212.1b

And by the coalition of Prana and other airs, a reaction (combination) ensues, and the heat generated thereby is known as the internal heat of the human system which causes the digestion of our food. The Prana and the Apana air are interposed within the Samana and the Udana air. And the heat generated by their coalition causes the growth of the body (consisting of the seven substances, bones, muscles, &c). And that portion of its seat extending to as far as the rectum is called Apana; and from that arteries arise in the five airs Prana, &c. The Prana air, acted on by the heat strikes against the extremity of the Apana region and then recoiling, it reacts on the heat. Above the navel is the region of undigested food and below it the region of digestion. And the Prana and all other airs of the system are seated in the navel. The arteries issuing from the heart run upwards and downwards, as also in oblique directions; they carry the best essence of our food, and are acted upon by the ten Prana airs. This is the way by which patient Yogins who have overcome all difficulties, and who view things with an impartial and equal eye, with their souls seated in the brain, find the Supreme Spirit, the Prana and the Apana airs are thus present in the body of all creatures. Know that the spirit is embodied in corporeal disguise, in the eleven allotropous conditions (of the animal system), and that though eternal, its normal state is apparently modified by its accompaniments,--even like the fire purified in its pan,--eternal, yet with its course altered by its surroundings; and that the divine thing which is kindred with the body is related to the latter in the same way as a drop of water to the sleek surface of a lotus-leaf on which it rolls. Know that sattwa, rajas and tamas, are the attributes of all life and that life is the attribute of spirit, and that the latter again is an attribute of the Supreme Spirit.