Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verse 16: “A hundred and one are the nerves of the heart, and of all these only one issues out through the head of a man: by this his soul mounts up to its immortal home, but the rest lead him to all sorts and conditions of births in his passing.”
Esoteric yoga psychology identifies numerous channels, called nadis, which act as the carriers of prana in the body. Prana in this sense is not the “breath” but the energy that enlivens the being and carries out mental action, life and physical activity. These are subtle channels, separate from the physical nerves and blood vessels. These subtle channels are connected to the various chakras which are subtle centers where energies are received, sent and distributed throughout the being. Each chakra has its own characteristic ability to receive and transmit specific types of energies. When a chakra is “open”, the specific energy associated with that chakra flows, using the nadis as the channels of movement.
Yoga texts speak of innumerable nadis (hundreds of thousands in some cases), but generally focus on a much smaller number of prominent nadis. The Katha Upanishad, as well as the Prashna Upanishad, refer to 101 of these channels, while yoga practitioners focus on three, the ida, pingala and sushumna. The sushumna nadi is said to run from the base of the spine up through the top of the head, and remains closed for most people. Through its activation, the practitioner receives enlightenment, and, as noted in the Upanishad, it is the flow of the prana out of the body at the time of death through the sushumna and the 1000-petaled chakra at the top of the head, that leads to immortality, that is, the unification of the Atman in the individual with the Divine.
If, at the time of death, the life-energy departs through any of the other nadis, the rebirth follows the energy carried by that nadi, related to the corresponding chakra, and rebirth will take place consistent with the energy and direction of that energy. Thus, someone attached to the sense of power may find his life-essence departing through a nadi associated with the navel chakra and take a suitable birth to follow up and work out that karmic destiny. Someone attached to sexual energy will similarly find that the next birth continues the process of working out that destiny, with the energy departing through a nadi that carries that energy. Those who open up the heart chakra and live a life of compassion and caring may similarly find that the life energy departs therefrom and they are reborn in an environment that suits such an energetic direction. Generally the complex, mixed nature of human life leads to a birth that encompasses a variety of motives and fixations to be addressed. While the primary chakras are seven in number, there are 101 prominent nadis among potentially hundreds of thousands of nadis in total, creating the ability of a complex action taking into account multiple energetic directions in the path to rebirth.
Thus, the nature, direction and intensity of the prana that acts within a being will determine, at the time of death, the outflow and subsequent result of taking birth in a form that is mixed to one degree or another, unless the flow is directed out through the sushumna and the 1000-petaled chakra at the top of the head, which leads directly to the divine consciousness, and thus, immortality.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129