THE LIFE FORCE
"Prāna circulates through the 72,000 conduits (nādī) of the body and maintains its diverse functions."
More specifically the life force circulates rhythmically through the two principal conduits that are situated to the left and right of the central channel and rise along it in helical fashion.
In a healthy person, this rhythm is approximately 60 to 90 minutes. For that period of time, the prānic current flows more strongly through, say, the left conduit and less strongly through the right, before it switches and flows more strongly through the right conduit and less strongly through the left. (In medicine, this phenomenon is known as nostril dominance.) At each switch, it flows for a few minutes through the central channel itself. These three kinds of flow can easily be detected by determining which nostril is more open (i.e., active) than the other. When the prānic flow is through the central channel, both nostrils are equally open.
The left and the right channel, which wind around the central channel in helical fashion, are respectively known as idā and pingalā. The symbolic designation of the former is “moon” and of the latter, “sun.” The former is thought to have a cooling and the latter a heating or stimulating effect on the nervous system. This hints at the fact that prāna is the “field” that translates mental impulses into neurological activity and vice versa."
Georg Feuerstein, Traditional Yoga Studies.