There are three main naadis (which can be understood as current in our system) in our body. The three Naadis are Ida, Pingla & Sushumna. These naadis are at subtle level and carries Prana in our body. The very name itself ” Naadi” is similar to “नदि” which means river which is responsible to sustaining the world. After-all all the civilizations mushroomed near rivers. So these nadis are responsible for our sustenance in the form of prana at a subtle level.
Ida Naadi is responsible for cooling as it is related to Moon and Pingala is responsible for Heating as it is related with Sun. Ida flows through the left nostril and Pingala through right nostril. Most of the time we are using Ida or/and Pingala. Sushumna activation something which all spiritualist and yogis crave for. The Ida and Pingala flows criss-crossing each other at certain points called Chakras. For further details read the following article: