In classical yoga, a Nadi (plural: Nadis) is described as a channel for prana or life energy. One could imagine that the total of nadis within the human being penetrates the subtle bodies (Sanskrit: koshas) in a manner similar to how the total of blood vessels or the nervous system penetrates the physical body.
The word nadi is derived from the Sanskrit root “nad”, which means “channeling”, or “flowing”.
Yogis claim that the nadis as a subtle-anatomical system underlie the manifestation of the physical body, and that the energy flow in that system allows / directs all physiological processes. Similar to the concept of nadis, as channels for prana, is the Chinese concept of meridians, which serve as channels for qi, which is the Chinese name for the universal life energy.
The most important nadis are the central channels, but in ancient yoga texts there are about 72,000 larger and smaller energy channels within the human being.
In the center of the body there are three main vertical channels forming luminous columns of energy from the base of the spine to the top of the head. The main currents of our life force flows along these central channels (nādīs) that are suṣumṇā (parasympathetic central channel), iḍā (left sympathetic channel) and piṅgalā (right sympathetic channel), which connect the chakras (energy centers).
The Ida and Pingala nadis are often used to refer to the halves of the brain. These two primary nadis, run from the mental chakra (on the top of the head) to the base chakra, in the pelvic floor. Pingala is the extraverted nadi (right channel), the nadi of the sun, and corresponds to the left hemisphere. Ida is the introvert nadi (left channel), the nadi of the moon, and corresponds to the right hemisphere.
The whole of energy channels is stimulated by the practice of the energy exercises of IP that are inspired by pranayama, kundalini yoga and energy healing. These exercises could be compared to fitness training, in the sense that they strengthen the subtle bodies and give the practitioner more and more control over and awareness of the energy flows.