Nāḍī (Sanskrit: नाडी, lit. 'tube, pipe, nerve, blood vessel, pulse') is a term for the channels through which, in traditional Indian medicine and spiritual knowledge, the energies such as prana of the physical body, the subtle body and the causal body are said to flow. Within this philosophical framework, the nadis are said to connect at special points of intensity, the chakras. The three principal nadis run from the base of the spine to the head, and are the ida on the left, the sushumna in the centre, and the pingala on the right.
The nadis play a role in yoga, as many yogic practices, including shatkarmas, mudras and pranayama, are intended to open and unblock the nadis. The ultimate aim of some yogic practises are to direct prana into the sushumna nadi specifically, enabling kundalini to rise, and thus bring about moksha, or liberation.
Nadi is an important concept in Hindu philosophy, mentioned and described in the sources, some as much as 3,000 years old. The number of nadis of the human body is claimed to be up to hundreds-of-thousands and even millions. The Shiva Samhita treatise on yoga states, for example, that out of 350,000 nadis 14 are particularly important, and among them, the three just mentioned are the three most vital. The three principal nadis are ida, pingala, and sushumna. Ida (इडा, iḍā "comfort") lies to the left of the spine, whereas pingala (पिङ्गल, piṅgala "tawny (brown)", "golden", "solar") is to the right side of the spine, mirroring the ida. Sushumna (सुषुम्णा, suṣumṇā "very gracious", "kind") runs along the spinal cord in the center, through the seven chakras. When the channels are unblocked by the action of yoga, the energy of kundalini uncoils and rises up the sushumna from the base of the spine.
Several of the ancient Upanishads use the concept of nadis (channels). The nadi system is mentioned in the Chandogya Upanishad (8~6 cc. BCE), verse 8.6.6. and in verses 3.6-3.7 of the Prasna Upanishad (second half of the 1 millennium BC). As stated in the last,
3.6 "In the heart verily is Jivātma. Here a hundred and one nāḍis arise. For each of these nāḍis there are one hundred nāḍikās. For each of these there are thousands more. In these Vyâna moves."
3.7 "Through one of these, the Udâna leads us upward by virtue of good deeds to the good worlds, by sin to the sinful worlds, by both to the worlds of men indeed." (PU Q3)
"The nāḍis penetrate the body from the soles of the feet to the crown of the head. In them is prāṇa, the breath of life and in that life abides Ātman, which is the abode of Shakti, creatrix of the animate and inanimate worlds." (VU 54/5)
In yoga theory, nadis carry prana, life force energy. In the physical body, the nadis are channels carrying air, water, nutrients, blood and other bodily fluids around and are similar to the arteries, veins, capillaries, bronchioles, nerves, lymph canals and so on.
In the subtle and the causal body, the nadis are channels for so called cosmic, vital, seminal, mental, intellectual, etc. energies (collectively described as prana) and are important for sensations, consciousness and the spiritual aura.
Yoga texts disagree on the number of nadis in the human body. The Hatha Yoga Pradipika and Goraksha Samhita quote 72,000 nadis, each branching off into another 72,000 nadis, whereas the Shiva Samhita states 350,000 nadis arise from the navel center, and the Katha Upanishad (6.16) says that 101 channels radiate from the heart.
The Ida and Pingala nadis are sometimes in modern readings interpreted as the two hemispheres of the brain. Pingala is the extroverted (Active), solar nadi, and corresponds to the right hand side of the body and the left hand side of the brain. Ida is the introverted, lunar nadi, and corresponds to the left hand side of the body and the right hand side of the brain (there is a contralateralization). These nadis are also said to have an extrasensory function, playing a part in empathic and instinctive responses. The two nadis are believed to be stimulated through different Pranayama practices, including nadi shodhana, which involves alternately breathing through the left and right nostrils, which would theoretically stimulate the left and right sides of the brain respectively. The word nadi comes from the Sanskrit root nad meaning "channel", "stream", or "flow". Special breathing techniques are practised to influence the flow of prana within these nadis. According to this interpretation, these techniques purify and develop these two energetic currents and may lead to the awakening of kundalini.
The Ida and Pingala nadis are often seen as referring to the two hemispheres of the brain. Pingala is the extroverted, solar nadi, and corresponds to left hemisphere. Ida is the introverted, lunar nadi, and refers to the right hemisphere of the brain. Ida nadi controls all the mental processes while Pingala nadi controls all the vital processes.
The medieval Sat-Cakra-Nirupana (1520s), one of the later and more fully developed classical texts on nadis and chakras, refers to these three main nadis by the names Sasi, Mihira, and Susumna.
In the space outside the Meru, the right apart from the body placed on the left and the right, are the two nadis, Sasi and Mihira. The Nadi Susumna, whose substance is the threefold Gunas, is in the middle. She is the form of Moon, Sun, and Fire even water also; Her body, a string of blooming Dhatura flowers, extends from the middle of the Kanda to the Head, and the Vajra inside Her extends, shining, from the Medhra to the Head.
The purpose of yoga is moksha, liberation and hence immortality in the state of samadhi, union, which is the meaning of "yoga" as described in the Patanjalayayogasastra. This is obstructed by blockages in the nadis, which allow the vital air, prana, to languish in the Ida and Pingala channels. The unblocking of the channels is therefore a vital function of yoga. The various practices of yoga, including the preliminary purifications or satkarmas, the yogic seals or mudras, visualisation, breath restraint or pranayama, and the repetition of mantras work together to force the prana to move from the Ida and Pingala into the central Sushumna channel. The mudras in particular close off various openings, thus trapping prana and directing it towards the Sushumna. This allows kundalini to rise up the Sushumna channel, leading to liberation.
Other cultures also work with concepts similar to nadis and prana.
Systems based on Traditional Chinese Medicine work with an energy concept called qi. Qi travels through meridians similar in description to the nadis. The microcosmic orbit practice has many similarities to certain Indian nadi shuddha (channel clearing) exercises and the practice of Kriya Yoga.
Tibetan medicine borrows many concepts from Yoga through the influence of Tantric Buddhism. One of the Six Yogas of Naropa is a cleansing of the central channel called phowa, preparing the soul to leave the body through the sagittal suture. The Vajrayana practice of Trul Khor is another practice used to direct and control the flow of energy within the body's energetic meridians through breath control and physical postures.
Sometimes the three main nadis are related to the Caduceus of Hermes: "the two snakes of which symbolize the kundalini or serpent-fire which is presently to be set in motion along those channels, while the wings typify the power of conscious flight through higher planes which the development of that fire confers".