Together with the chakras, the nadis - variously translated as "conduits", "nerves", "veins", "vessels" or "arteries" - constitute the composition of the subtle or yogic body in Tantra. Like the Chinese meridians, the nadis constitute channels of flow of subtle vital force (prana)
The idea of nadis first appears in the earliest Upanishads (7th-8th century b.c.e.). The heart for example was said to be the centre of the 72,000 nadis. The concept was developed in the later Upanishads - from 2nd century b.c.e. to the 2nd century c.e. and later - and the nascent Yoga and Tantric schools. The Kshurika-Upanishad and later the Hathayogapradikpa mention the 72,000 nadis, especially the Ida, Pingala, and Sushumna channels. [Arthur Avalon, The Serpent Power, p.261]. Other figures have been proposed - 80,000 (Trisikhibramanopanishad), 200,000, or 300,000 (e.g. Siva Samhita, Goraksha Sataka, Goraksha Paddhati)
The symbolism regarding the Ida, Pingala, and Sushumna nadis is especially richly developed in tantric texts. The Ida on the left is generally associated with the moon, white in colour, and with the prana or rising vital breath, semen and Shiva (male) ; the Pingala on the right with the sun, the colour red, blood or ovum and Shakti (female) (Gorakshasiddhantasamgraha). The Ida is also sometimes associated with the prana or rising vital breath, and the Pingala with the apana or descending breath.
Clearly, the correspondences vary according to the school and the text. In the Laya Yoga tradition of the Sat-Cakra-Nirupana, the Padaka-Pancaka, and the commentary by Sir John Woodroffe (Arthur Avalon) (The Serpent Power), the Ida or lunar channel on the left is associated with Shakti-rupa or the female principle; the Pingala or solar channel on the right with the masculine principle (according to the Sammohana-tantra the purusha). The central channel or Sushumna is associated with fire and the union of the two.
Although it is generally agreed that the Ida terminates at the left nostril and the Pingala at the right nostril (so much so that the practitioner is advised to breathe alternately through each nostril to purify his or her nadis), there are two interpretations regarding the remaining position of these two primary nadis in the body. Perhaps the standard traditional interpretation is that they are strung like a double bow, the Ida wholly on the left, the Pingala completely on the right, and the Sushumna supporting the chakras in the center. A rival interpretation, and one that has become very popular in the West, is that the Ida and the Pingala alternate, crossing over the the Sushumna at various points, thus giving rise to the image of the Caudicus. (see the background grafic on these pages, and also the following image, showing the nadis and the chakras equated with the five elements)
above, Stylised "caudicus" diagram showing the the Ida (blue), the Pingala (orange), and the Sushumna nadis, the six chakras (excluding the Sushumna) and the elemental associations of each chakra.
from Layayoga - an Advanced Method of Concentration by Shyam Sundar Goswami
In addition to the various primary and minor nadis, the Shakta Tantra and Kundalini/Laya Yoga traditions emphasis was placed on the central nadis which represented concentric (hence increasingly subtle) channels located vertically along or in front of the spine or backbone, and along which are strung the seven chakras.
These four nadis are:
Sushumna-nadi starts from
the Kanda-mula, lying just below the Muladhara
chakra, and goes upward centrally within the vertebral column.
Vajra-nadi starts from the starting point of Sushumna and goes upward, lying within Sushumna.
Chitrini-nadi starts at the starting point of the Vajra nadi and goes upward, lying within the Vajra nadi.
Brahma nadi or Brahmarandhra nadi starts from the orifice of Swayambhu-linga in the Muladhara and goes upwards, lying within Chitrini
The chakras are actually said to be strung along the fine Chitrini nadi, rather than the Sushumna as is commonly stated in Western books on the subject.