The eight extraordinary vessels is a collective term for the governor vessel, conception vessel, thoroughfare vessel, belt vessel, yin link vessel, yang link vessel, yin heel vessel and yang heel vessel.
Compared with the twelve meridians, there is no external and internal relation among the eight extraordinary vessels. In terms of visceral association, the eight extraordinary vessels are in close relation with the extraordinary fu organs. In terms of the distribution, they do not have their own distributing routes and just run around among the twelve meridians except the governor and conception vessels. In terms of the acupoints, they do not have their own specific acupoints except the governor and conception vessels. In terms of the functions, they do not transport qi and blood like that of the twelve meridians, just accumulate and regulate qi and blood in the twelve regular meridians.
The functions of the eight extraordinary vessels are reflected in two aspects. One is to associate the twelve meridians to command qi and blood in certain meridians as well as to regulate yin and yang. The other is to accumulate, effuse and regulate qi and blood in the twelve meridians. “To accumulate” means that the extraordinary vessels can store up qi and blood in the twelve meridians when they are superabundant. “To effuse” means that the extraordinary vessels can provide qi and blood in case of need during the activity of the body or insufficiency of qi and blood in the twelve meridians to regulate the conditions of qi and blood as well as yin and yang related to the twelve meridians.