Meridians refer to the routes that transport qi and blood, regulate yin and yang, connect the zang-organs with the fu-organs, associate the external with the internal as well as the upper with the lower.

Meridians include both meridians and collaterals. Meridians refer to the longitudinal trunk parts of the meridian system, while collaterals refer to the branches of meridians. Collaterals can be further divided into sub-collaterals. The shallow collaterals are called superficial collaterals which distribute all over the body. In this way meridians and collaterals form a network connecting all parts of the body, including the viscera, five sensory organs, nine orifices, four limbs and skeleton, into an organic whole.

Meridians not only connect all parts of the body, but also coordinate their functions. By means of regulating their own conditions of qi and blood, the meridians influence the states of the viscera and regulate the physiological activities between the zang-organs, the zang and fu organs, the viscera and five sensory organs and nine orifices as well as the four limbs and skeleton to coordinate the functional activities of different parts of the body. Under morbid conditions, the meridians also influence various complicated pathological changes. The basic tenets of acupuncture and moxibustion in treating disease lie in their regulating functions through activating meridian qi to adjust the states of the viscera.