The occurrence of exogenous disease is usually caused by invasion of pathogenic factors which first attack the surface of the body, then invade the collaterals with scanty defensive qi, and gradually get into the internal part of the body. The occurrence of endogenous disease is caused either by insufficiency or imbalance of qi, blood, yin and yang. If qi, blood, yin and yang are insufficient, the meridians and collaterals will become empty; if qi, blood, yin and yang have lost balance, the meridian qi will be stagnated or in disorder, leading to the occurrence of disease.

Since the meridians run along certain routes and pertain to certain viscera, there is a special relationship between different parts of the body and the internal organs. Clinically the relationship between the pathological location or the disease and the meridians can be used to decide which meridian and viscus are involved so as to make an accurate diagnosis. For example, the liver meridian distributes over the hypochondrium, so hypochondriac pain indicates liver disease; the lung meridian emerges from the supraclavicular fossa, pain in the supraclavicular fossa indicates lung disease.

The theory of the meridians and collaterals is extensively used in clinical treatment, especially in acupuncture and moxibustion, massage and drug treatment.