Functions of Yin and Yang
Yin functions
To cool: | Yin must be able to cool the body adequately, so that Yang doesn't warm the body excessively. E.g.: The Yin of the Kidney cools the Yang (fire) of the Heart.
To moisten: | Yin moistens the body, including the skin, hair, muscles, mucous membranes etc. E.g.: The Yin of the Lungs moistens the skin.
To nourish: | Yin nourishes the body. E.g.: The Yin (Blood) of the Liver nourishes the muscles and the tendons.
To calm: | Yin calms the activities of Yang. The calming function of Yin protects us from excessive activity and restlessness. E.g.: The Yin of the Heart calms the spirit.
If Yin is weak, the functions of Yin can not be maintained. As a result, Yang will dominate in the body.