from the balance between yin and yang and that the close relationship between histological structure and physiological functions signifies the opposition and the unity between yin and yang. The histological structure of the body, including the viscera, the meridians, qi, blood and body fluid, all pertains to yin because they are all substantial. However, their functions all pertain to yang. Thus there is an intrinsical relationship between substances that pertain to yin and functions that pertain to yang, i.e. the mutual opposition, interdependence, wane and wax as well as transformation of yin and yang. In terms of the interdependence between yin and yang, the human body relies on the viscera, the meridians, qi, blood and body fluid to perform and maintain its physiological functions. 

On the other hand, the metabolism of qi, blood and body fluid depends on the functional activities of the related viscera. Similarly, the performance of various functional activities will inevitably consume qi, blood and body fluid. This process is marked by wax of yang and wane of yin. Besides, constant transformation and generation of nutrients must rely on the functional activities of the viscera and consume certain amount of energy. 

This process is characterized by wax of yin and wane of yang. This process of metabolism is also accompanied by mutual transformation of yin and yang between substances and functions. Only when the balance between yin and yang is ensured in this series of physiological process can normal state of life be maintained.