The theory of the five elements explains the physiological characteristics and functions of the five zang-organs according to the attributes of the five elements, pairing each of the five zang-organs with the corresponding one of the five elements. For example, wood is characterized by free development while the liver prefers freedom to stagnation, so the liver pertains to wood; fire is hot and tends to flame up while heart-yang warms the whole body.
So the heart pertains to fire; earth receives and generates while the spleen transforms food nutrients and is the source of qi and blood, so the spleen pertains to earth; metal depurates and astringes while lung-qi maintains inside and normally descends, so the lung pertains to metal; water moistens and closes while the kidney stores essence and manages water metabolism, so the kidney pertains to water.
Some of the intrinsic relationships among the functional activities of the five zang-organs also reflect the relationships of inter-promotion and inter-restraint. So they can be explained with the theory of inter-promotion and inter-restraint among the five elements. In terms of inter-promotion, the state of wood promoting fire corresponds to the state of the liver promoting the heart because liver-blood can nourish heart-spirit; the state of fire promoting earth corresponds to the state of the heart promoting the spleen because heart-spirit can regulate the transporting and transforming functions of the spleen;
The state of earth promoting metal corresponds to the state of the spleen promoting the lung because the spleen can transports and transforms food nutrients to nourish the lung; the state of metal promoting water corresponds to the state of the lung promoting the kidney because the lung depurates and descends qi to help the kidney receive qi; the state of water promoting wood corresponds to the state of the kidney promoting the liver because kidney-yin can nourish liver-yin.
In terms of inter-restraint, the state of wood restraining earth corresponds to the state of the liver restraining the spleen because liver-qi relieves stagnation of spleen-earth to bring the transporting and transforming functions of the spleen into full play; the state of wood restraining water corresponds to the state of the spleen restraining the kidney because the spleen governs metabolism of water to prevent edema due to abnormal flow of kidney-water.
The state of water restraining fire corresponds to the state of the kidney restraining the heart because sufficient kidney-yin can assist the heart to control hyperactivity of heart-fire; the state of fire restraining metal corresponds to the state of the heart restraining the lung because the warmth of heart-yang can prevent lung-qi from excessively depurating and descending; the state of metal restraining wood corresponds to the state of the lung restraining the liver because lung-qi depurates and descends to prevent liver-qi from excessively ascending.