In Chinese medicine the Lung rules the circulating and descending of Qi. It is also called the "tender organ" because it is the Yin Organ most easily affected by external pernicious influences (generally a susceptibility of Yang Organs).
The Lung is the Yin Organ of the element Metal and is paired with the Large Intestine, the Yang Organ of Metal. Metal is concerned with energy concentration and transfer and symbolizes connection with the environment. Whilst the Lung receives the life energy, the Qi of heaven, the Large Intestine (together with the skin and lung) is the eliminator of waste from the body into the environment. This applies equally on the mental and spiritual planes as on the physical.