Functions of the Lungs
The Lung is known as the "canopy" of all the internal organs as it is the superior-most organ in the body. The flow of the lung qi therefore is always down and out. In addition, the lung is the middle man between the body and its environment. It is internally-externally related to the Large Intestine. Its element is Metal.
Governs the Qi and Respiration
The lung takes in the qi of the air (kong qi) and in the chest this is what is combined with the food qi to make true qi. After that it exhales the waste, and then circulates the qi througout the body.
Controls Descending and Dispersing
The lungs circulate the finished product of true qi througout the body. The lungs are responsible for dispersing the qi to the skin and surface as well as to within the channels and blood (yup there's qi in blood too). Thus if anything attacks the lungs (external pathogens) the lungs are the first thing effected. In TCM, the lungs are considered the uppermost organ of the body, so aside from out, the qi has to descend, and it is the lungs that do this. This function is also key because here it has to communicate with the kidneys which receives the qi from above (very important with all breathing problems: it is said the kidneys govern inhalation while the lungs govern exhalation). When functioning properly: strong defensive qi, abundance of qi in channels, lustrous skin. When not functioning properly:weak defensive qi (easy invasion of pathogens), dull skin, nasal obstructions, poor breathing.
Regulates Water Passages
Descending and dispersing of water are also of the realm of the lungs. The spleen sends pure fluids up to the lungs who then spreads it to the rest of the body.
Opens into the Nose
No nose, no breathing, you figure it out. Particularly the qi of the lungs effects the opening and closing of the passages and are thus effected when external pathogens are attacking.
Controls the Skin and Hair
Since the lungs spread both qi and moisture to the outer areas, this determines how nourished the skin and hair are. Additionally, it gives the lungs another opening to regulate the water passages; the pores.
Houses the Corporeal Soul (Po)
The corporeal and ethereal souls are the yin and yang of a persons sould. The corporeal is the yin or material aspect of it. It is most affected by grief and sadness. This is the key to the relationship between the breath and the emotions. Possibly why easy breathing calms the soul, and constriction in the chest can constrict the emotions.