The main physiological functions of the lung are dominating qi and managing the regulation of water passage.
Dominating Qi
The physiological function of the lung to dominate qi covers two aspects, to dominate respiration and to control qi through out the body.
Dominate Respiration
The function of the lung to dominate respiration means that the lung controls the respiratory movement.The lung is the main organ involved in respiratory movement and the place where air in and out of the body exchanges. The body inhales fresh air and exhales waste air through the respiratory movement of the lung so as to maintain the normal activities of life.
The normal function of the lung to dominate respiration, apart from the moistening function of lung-yin, mainly relies on the dispersing and descending functions of lung-qi. Waste air is exhaled out of the body by means of the dispersing function of lung-qi and fresh air is inhaled into the body through the descending function of lung-qi. Thus the dispersing function and descending function of lung-qi depend on each other and restrict each other to maintain the normal respiratory function of the body. Physiologically, harmonious dispersing and descending functions of lung-qi ensures smooth and regular respiration. If lung-qi fails to disperse and descend, it will affect respiration, leading to chest oppression, cough and dyspnea.
Dominate Qi
The function of the lung to dominate qi all through the body means that the lung controls and regulates qi in the whole body. Such a function is reflected in the following three aspects:
The first aspect is the production of the pectoral qi (or thoracic qi). The pectoral qi, part of qi in the body, is synthesized with the nutrients of food refined by the spleen and fresh air inhaled by the lung. When it is produced, the pectoral qi is transported and distributed, by means of the dispersing and descending functions of the lung as well as the function of the heart to control blood, to the whole body to warm and nourish the internal organs and tissues to maintain their normal physiological functions. Since fresh air in the natural world is a necessary condition for the production of the pectoral qi and is in- haled into the body by the lung, the respiratory function of the lung directly influences the production of the pectoral qi and qi in the whole body.
The second aspect is the regulation of qi activity. Qi is flowing constantly in the body. The movement of qi is characterized by ascending, descending, going-out and entering. The lung functions to disperse and descend, directly influencing the activity of qi. The respiratory function of the lung, reflecting the ascending, descending, going-out and entering activities of qi, also exerts great effect on the movement of qi in the whole body. If the lung is normal in dispersion and descent as well as respiration, the visceral qi and the meridian qi in the whole body will be normal in ascending, descending, going-out and entering. It is clear that the lung regulates the activities of qi in the whole body.
The third aspect is to assist the heart to promote blood circulation. The heart controls blood vessels and the heart beat is the basic motive power to propel blood to circulate in the vessels. Besides, the lung is closely related to the vessels in the whole body. Because through the vessels blood from the whole body converges in the lung and then is distributed to all parts of the body. That is why it is said in Huangdi Neijing that "the lung is connected with all the vessels". It is just the convergence of blood from the whole body in the lung that makes it possible for the lung to assist the heart to promote blood circulation and to accomplish the activity of respiration.
As it is mentioned above, the function of the lung to govern qi in the whole body is closely related to the function of the lung to control respiration. The latter is the base of the former and the former is the aim of the latter. If the lung is abnormal in governing respiration, it will affect the production of the pectoral qi, the regulation of qi activity and the circulation of blood, leading to insufficiency of the pectoral qi, disturbance of qi activity and disorder of qi and blood. The function of the lung to govern respiration will be affected if the production of the pectoral qi is insufficient or if the lung is abnormal in dispersion and descent or if blood circulation is in disorder.
Regulate Water Passage
Regulation here means dredging and readjusting. Water passage refers to the route for transmitting and discharging water. The lung governing the regulation of water passage refers to the function of the lung in propelling, adjusting and discharging water.
After taken into the body, water is absorbed by means of transportation and transformation of the spleen, and then transmitted upwards to the lung. On the one hand, lung-qi, by means of dispersing to the upper and external, transmits water to the surface of the body and transforms it into sweat to be discharged. At the same time, part of the water is excreted through respiration. On the other hand, lung-qi, by means of descending to the lower and the internal, transmits water to the viscera and then to the kidney where it is transformed into urine to be excreted out of the body.
Besides, the descending function of the lung assists the large intestine in transmission, through which part of the water is discharged in defecation. Since the location of the lung is supreme, like a canopy, and because it also participates in the metabolism of water, the lung is called "the upper source of water" in TCM. If the lung fails to regulate water passage due to its disorder in dispersion and descent, it will affect the distribution and discharge of water, leading to production of phlegm, rheum, edema and disorder of urination.