KIDNEY 1

The Classic states: 'Whenever we treat a disease, we must approach it at the base.' Base here means root or source. Every stream on earth has a source, and every plant has a root. If all murky sediments settle at the source, the downstream waters will naturally be clear and fresh, and if we water a root, it will grow and branches will sprout; these are the laws of nature. The experienced physician, therefore, will always consider the source.

However, the body's source is differentiated into a prenatal and a postnatal aspect. The prenatal source is the kidney; the kidney is associated with the direction north and the phase element water-water being the first offspring, in Taoism, of heavenly Oneness. The postnatal source of the body is the spleen; the spleen is known as the Central Palace and associated with the phase element earth-earth being the mother of every living thing.

From Zhuang Yuanchen, Shujuzi: Inner Chapters (Shujuzi Neipian), Ming Dynasty:

The kidney is the ocean of the human body. Since oceans are situated on a lower level than the earth's streams and rivers, they draw every one of them to form one large body of water. Oceans may appear vast and inexhaustible, yet they still drain off some of their seemingly unlimited supply. One way of drainage is called 'going to ruins,' meaning the water drains down into the earth from where it will not return. The other way of drainage is called 'dwelling with the stars,' meaning the water steams toward the sky and later rains down to earth again, where it dissipates into rivers and streams and eventually returns to the ocean. This is the water that circulates between heaven and earth, always striving to keep an equilibrium between the extreme states of drought and flooding. 

In the context of the human body only the kidney can be compared to the workings of this natural cycle. All the essences and fluids of the body's various pathways pour into the kidney. After the kidney has assembled the essential fluids of the body's vessels, it also experiences two ways of drainage: one way is through the sexual urge which draws the essence downward to the sexual centers; once it exits from here it cannot come back into the system, so this is just like the ocean "going to ruins." The other way is the upward dispersal by way of the suctioning affect of true qi, which draws the body's combined essences all the way up to the flower pond (mouth); from here it moves down through the throat into the stomach, lubricating the five organ networks, nourishing all of the body's pathways, and finally returning to the kidney. This is the microcosmic process of ascending and descending that can be compared to the ocean 'dwelling with the stars.' 

Those who are knowledgeable in the art of nourishing life take care to shut off the lower exit [of jing via ejaculation] while striving to keep the upper pathway [of jing, nourishing the organs and brain] open and unobstructed. In this fashion, there will be a nourishing cycle that is free of leaks. Physical vitality (jing) and mental clarity (shen) will be abundant, nutritive qi (ying) and protective qi (wei) will be strong, inside (water essence) will be sufficient to control fire, and outside (qi) will be sufficient to ward off noxious influences. This is what the art of expelling disease and the art of longevity is all about.