Traditional Chinese Medicine (TCM) holds that the "kidney" belongs to "water" in the Five Phases, governing the regulation of body fluids. Neijing-Shanggu Tianzhen Lun states "the kidney governs water," and Neijing-Nitiao Lun mentions "the kidney is the organ of water, responsible for body fluids." The "water" managed by the kidney is also related to various fluids secreted by exocrine glands, as stated in Nan Jing-Nan 49: "The kidney governs fluids: entering the liver as tears, the heart as sweat, the spleen as saliva, the lungs as mucus, and itself as saliva." This means that the production of tears, sweat, saliva, and mucus all originates from the "water" controlled by the kidney. This aligns with modern medical findings that the kidney itself regulates blood pH, filters and excretes metabolic waste, and maintains an appropriate amount of water in the body, ensuring a clean and suitable fluid environment for cell survival.
The "water" in the human body can be roughly divided into fluids with lower functional and nutritional value, such as sweat, urine, and mucus; and fluids with higher functionality, such as lubricating fluids secreted by mucous membranes and digestive juices secreted by the stomach and intestines; or fluids with higher nutritional value, such as solutions containing glucose, nucleic acids, amino acids, proteins, fatty acids, minerals, or trace elements. The nourishing "kidney water" or "kidney yin" typically refers to the latter.
General bodily functions can operate by burning glucose for energy, but reproduction requires the production of a large number of gametes (sperm or eggs), growth necessitates the generation of a large number of cells and the construction of many tangible tissues, and learning and memory require the weaving of complex neural fiber networks in the brain. The formation of these all consumes a large amount of basic materials—namely, "kidney water."
At first glance, the functions of urination, reproduction, and growth and development seem unrelated, so why are they grouped together in the kidney system in TCM? Because they are all closely related to "water." The function of urination primarily regulates the body's water balance, while the nurturing and maintenance of life primarily depend on body fluids. All cells of terrestrial life (including humans) are enveloped in a fluid similar to seawater. Water is the cradle of life, and the reason terrestrial life can survive is that it can carry water with it. Moreover, the main purpose of life on Earth is survival and reproduction. To this end, many lower life forms rely on potent reproductive capabilities, producing tens of thousands of gametes at once, the generation and dissemination of which require "kidney water" and environmental fluids. Higher animals, such as humans, dominate the Earth due to their brains, which are also nourished by "kidney water," as seen in "Kidney Govern Bones and Generate Marrow."