The Neijing‧Shanggu Tianzhen Lun mentions that "the kidney governs water and receives the essence of the five zang and six fu organs to store it," meaning that the kidney stores the nutritional essence absorbed by the stomach and intestines for future use. When the body's daily needs are insufficient, it can be extracted and utilized. This is quite similar to the concept of "the liver storing blood," where the liver stores a large amount of glycogen, fats, vitamins, and minerals for the body's emergency needs. The difference between the two lies in:

• the kidney stores innate essence, also known as "kidney water" or "kidney yin." "Essence" is a general term for various essential substances, primarily used for producing gametes required for reproduction, constructing or repairing the body's tangible tissue cells, and weaving the brain's neural networks. The substance of "essence" includes proteins, nucleic acids, minerals, trace elements, etc.;
• the liver stores acquired nutrient-blood, which is used for the body's daily energy production needs. The substance of "nutrient-blood" is mainly glucose and fats.

The kidney itself is small and cannot store much "essence." These "kidney essences" are actually stored in various forms throughout the body, and the kidney mobilizes them through "kidney qi." The mobilization of kidney essence by "kidney qi" essentially involves various hormones secreted by endocrine glands. For example, the growth hormone secreted by the pituitary gland can stimulate the development and maturation of various tissues and organs during childhood, requiring the mobilization of a large amount of kidney water for the construction of various tissues. Another example is the adrenal gland, which regulates the metabolism of water, electrolytes, sugars, proteins, fats, and other substances in the body. When the body faces stress, it is activated and mobilizes many stored resources for emergency use.