Tiangui is stored in the kidney and changes with the physiological growth and decline of kidney qi. At the beginning of kidney qi, the sky is weak; Kidney qi is abundant, and Tiangui is extremely active; Kidney qi gradually declines, and the sky is exhausted. Su Wen Gu Naive Theory points out that in the general population, women are twenty-seven (14 years old) and exhausted, while women are seventy-seven (49 years old); Men are 28 years old (16 years old), 78 years old (56 years old) and 88 years old (64 years old). Male fertility lasts longer than female fertility. It is also pointed out that the age of 49 for women and 64 for men is not an absolute limit. Some people can still have fertility beyond this age if they keep healthy. In a word, Tiangui is closely related to reproductive function, which originates from kidney and is independent in essence and function. The function of kidney governing reproduction is embodied by Tiangui. At a certain age, with the attenuation of kidney, liver and other five internal organs, Tiangui gradually fails, women's menstruation no longer comes, men's sperm quality becomes worse, and fertility declines. It is the premise of conception and also plays a leading and key role. This is a theory in Neijing, which has been introduced before. This passage shows that from childhood, with the gradual filling of essence in the kidney, Tiangui came into being. The qi transformed by kidney essence is kidney qi. The so-called Tiangui is a subtle substance produced when human kidney essence is filled to a certain age. The ancients called it "invisible water". It is related to the growth, development and reproduction of human body. Prompt the woman to let the pulse pass, the pulse is too strong, and the menstruation will come on time, so that she can get pregnant and urge the man to have normal reproductive function and normal semen. At this time, Yin and his wife lived in harmony and had children. It can be seen that Tiangui is similar to the neuroendocrine hormone that regulates reproductive function in western medicine. The innate essence needs the constant supplement of the acquired essence to maintain the life activities of the body. Therefore, Tiangui also has an important relationship with the acquired spleen and stomach.