Tiangui is the nutrient substance stored in the kidney to promote genital development and maintain the reproductive function when essential qi in the kidney amounts to a certain extent. Specifically, the production of tiangui is based on the successive accumulation of kidney essence and kidney qi when a person is born. Tiangui promotes the growth and maturation of reproductive organs and maintains the reproductive mechanism of the human body. When tiangui comes, a female will experience regular menses and ovulation and a male will experience sperm emission, indicating the maturation of their sexual organs and their capability to reproduce offspring. When tiangui goes, atrophy of reproductive organs will occur and their fertility declines, indicating the arrival of advanced years.