A substance produced when the essence in the kidney is filled to a certain extent, which can promote the maturity of human reproductive organs and maintain reproductive function. "Heaven" means that it comes from nature, and "Gui" means that its essence belongs to Guishui in the dry weather, that is, Yin in the yang. Zhang Jingyue said, "A man with a husband and wife is the water of heaven, and his name is dry. ..... Therefore, Tiangui, Yinqi Tianer. Gasification into water, because the name is expensive, ... In the human body, it is called Yuan Yin, also known as vitality "(Jing Lei Tibetan Elephant Class).