Kidney stores essence

Growth, development, and reproduction rely on the essential qì stored by the kidney. Kidney essential qì is derived from the reproductive essence of the parents (congenital essence), out of which the embryo develops. After birth, it is gradually nurtured by the essence of food (acquired essence) and reaches fullness in puberty, when men are able to produce semen and women begin menstruating—important signs of the reproductive function coming to maturity. In old age, the kidney essential qì weakens so that the reproductive function gradually fades away, and the body degenerates. Plain Questions (素问 sù wèn, shàng gǔ tiān zhēn lùn) states, [In the male] at the age of two eights [i.e., sixteen] kidney qì is exuberant, the heavenly tenth tiān guǐ arrives, essential qì flows forth, yīn and yáng are in harmony, and [he] can beget offspring... At the age of seven eights [i.e., fifty-six] heavenly tenth is exhausted, essence diminishes, the kidney grows weak, and the body loses its tone; at eight eights, the teeth and hair fall out. [In the female] at the age of two sevens, heavenly tenth arrives, the controlling (rèn) vessel flows, and the thoroughfare vessel fills, the menses come according to their times, and [she] can bear offspring... At seven sevens, the controlling (rèn) vessel empties, the thoroughfare vessel weakens, heavenly tenth is exhausted, the passages of the Earth are cut, the body deteriorates, and she can no longer bear children. In this quotation, tiān guǐ is explained as meaning viability of the reproductive function. See heavenly tenth.