The kidney's primary function is storing and controlling 'essence', or "jing". "Jing" is the essence of "qi" and the basis for body matter and functional activities. There are two types of "jing", congenital jing and acquired jing, which are stored in the kidney and known as kidney jing. Unlike qi, "jing" circulates in long cycles (seven years for females and eight years for males) governing developmental stages. The function of "jing" is to promote growth, development and reproduction, provide the basis for kidney qi, produce marrow, and provide the basis for jing, qi and shen (mind).