The Physiological Functions of Kidney Essence (jing)
	Promote reproduction: Kidney essence is necessary for maturation of the reproductive system and is the primitive material needed for embryo development. When there is mutual nourishment between acquired and congenital jing, kidney essence becomes gradually more abundant as a child ages. During puberty, the kidneys create a substance for promoting sex functions, which is called "tian-gui" from the abundance of stored kidney essence. As a result, a male will produce sperm and a female will begin menstruation signaling that the sex organs are mature and capable of reproduction. Thus the development of the sex organs is mostly dependent on the kidneys, which are the root of reproduction.
	Promote growth and development: Birth, growth, development, aging, and death are natural human processes of that are closely related to the abundance and decline of kidney essence. In childhood when there is an abundance of kidney essence stored in the kidneys, the body is able to undergo changes such as getting adult teeth and growing hair. During puberty, the prime time of life, the kidney essence is even more plentiful resulting in a healthy body with stronger bones and tendons. In old age, the kidney essence becomes depleted and manifestations of aging appear, such as stiffness of joints, bones and tendons, loose teeth and hair loss. Thus the amount of stored kidney essence will determine how quickly the natural processes of birth, growth, development, aging, and death occur. The longer a significant amount of kidney essence is stored in the kidneys, the healthier an individual will be.
	Promote the transformation of blood: Ancient TCM classics commonly mention that "Kidney essence infuses into the bones and transforms into marrow;" "If the bone is strong and marrow is abundant, blood and qi will follow;" and "Essence cannot be excreted from the body and excess essence is stored in the liver as pure blood." "Ruling the bones" is one of the major functions of the kidneys and one reason is that kidney-essence is necessary for blood production. Kidney essence is transformed into the marrow that occupies the bone cavities where it changes into blood. The liver also plays an important role in the process of transforming essence into blood because it stores the excess blood that is produced from the marrow. When either the kidneys or liver are under functioning, the production of blood will be affected and individuals can acquire a blood-deficiency syndrome.
	Prevent the invasion of exogenous evils: Exogenous evils in TCM are disease-causing factors that invade the body and are classified into six different types: wind, heat, cold, dampness, dryness, and summer heat. An abundance of kidney essence helps promote our vitality and the function of protective-qi thereby strengthening the body's resistance to these evils so they cannot invade and cause disease. In contrast, when kidney essence is depleted, weakness in vitality and resistance to protective-qi result, meaning that pathogens can invade easily. The ancient classics claimed that, "When essence is stored properly, no seasonal disease occurs in spring" and "When essence is not stored properly in autumn, then seasonal disease will occur in spring." Kidney essence's ability to prevent exogenous evils invading the body is closely linked with the role of vital energy (qi) in humans. As a result, sayings such as: "When there is vital energy (qi) inside the body, no evils can interfere" and "When evils accumulate inside the body, there is a weakness in vital energy," are common.