The Kidneys' principal role from a Chinese Medicine standpoint is to store and release an Essence called "Jing" (精). Jing is both inherited from one's parents (i.e. the so-called "Pre-Heaven Essence") and acquired via food and various forms of stimulation like exercise or study ("Post-Heaven Essence"). The Kidneys are the root of Pre-Heaven Essence, which can be compared to a Chinese Medicine version of DNA.
The role of Essence or Jing is to govern one's growth and development as well as reproduction. As such someone with a strong Jing will for instance have an unimpeded development process during their childhood and teenage with good bone structure and good teeth. By the middle of one life, Jing will progressively decline. This is the main Chinese Medicine explanation for menopause syndrome .
The Kidneys are also considered to be the root of Yin and Yang for all other Organs and all the Yin and Yang energies of the entire body. Therefore, Kidney Yin and Kidney Yang are also called ‘Primary Yin’ and ‘Primary Yang’. Kidney Yin provides the material foundation for Kidney Yang, while Kidney Yang provides the driving force that transforms Kidney Yin. Since they have the same source, they are interdependent. Therefore, the reduction of one leads to a Deficiency in the other.
Another important role of the Kidneys is to control the metabolism of water and notably to separate it into 'clear' and 'impure' parts. The latter is expelled from the body as urine and the rest is distributed to other Organs.
The Kidneys open into the ears and lower Orifices. This means that someone with a hearing problem or a urinary tract disorder (like incontinence) might have Deficient Kidneys. The kidneys' condition can also be reflected in the hair. As such, someone who loses their hair might also find the cause in their Kidneys.
The Kidneys stores Essence which is composed of both Pre-Heaven Essence and Post-Heave Essence. Pre-Heaven Essence is inherited from both parents at the time of conception and defines the constitutional strength and resistance of the body. It can be compared with a Chinese Medicine version of DNA. Pre-Heaven Essence can not grow, but it can be supplemented by Post-Heaven Essence originated from the Grain Qi produced by the Spleen.
Post-Heaven Essence travels throughout the body to support physiological activities. When there are excessive amounts of Post-Heaven Essence, it is stored in the Kidneys for future use. When there is insufficient Essence in the body, the Kidneys' reserve is used. If this process continues, the inherited Pre-Heaven Essence will also get used and it might cause Kidney Essence Deficiency.
Essence rules the processes of reproduction, growth, development, sexual potency, conception, pregnancy, menopause, and ultimate physical decay of the body. It is the root for constitutional strength, vitality, and immune system.
During childhood, Essence is responsible for the growth of bones, teeth, hair, brain, and sexual maturation. In maturity, it controls reproductive function, fertility, conception, and pregnancy. Kidney Essence is also the foundation of sexual life, and the material root for the creation of sperm in men and ova and menstrual Blood (also called Tian Gui) in women.
In case of Essence Deficiency, some symptoms appear such as improper bone formation, retarded mental or physical growth, delayed sexual maturation, premature aging, infertility, sterility or impotence. Essence declines with age, therefore over time physical and mental abilities decline such as sexuality, fertility, hair growth, skin smoothness, memorization, and so forth.
The Kidneys control water circulation and balance Body Fluids in the body. They are the root of water metabolism in the body, strengthened by the Spleen's function of transforming and raising Fluids and the Lungs' function of circulating and descending Fluids to the Lower Burner.
When it comes to water, the Kidneys' task is to separate it into 'pure' and 'impure' parts. Here Kidney Yang plays the key role. It is like a gate. When opened, impure Fluids get excreted. When closed, pure Fluids get retained for other Organs. When the Kidneys' gate function normally, the body produces normal urination. However, scanty dark urine and edema occur when the gate closes more than necessary. On the other hand, profuse and frequent pale urination appear when the gate opens too often.
Some other Organs that deal with Fluids receive assistance from the Kidneys. The Bladder receives the Qi from the Kidneys to store and transform urine. The Intestines receive Kidney Yang to filter pure Fluids. The Spleen also receives Kidney Yang to help transforming and transporting Body Fluids.
According to Chinese Medicine, Marrow (Sui-髓) does not refer to just the bone marrow that we're familiar with in Western Medicine. The Chinese Medicine Marrow also includes bones, bone marrow, the brain, and the spinal cord. The Marrow nourishes them and defines their strength, development, and recovery.
Essence stored in the Kidneys plays a significant role in the creation of Marrow. If it is insufficient during childhood, it causes soft, maldeveloped, and weak bones or a late closure and softness of the fontanel. For adults, lack of Marrow causes weak legs and knees, fragile bones, or stiffness of the spine.
The Lungs rule respiration and the Kidneys help this process by receiving the Clean Air from the Lungs and holding it down. In case of Kidneys Deficiency, Qi can not travel down and it "rebels" up, creating congestion in the chest. It results in breathlessness, chronic asthma, panting, or difficult breathing.
The Ears condition reflects the state of the Kidneys. In the case of Kidneys Deficiency, hearing problems appear such as deafness, frequent or chronic earaches, and tinnitus.
The urethra, the anus, and men's spermatic duct belong to the lower orifices. Kidney Qi plays a role in the Lower Burner's Body Fluids regulation. It also controls the opening and closing of these orifices. In the case of malfunctioning Kidney Qi, there will be urinary incontinence, diarrhea, spontaneous defecation while eating or spermatorrhea.
The Kidney condition can be reflected by the body hair, especially head hair. It is because Blood nourishes the hair and Blood is partly created by the Essence stored in the Kidneys. When the Kidneys have adequate Essence, the head hair is moist, smooth, thick, abundant, and stays at its original color. In case of Kidney Essence Deficiency, the head hair is withered, thin, prematurely grey. It can also cause frequent loss of hair or baldness.
According to Chinese Medicine, Willpower (Zhi - 志) resides in the Kidneys. Zhi is the driving force to get things started and complete them. It creates power, skill, and ability. It is motivation and determination, the deep inner power and endurance of a person. Zhi gives purpose to life and supports the person to achieve their goals.
The combination of a strong Zhi (housed in the Kidneys) and Shen (housed in the Heart) creates a personality full of charisma, vigor and determination. If the Kidneys are strong, the Willpower is strong as well. Our Mind will also stay focused to complete the goals it sets. However, if the Kidneys are weak, Willpower weakens and the Mind will be easily distracted and discouraged.
The right Kidney was originally considered to be the Gate of Life (also called Gate of Vitality). As such it was thought to supply Yang functions in the body while the left Kidney was both Yin and Yang. Nowadays, both Kidneys are said to supply Yang functions, which is the foundation of Original Qi (Essence transformed into Qi). It is also the root of body Fire which supplies needed Heat to all the internal Organs in order to support their daily functions.
It plays an important role in warming the Stomach and the Spleen to assist digestion. It also warms the Lower Burner and the Bladder to assist transformation and excretion of Body Fluids. Finally, it supports sexual function and the Uterus to aid fertility, puberty, menstruation, and sexual performance. If impaired, symptoms such as poor digestion, diarrhea, tiredness, feeling cold, cold limbs, edema, impotence, infertility, lowered sexually drive and leukorrhea can appear.
Fear, fright, and paranoia can result from a Kidney imbalance. Inversely, excessive or prolonged exposure to any such emotion may injure the Kidneys (for instance if someone lives in an environment where they fear for their life).
The Kidneys loathe Dryness as it is the Organ that rules water metabolism. Kidney Yin can be damaged by prolonged exposure to dry weather or Internal Dryness that results from profuse and continued Body Fluids loss such as sweating or diarrhea. Stomach Deficiency or excessive smoking can also cause Internal Dryness.