The kidneys are located at either side of the Lumbarspine, which is therefore described as "the home of the kidney". The Kidney Meridian connects with the bladder with which it is internally-externally related. Its main functions are: to store essence and dominate human reproduction and development, dominate water metabolism and the reception of qi, produce marrow to fill up the brain, dominate anterior and posterior orifices, open into the ear, manifest in the hair, dominate bones, manufacture blood.

Storing essence and dominating development and reproduction

"Essence" is the material base of the human body and of many of its functional activities. Kidney essence consists of two parts: congenital and acquired. Congenital essence is inherited from the parents, and acquired essence is transformed from the essential substances of food by the spleen and stomach. The congenital and acquired essences rely on, and promote, each other. Before birth, congenital essence has prepared the material base for acquired essence. After birth, acquired essence constantly replenishes congenital essence. Of the two, acquired essence is the most important.

The function of the kidney in reproduction and development relies entirely on kidney qi. In other words, the ability to reproduce, grow and develop is related to the prosperity or decline of the essential qi of the kidney.

In childhood, the essential qi of the kidney develops gradually. During adolescence, it flourishes. There will be ripening of sexual function in the form of seminal emission in males and the onset of menstruation in females. In old age, essential qi of the kidney declines, reproductive ability and sexual function disappear and body begins to wither.

Due to its function of dominating human growth, development & reproduction, it is considered to be " the congenital foundation".

The essential qi of the kidney involves both kidney yin and kidney yang. Kidney yin is the foundation of the yin fluid of the whole body, which moistens and nourishes the zang-fu organs and tissues. Kidney yang is the foundation of yang qi of the whole body, which warms and promotes the functions of the zang-fu organs and tissues. Yin and Yang are both lodged in the kidney, which was therefore said to be "the house of water and fire" by the ancients. According to their major, essence is yin, and qi is yang. So kidney essence is sometimes called "Kidney Yin" and kidney qi is sometimes called "Kidney Yang".

Kidney yin and kidney yang both restrict and promote each other in the human body so as to maintain a dynamic physiological equilibrium. Once this equilibrium is disrupted, pathological changes due to imbalance of yin and yang in the kidney will manifest.

If kidney yin is deficient through exhaustion, it will fail to control yang, which becomes hyperactive. Typical symptoms are heat sensation of the chest, palms and soles, afternoon fever, night sweat and seminal emission in males and sexual dreams in females. If kidney yang is deficient, leading to failure in warming and promoting, there maybe symptoms such as lack of spirit, coldness and pain in the lumbar region and knees, aversion to cold, cold limbs, and impotence in men and frigidity and infertility in women. If kidney deficiency is not accompanied by obvious cold symptoms, it is usually called "deficiency of kidney qi" or "deficiency of kidney essence".

Dominating water metabolism

Dominating water metabolism is the function of kidney, which regulates water circulation and helps maintain fluid balance in the body. Water metabolism of the human body has two aspects. One is to disseminate body fluid which has been derived from food essence and has nutritive and nourishing functions to the tissues, Zang-fu organs throughout the body. The other is to discharge from the body turbid fluid produced by all the Zang-fu organs after metabolism. All these rely mainly on the kidney's function of regulating water metabolism.

Lungs participate in maintaining water metabolism by sending the water from food essence down to kidney & distributing clear body fluid after receiving it from the kidneys. The kidney has the function of controlling the opening and closing the gate of water. Opening the gates makes the water excreted, while closing the gate helps retain the water needed by the organ. If the function of regulating water metabolism is normal, opening or closing the gate of water will be properly regulated, resulting in normal urination. If this function is abnormal it can lead to inappropriate opening or closing of the gate of water. When closing occurs more often than opening, oliguria and edema occur. But when the reverse occurs, polyuria and frequent urination are apparent.

Receiving qi Receiving qi means that the kidney assists the lung in its function of receiving and descending the qi. In other words, respiration depends not only on the descending function of the lung, but also on the kidney's function of reception and control. Only when the kidney qi is strong can the passage of qi in the lung be free, and the respiration smooth and even. If kidney qi is weak, the root of the qi is not firm, kidney will fail to receive qi, giving rise to shortness of breath and difficult inhalation that is worse after movement.

TThe kidney stores essence, which produces marrow. The marrow develops in the bone cavities and nourishes their growth and development. When kidney essence is sufficient, the bone marrow has a rich source of production and the bones are well nourished, firm and hard. If the kidney essence is deficient, it will fail to nourish the bones, leading to weakness and soreness of the lumbar region and knees, weakness or atrophy of the feet, and maldevelopment. Since the kidney dominates bone, and the teeth are the surplus of bone, ample kidney essence will result in strong healthy teeth. While deficiency of kidney essence will lead to loose or even falling teeth.

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The marrow consists of two parts: spinal marrow and bone marrow. The spinal marrow ascends to connect with the brain, which is formed by the collection of marrow. "The brain is called the sea of marrow". Essence and blood promote each other. When the essence is sufficient the blood will flourish.

The nourishment of the hair is dependent on a sufficient supply of blood, but its vitality is rooted in the kidney qi. The hair, therefore, is both surplus of blood on one hand, and the outward manifestation of the kidney on the other. Growth or loss of hair, its lustre or withering, are all related to the condition of the kidney qi. During the prime of life, the kidney qi is in a flourishing state and the hair is lustrous; in older age the kidney qi declines and the hair turns grey and falls.

Opening into the ear and dominating anterior and posterior orifices The function of the ear in dominating hearing relies on nourishment by the essential qi of the kidney. The ear therefore pertains to the kidney. When the essential qi of the kidney is sufficient, the ear is well nourished and hearing is accurate. When the essential qi of the kidney is deficient, it will fail to ascend to the ear leading to tinnitus and deafness.

"Anterior orifice" refers to the urethra and genitalia, which have the function of urination and reproduction.

"Posterior orifice" refers to the anus, which has the function of excreting the feces.

Although the discharge of urine is a function of bladder, it also relies on kidney qi, as do the reproductive function and the excretion of feces. Decline or deficiency of kidney qi, therefore, may give rise to frequency of micturition, enuresis, oliguria and anuria, seminal emission, impotence, premature ejaculation and infertility and prolonged diarrhea with prolapse of rectum or constipation.

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Note: From the kidney's physiology and pathology stated above we can see that the kidney in TCM basically Includes the functions of the urinary system, reproductive system, and part of the endocrine system and nervous system.