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The kidneys are located at either side of the lumbus, 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 bone, manufacture blood, manifest in the hair, open into the ear, and dominate anterior and posterior orifices.

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 essence 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 and manifests in changes in the skin and hair. It flourishes in adolescence and at this time males will have seminal emission, and females the onset of menstruation, reflecting the ripening of the sexual function. In old age the essential qi of the kidney declines, reproductive ability and sexual function finally disappear, and the body begins to wither. The first chapter of Plain Questions says: "At the age of fourteen, a woman will begin to menstruate, her Ren (Conception Vessel) Meridian begins to flow, and the qi in the Chong Meridian begins to flourish. That is why she is capable of becoming pregnant ... At the age of forty-nine, the qi of the Ren Meridian declines, the qi of the Chong Meridian becomes weak and scanty, the sexual energy becomes exhausted and menstruation stops, with the result that her body becomes old and she can no longer become pregnant."

It also says: "At the age of sixteen, the kidney qi of a man becomes even more abundant, his sexual function begins to develop, and he is filled with semen that he can ejaculate. When he has sexual intercourse with a woman, she can have children ... At the age of fifty-six, sexual energy begins to decline, the semen becomes scanty, and the kidney weak, with the result that all parts of the body begin to age. At the age of sixty-four teeth and hair are gone."

These quotations clearly reflect the role played by the kidney in dominating human growth, development and reproduction. This is why the kidney is considered to be "the congenital foundation" and why traditional Chinese medicine attaches such great importance to it.

The essential qi of the kidney includes kidney essence and the kidney qi transformed from kidney essence. The transformation of kidney qi from kidney essence relies on the evaporating function of kidney yang upon kidney yin. Both kidney yin and kidney yang take the essential qi stored in the kidney as their material base. The essential qi of the kidney therefore 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 the 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 nature, 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 equilibrium. Once this 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 sensations of the chest, palms and soles, afternoon fever, night sweats, and seminal emission in males or sexual dreams in females. If kidney yang is deficient, leading to failure in warming and promoting, there may be 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 means that the kidney plays an extremely important role in regulating the distribution of body fluid. Such a function relies on the qi activity of the kidney. When the qi activity of the kidney is normal, then the "opening and closing" of the kidney will also be normal. Water is first received by the stomach, and then transmitted by the spleen to the lung which disperses and descends it. Part of the fluid reaches the kidney where it is further divided into two parts, the clear and the turbid by the qi activity of kidney yang. The clear fluid is transmitted up to the lung from which it is circulated to the zang-fu organs and the tissues of the body. The turbid flows into the bladder to form urine which is then excreted. The function of the kidney dominates this whole metabolic process. If the kidney fails to open and close, then disturbance of water metabolism such as oedema or abnormal micturition will occur.

Receiving qi

Receiving qi means that the kidney assists the lung in its function of receiving and descending the qi. The book Direct Guidebook of Medicine states: "The lung is the governor of qi and the kidney is the root of 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, and the kidney will fail to receive qi, giving rise to shortness of breath and difficult inhalation which is worse after movement.

Dominating bone, manufacturing marrow to fill up the brain and manifesting in the hair

The 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 even 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, whilst deficiency of kidney essence will lead to loose or even falling teeth. 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 thirty-third chapter of Miraculous Pivot therefore states: "The brain is the sea of marrow."

Essence and blood promote each other. When the essence is sufficient, then 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 the surplus of blood on the 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 old age the kidney qi declines and the hair turns white and falls. The tenth chapter of Plain Questions states: "The kidney dominates bone and manifests on the hair".

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 acute. 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 faeces. Although the discharge of urine is a function of the bladder, it also relies on the qi activity of the kidney, as do the reproductive function and the excretion of faeces. 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 in reproduction; and prolonged diarrhoea with prolapse of rectrum or constipation.

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