The physiological functions of the kidney are composed of six parts: to govern growth and development, to govern reproduction, to govern water, to govern reception of qi, to produce marrow to enrich the brain and transform blood and to nourish and warm the internal organs.

The kidney stores essence, qi, yin and yang. The growth and development of the body mainly depend on the kidney-essence, kidney-yin and kidney-yang. After birth, the kidney-essence gradually becomes abundant with the development of the body. During the childhood, the abundance of the kidney-essence is marked by dental transition, growth of hair and body. During youth period, the kidney-essence is further enriched, marked by full development of all organs and maturity of reproductive function. 

During the mid-age and prime period of life, the kidney-essence has developed to its peak, characterized by sturdy build, rich vitality and strong tendons and bones. After the mid-age and during the old-age,the kidney-essence gradually declines, and so are the functions of the five zang-organs and six fu-organs, leading to a series of signs of senility, such as loss of hair and teeth, hunch-back, poor hearing and gradual loss of reproductive function. At different stages of life process, the body and its physiological functions vary with the changes of the essence in the kidney. 

That is to say that the essence, qi, yin and yang in the kidney decide the growth and development of the body. If these substances in the kidney are deficient, the growth and development of the body will be affected, leading to a series of pathological manifestations, such as hypoevolutism, feeblemindedness, atrophy and flaccidity of tendons and bones as well as senilism.

The reproductive function includes the development of sex organs, maturity and maintenance of sex as well as fertility which are all closely related with the kidney-essence, kidney-yin and kidney-yang. When the body develops to the period of youth, the essence in the kidney enriches to a certain degree and produces a kind of reproductive substance known as tiankui which can promote the development of sex organs and maintain normal sex function. Consequently man has sperm and experiences seminal emission, and woman has menarche. These physiological changes indicate genitality. 

Man and woman maintain such a sex function and genitality after their middle age. At the period of old age, the kidney-essence gradually declines. Subsequently tiankui becomes exhausted and, accordingly, genitality declines and disappears in the end. This shows that the essence, qi, yin and yang in the kidney play a key role in the genitality and reproduction. If these vital substances are deficient, the genitality and reproduction will inevitably be affected, leading to maldevelopment of sex organs, weakness of sex function and sterility.

To govern water means that the kidney controls the metabolism of water.The metabolism of water is related to many organs like the lung that regulates water passage, the spleen that transports and transforms water, the liver that promotes the metabolism of water and the triple energizer that serves as the passage of water. However, the function of the kidney to govern water is key to the metabolism of water. 

When taken into the stomach, the water is transmitted to the kidney with the collective action of the spleen, the lung, the liver and the triple energizer. The kidney regulates the whole process of transporting and excreting water, playing a very important role in maintaining a holistic balance of water metabolism. It is said in Huangdi Neijing that "the kidney is the door of the stomach. If the door is not properly closed, water will be accumulated inside the body."

 

The function of the kidney to govern water is accomplished with its function of qi-hua (qi-transformation). After absorbed by the spleen, the water is transported to the lung and distributed to all parts of the body with the dispersing and descending functions of the lung. After being used by all the tissues and organs in the body, the water is collected into the kidney. With its function of qi-transformation, the kidney separates the lucid part from the water and elevates it again to the lung to keep certain amount of water inside the body on the one hand, and transforms the rest part into urine to be transported to the bladder and excreted out of the body on the other.

 

The kidney depends on kidney-yang and kidney-qi to transform water. With the steaming and fixating functions of kidney-yang and kidney-qi, the kidney elevates the lucid part of water to the heart and the lung. With the warming function of kidney-yang and propelling function of kidney-qi, the kidney descends the turbid part of water and transforms it into urine to be excreted out of the body. So deficiency of kidney-yang and dysfunction of qi-transformation of the kidney will lead to profuse urine, enuresis and incontinence of urine due to weakness in steaming and fixating on the one hand, and scanty urine, anuria and edema due to weakness in warming and propelling on the other.

 

Reception means acceptance and storage. To govern reception of qi means that the kidney can receive fresh air inhaled by the lung to assist the lung to govern respiration.

The respiratory activity is controlled by the lung. But the fresh air inhaled by the lung from the natural world has to be descended into the kidney to ensure free and smooth respiration and keep the respiration into a certain depth so as to meet the need of various viscera and tissues for fresh air. There is a close relationship between the function of the lung to govern respiration and the function of the kidney to receive qi. The former is prerequisite to the latter and the latter is the basic condition of the former. Normal respiration is accomplished by coordinative action of both the lung and the kidney.

The function of the kidney to receive qi is accomplished with the fixating function of kidney-qi. Only when kidney-qi is abundant and normal in receiving qi can the lung respire evenly and the air passage be clear. If kidney-qi is deficient and the function of the kidney to receive qi is weak, the fresh air inhaled by the lung cannot be descended to the kidney, bringing on various pathological changes like more exhalation and less inhalation, shortness of breath and frequent asthma.

     

Produce Marrow 

The kidney stores essence and the essence stored in the kidney can produce marrow. The marrow includes cerebral marrow, spinal cord and bone marrow. When transported into the bones, the marrow can nourish the bones. That is why TCM believes that "the kidney governs the bones". The spinal cord is connected with the brain, nourishes the brain and maintains the physiological functions of the brain together with the cerebral marrow. That is why the brain is called "the sea of marrow" in TCM. If the kidney-essence is insufficient, the production of cerebral marrow will be reduced, leading to various pathological changes, such as headache, dizziness, amnesia and retard response.

 

The production of blood, apart from the transformation of food nutrients by the spleen and stomach, is related to the kidney-essence. Physiologically the kidney-essence and blood promote each other and transform into each other. So sufficient kidney-essence ensures abundant production of blood while deficient kidney-essence leads to insufficient production of blood. Clinically blood deficiency syndrome due to deficiency of kidney-essence must be treated by the therapy for nourishing the kidney and enriching the essence.

 

Kidney-yin and kidney-yang restrain each other and depend on each other, playing an important role in maintaining the normal functions of the kidney. Kidney-yin, also known as primordial yin or genuine yin, can nourish yin in all the viscera, serving as the source of yin-fluid in the body. Kidney-yang, also known as primordial yang or genuine yang, can warm yang in all the viscera, serving as the source of yang-qi in the whole body. Since kidney-yin and kidney-yang are the sources of yin and yang in the other viscera, the deficiency of kidney-yin and kidney-yang may lead to deficiency of yin and yang in the other viscera. 

Clinically, deficiency of kidney-yin mainly leads to deficiency of liver-yin, heart-yin and lung-yin deficiency of kidney-yang mainly brings on deficiency of spleen-yang and heart-yang. On the other hand, yin-essence and yang-qi in the kidney depend on the essence, yin and yang in the other organ to nourish and foster. Thus insufficiency of yin-essence or yang-qi in the other viscera will eventually involve the kidney or consume yin-essence or yang-qi in the kidney, or prevent the kidney from storing essence, consequently leading to deficiency of essence, yin and yang in the kidney. This is what "impairment of the five zang-organs inevitably exhausting the kidney" means.