Physiological Functions of the Kidney
1) Storing Essence, Dominating Growth, Development and Reproduction
The essence is one of the most valuable materials of the human body. This concept is extensively used in TCM. Its implications consist of two aspects. The first one refers to the basic material constituting a human body, which forms the viscera, tissues, skin,hair, tendons and muscles of the human body. Only on the basis of the "essence", can the embryo grow and develop, and can the life come into being. The second one implies that the essence is the material basis for growth, development and the various physiological activities of the human body. During a person's entire life, from childhood, youth,prime of life, to old age, material metabolism has always been continuously carried out producing various kinds of essential materials which are consumed through physiological activities. The "essence" therefore, is a generalization of the essential material and nutritional composition of the body.
Storing essence is the main physiological function of the kidney. It preserves and stores the essence, thus Chapter 9 in the Plain Questions states: "The kidney dominates dormancy, and is the basis of preserving the function and the place where the essence is stored. " The essence stored by the kidneys consists of two parts, congenital and acquired. Congenital essence is inherited from both parents, it is the reproductive essence of parents. It forms original substance in the development of the embryo, and is the basic function of reproduction. This essence determines the individuals constitution, physical development, and to some extent, eventual life-span. After birth, the congenital essence is stored in the kidney, becoming part of the kidney-essence. It will be the material base of reproduction.
Acquired essence denotes the essence of the viscera, and is derived from the essential substances of food and drink. As it is transformed from the essential substances of diet by the spleen and stomach following birth, it is then called "acquired essence. " Its nutrient substance maintains the life activities of the body. The acquired essence is distributed to the five zang-viscera and six fu-viscera, skin, hair, and tendons to provide nourishment.The surplus to what is acquired for metabolism, is then transported to the kidney and forms a part of the kidney-essence.
The formation of acquired essence relies on the replenishment of vitality from the congenital essence while congenital essence, in turn, relies on the supplement and rein forcement of acquired essence. Both rely on the supplementation and promotion of each other in order to ensure sufficiency of the kidney essence. Although their resources are different, both essences are stored in the kidney, and are in fact inseparable. Therefore the prosperity or decline of the kidney-essence is closely related to the abundance of acquired nutrition, apart from its relationship to the congenital condition.
The physiological function of the kidney in storing essence is extremely important. It is the foundation for the preservation and the production of the human body. The kidneyessence can be changed into the vital energy, called kidney-qi, and is responsible for the continuing health of the body. Hence, the prosperity and decline of the kidney-essence determines the conditions of growth, development, and reproduction of the human body.
The kidney dominates reproduction: Reproduction functions in two ways: sexual function, and reproductive ability. Traditional Chinese medicine believes that the reproductive function is related primarily to the kidney. This is due to the kidney's ability to store essence, and provide the material basis for the origin of life. On the other hand, the kidney-essence can also promote genital development, and maturation of the sexual instinct, thus maintaining the exuberant reproductive function. Following birth, as the congenital essence is nourished by the acquired essence, the essential qi of the kidney gradually develops, and reaches fullness in puberty. This causes the human body to produce a substance which promotes a process of maturity of the reproductive function, and is known as "tiangui" in TCM.
"Tiangui" refers to a kind of substance necessary for promoting the development and maturity of the gonad. It is derived from the kidney-essence of the parents, and is transformed mainly by congenital essence. It also receives the nourishment and supplement of the acquired essence. Tiangui functions in promoting growth, development, and reproduction in the human body. When it reaches a certain level, men are able to produce sperm, and women reach menstruation. Soon, the reproductive function for both males and females mature and the ability to reproduce is fully developed. With age increase, the kidney-essence may change from abundant to insufficient. Similarly, tiangui gradually decreases and eventually fades away.
Prosperity or decline of kidney essential qi directly affects the reproductive function of the human body: Should there be a disturbance of the kidney's physiological function in storing essential qi, there may result disorders of sexual function and reproductive ability. In treating these conditions, formulas and herbs for kidney reinforcement are often adopted.
The kidney dominates growth and development of the human body: This process is
closely related to the physiological function of the kidney's ability to store essence, and is affected directly by the prosperity or decline of the essential qi of the kidney. In childhood, the essential qi of the kidney develops gradually allowing the body to grow and develop rapidly with exuberant vitality. By the age of seven or eight the baby teeth are replaced by permanent teeth, and hair grows longer and faster. During youth, and in the prime of life stage, tiangui reaches a threshold. The reproductive organs reach maturity,the essential qi of the kidney flourishes, and the human body is healthy, its bones and teeth strong, its spirit vigorous, and hair shiny. In old age, the essential qi of the kidney declines, the reproductive ability and sexual function disappear gradually, and the body degenerates. This manifests in grey hair, looseness of teeth, humpback, hearing impairment, pale complexion, and difficulty walking.
While the prosperity and decline of the essential qi of the kidney determine growth,development and reproduction, an insufficiency of kidney-essence may lead to mal-development of bones and intelligence. Shortness, and slight figure, low intelligence, or five types of retardation (retardation in standing, walking, hair-growing, teeth eruption, and difficulty of speech), and five kinds of softness (softness of hand and feet, head, nape,muscle, and softness in mastication) may result from lack of kidney-essence.
In adults, excessive impairment of the kidney-essence may cause premature senility,characterized by loss of hair, looseness of teeth, dizziness, tinnitus, poor memory, decline of sexual function, etc. In clinical practice, the method of reinforcing kidney-essence is usually used. In addition, under the guidance of the theory of the kidney dominating growth and development, physicians of successive dynasties have attached a great importance to the regulation and reinforcing of the kidney as a preventative method of senility.Presently there is much research on the use of herbs for anti-aging, and within this, great importance is being placed upon herbs which tonify the kidney.
The kidney stores the essence which then transforms itself into kidney-qi. Thus kidney-essence and kidney-qi are mutually reciprocal. This is why the two together are often called essence-qi. The essential qi of the kidney is the foundation of life activities and plays an extremely important role in all physiological activities of the human body. Thus, the kidney is considered to be "the congenital foundation. " Relating this to the nature of yin and yang, the essence pertains to the substance, being yin, while the qi to function, being yang. The two are also called the primordial yin and primordial yang. Kidney-yin is the foundation of the yin-fluid of the entire body. This moistens and nourishes the viscera, tissues and organs. Kidney-yang is the foundation of the yang-qi of the entire body, which functions in warming and promoting the growth of tissues and organs in the human
body. Yin and yang are both rooted in the kidney, and the kidney is therefore said to be "the house of water and fire. "
Under normal conditions, kidney-yin and kidney-yang restrict, promote, and depend on each other in order to maintain the dynamic balance of yin and yang. Once this balance is disrupted, pathological changes caused by the imbalance of yin and yang occur. If the kidney-yin is deficient, it will fail to control yang, and becomes the syndrome of"flaring of fire due to yin deficiency. " Such manifestations as hot sensations in the five centers, afternoon fever, night sweats, soreness and weakness in the lumbar region and knees, vertigo, tinnitus, nocturnal emission in males or sexual dreams in females, result. If the kidney-yin is deficient, it will fail to nourish all the other internal organs, giving rise to disorders of imbalance of yin and yang. For example, deficiency of kidney-yin may lead to a deficient liver-yin, deficient heart-yin, deficient lung-yin, etc.
If the kidney-yang is deficient, it will lack in its warming and promoting function,resulting in deficiency of yang syndromes, such as symptoms of lack of spirit, coldness and pain in the lumbar region and knees, aversion to cold, cold limbs, impotence, frigidity, and infertility in women, etc. Simultaneously, deficiency of kidney-yang may fail to warm and promote all other viscera, and other disorders may manifest. An example is if the spleen is not warmed by the kidney-yang, it will manifest in the syndrome of deficient spleen-yang, leading to dysfunction of transportation and transformation of the spleen, finally resulting in deficiency of both the spleen and kidney. Conversely, if disharmony of yin and yang in other viscera lasts long,it will involve the kidney and consume the essential qi of kidney,leading to disharmony of yin and yang of the kidney. Therefore,there is
a saying "The prolonged diseases finally affect the kidney. "
2) Dominating Water Metabolism
Kidney dominating water metabolism implies that there is a physiological function on the kidney's part in managing and regulating the water metabolism of the human body.Although this regulation is also in relation to the lung, spleen, liver, kidney and triplejiao, etc. , the kidney is the leader in water metabolism. Its regulating function is inherent in the process from beginning to end.
The kidney's capacity to fulfill this function is mainly attributable to the transformative function of kidney-qi. Here, "the transformative function of qi" refers to the evaporation and fractional distillation of kidney-yang. Kidney-yang evaporates the body fluids transforming them into the qi, which in turn is accumulated into water, thus ensuring ascendance, descent, exit and entrance, as well as distribution and excretion of water in the human body. Normal water-fluid metabolism will therefore be maintained. Concretely speaking, the functions of the kidney manifest in the following three aspects:
The first manifestation is in the sending up of the clear, and the sending down of the turbid. The body fluid is divided into two types, the clear and the turbid during the metabolism of water-fluid. The clear refers to the part of the fluid which contains the composition of nutrition, while the turbid implies that part of the fluid containing metabolic wastes. The basic law of regulating water metabolism in the human body revolves around ascendence of the clear and descendence of the turbid. Metabolized water is received, digested, and transported by the stomach and spleen, then the essential part is conveyed to the lung which sends the clear upward, and the turbid downward to the kidney through the dispersing and depurative descending function of the lung. The part of the fluid reaching the kidney is also called "the turbid fluid. " This fluid still contains the clear composition, and is further divided into the clear and the turbid during evaporation and distilling functions of kidney-yang. The clear fluid is transported up to the lung from which it is circulated again to the entire body. Such physiological process is called the function of the kidney in sending the clear upward. The turbid flows into the bladder forming urine which is then excreted, and such process is known as "function of the kidney in'sending the turbid downward. " Hence, under the transformative function of the kidney qi, sending up the clear and descending down the turbid promotes metabolism of the body fluids so as to maintain the balance of water metabolism.
The second function of the kidney is to control the opening and closing of the bladder. The main function of the bladder is to store and discharge urine, which is closely related to the transformative function of kidney-qi. The storage of urine, as well as the discharge of urine, is reliant upon the retentive ability of the kidney-qi. Such function is known as "the kidney controls the opening and closing of the bladder. " Opening excretes urine outside the body, while closing enables the body fluid needed by the organs to be retained within the body. This allows for a relatively constant water-fluid level to be maintained.
Additionally, the kidney plays an active role in promoting the functional activities of the lung, spleen, liver and triple-jiao, etc. Since kidney-yang is the root of all yang in the body, and the motive power of the functional activities of the viscera, it is only by relying on the warming and evaporating function of the kidney-yang that the spleen can transport and transform water-dampness, the lung regulate water passage, the liver promote flow of water normally, the triple-jiao regulate the passages of water, and the bladder control the opening and closing. Therefore, all internal organs can faithfully discharge of their own duties, and be harmonious in the balance of water metabolism.
However, if the kidney is diseased, it fails to control the water metabolism, giving rise to dysfunction of water metabolism. Such manifestations as oliguria and edema arise.
If the kidney-yang is deficient, it fails to warm and evaporate the body fluids, forming an increase in clear urine, polyuria, and frequency of urination, etc.
3) Controlling the Reception of Qi
That the kidney controls the reception of qi implies that the kidney has a physiological function in receiving clear qi inhaled by the lung, and regulating respiration in order to prevent shallow breath and to maintain a normal exchange of gases outside and inside the body.
The respiratory function depends on the function of kidney-yin in its receiving of qi,although it is controlled by the lung. The clear qi inhaled by the lung must descend to the kidney. Here it is received and absorbed by the kidney. In this way, the depth, smoothness, and evenness of the respiratory movement can be maintained, ensuring a normal exchange of gases both inside and outside the body. Only when the lung and kidney are in coordination, will the respiratory function be normal. This function of the kidney is the incarnation of its preserving function within the entire respiratory movement. If this function is normal, respiration will be even and harmonious. If it is diminished, the clear qi inhaled by the lung ascends adversely, and cannot descend, creating shortness of breath, rapid breathing with asthma, more expiration than inspiration, dyspnea, etc.
These manifestations are frequently seen in chronic bronchitis, pulmonary emphysema,and pulmonary-cardiac disease cases. For the case of "failure of the kidney in receiving qi," the reinforcement of kidney and its reception of qi is usually the principle treatment.
