In modern medical science, the functions of the kidneys are maintaining the balance of water and electrolyte, regulating acid-base concentration, filtering the blood and excreting metabolic wastes as urine. Such functions are reduced to the function of controlling water in the traditional Chinese medical science. The book “Su Wen” (素问), one of the traditional Chinese medical classics, says, “The kidneys are organs of water, and control bodily fluid.” The active substance of the kidneys is called “kidney-Qi” (肾气), and it acts as prerequisite activating catalyst in the metabolism of water. With the catalysis of the kidney-Qi, needless water and metabolic wastes are transformed into urine and expelled out. If the kidney-Qi is deficient, the kidneys can’t maintain enough water, so the frequency of urination and urinary output will be much increased. If the kidney-Yang (肾阳, the substance is deemed to warm the body and maintain sexual functioning) are deficient, much needless water will be blocked, so urine will be few and the person may suffer from edema.

In the traditional Chinese medical science, respiration needs not only the functioning of lungs, but also the functioning of kidneys. In the process of respiration, though lungs act on both of exhaling and inhaling, but the inhaling function of lungs needs the maintaining function of kidneys. If the kidney-Qi is deficient, the maintaining function of kidneys will be impaired, and the person will suffer from short and shallow breathing, and he will always wheeze as he does intense movements.

The emotion of fear is deemed to originate from the kidneys. Why does someone incontrollably urinate when he is very frightened? The cause is that frightening emotion seriously disorders the functioning of controlling water of the kidneys, so that the kidney-Qi abnormally moves downwards, and then urine is leaked without control. Accordingly, the book “Su Wen” says, “The emotion of fear can impair the kidneys.”

In the traditional Chinese medical science, saliva is distinguished into two kinds. The thin saliva (涎) which is secreted by the parotid gland originates from the spleen, and the thick saliva (唾) which is secreted by the sublingual gland originates from the kidneys. The thick saliva is deemed to be transformed from the kidney-Qi, so swallowing it is beneficial to nourish the kidney-Qi. In the practice of the Qigong (气功, a traditional Chinese sport which keeps health), every morning, the exerciser faces the rising sun of the east and closes eyes, and keeps the tip of the tongue touching the roof of the mouth. After a long time, the thick saliva will be full of the mouth, and then he swallows it slowly. Such saliva is deemed to be formed from the kidney-Qi and the sun’s energy. The thick saliva is not phlegm, so don’t spit it out. If the thick saliva is much lost, the kidney-Qi will be impaired.

The kidney-Qi is greatly important to the growth of a human. The book “Su Wen” says that, when a girl is 14 years old or a boy is 16 years old, a substance called “Tiangui” (天癸) starts to be secreted. “Tiangui” can promote the growth of reproductive organs and the presence of secondary sex characteristics and reproductive capacity. The Western Medical Science proves that children at such age arrive at puberty, so “Tiangui” is similar to sex hormone. When a woman is 49 years old or a man is 56 years old, the kidney-Qi is very deficient, and the secretion of “Tiangui” is stopped. A woman at the age of 49 has no menstruation and reproductive capacity any longer. The Western Medical Science calls such age climacteric.

As described previously, the functioning of external organs of sex and excretion are related to the kidney-Qi. The evacuation of bowels is also stimulated and controlled by the kidney-Qi. The deficiency of the kidney-Qi may cause constipation or loose bowels. So the book “Su Wen” says, “The two private parts are the passages of kidneys.”

Bones, teeth, marrow, ears, and hair are nourished by the kidney-Qi. The book “Su Wen” says, “The kidneys produce bones and marrow”, “The kidneys master the bones and marrow of the entire body”. If the kidney-Qi of a baby is seriously deficient, his fontanels will close very late, and his bones can’t grow well. The kidney-Qi of old people is deficient, so their bones are fragile and they often suffer from osteoporosis. Teeth are the extensions of bones. The deficiency of the kidney-Qi can cause the loosening and even loss of teeth, and this is why old people generally suffer from the loss of teeth. If the kidney-Qi of a child is deficient, his teeth grow slowly. Marrow is deemed to originate from the kidney-Qi, and the brain is nourished by marrow. Ears also need the nourishing of the kidney-Qi and marrow. If the kidney-Qi is deficient, the marrow is also deficient, and the person will suffer from amnesia and dizziness, and there are changes of his hearing: tinnitus, a partial loss of hearing, or even deafness. Hair is deemed to originate from blood and be nourished by the kidney-Qi. So the deficiency of the kidney-Qi is also characterized by the loss of hair or the change in hair color.