1. Storing Essence, Controlling Human Reproduction Growth and Development
Essence is defined as the basic substance both constituting the human body and maintaining its functional activities. As described in the Suwen, "Essence is the foundation of the human body." Essence consists of two parts: congenital essence inherited from the parents and acquired essence transformed from food. Essence is stored in the kidney and is known as kidney qi. It greatly influences the ability of reproduction, growth, and development.

According to traditional Chinese medicine, kidney qi plays an essential role in the function of reproduction, growth, and development. If this function is abnormal, infertility, infantile underdevelopment, maldevelopment, weakness of bone development, etc. will manifest.

Kidney essence is classified as yin, while qi is yang. Known as kidney yin and yang, they both restrict and depend on each other in order to maintain a dynamic physiological balance. If this balance is disrupted, pathological changes of hyperactivity or hypoactivity of kidney yin and yang will occur.

Clinically, a kidney yin deficiency may be manifested soreness, aching and weakness of the lumbar region and knees, blurred vision, poor memory, etc. A yin deficiency leading to blurred vision, poor memory, etc. A yin deficiency leading to yang preponderance will produce tidal fever, night sweating, dizziness, ringing in the ear (tinnitus), spermatorrhea, and sexual dreams. Kidney yang deficiency decreases the warming function of the kidney bringing on the symptoms of lassitude, coldness and pain in the lumbar region and the knees, cold extremities and frequent urination, leading to pathological conditions such as inadequate reproductive ability, impotence, premature ejaculation and coldness of the uterus. If a certain degree of kidney yin or yang deficiency is reached then either may injure the other resulting in a loss of the body's dynamic physiological balance.

2. Controlling Water Metabolism
The kidney plays an essential role in the distribution, regulation, and metabolism of water. As the Suwen says, "The kidney is the organ of water in charge of fluid." Water is received by the stomach, transformed and transported by the spleen. Par of the fluid is sent down by the descending function of the lung until it finally reaches the kidney and is divided by the qi activity of the kidney yang into two parts: clear and turbid. The useful clear fluid is sent back up to the lung a jin, and the turbid waste fluid flows down into the urinary bladder to form urine, which is excreted. The distribution of water is related to the functions of the stomach, spleen, small intestine, large intestine, lung, urinary bladder, and sanjiao, but they all rely on the warming and pushing function of kidney yang. If kidney yang is insufficient, this may lead to retention of water resulting in scanty urination, retention of urine or frequent urination, enuresis, etc.

3. Controlling Bone, Producing Marrow, and Influencing Hair Luster
Nourishment of the bone structure requires marrow, a product of kidney essence. The term "marrow" includes the bone marrow and the spinal cord. The brain is known as the "sea of marrow." Sufficiency of this essence produces a well developed and functioning skeletal system. Inadequate essence, however, may lead to a variety of syndromes: a sore, aching, and weak lumbar region and knees, weakness or atrophy of the lower limbs, infantile maldevelopment, delayed closing of the fontanelle, etc. Furthermore, according to traditional Chinese medicine, "teeth are the remainder of bone." Poor nourishment by kidney essence also caused looseness and loss of teeth.

Essence and blood generate each other. Ample essence makes sufficient blood. Hair is nourished by the blood and rooted on the basis of kidney qi. Therefore luster, moisture, dryness, roughness growth and falling out of hair is related to the sufficiency of kidney essence. As the Suwen says, "The kidney is in tune with the bones, its prosperity is reflected in the luster and moisture of the head hair."

4. Opening into the Ear, Perineal Ante-tract and Perineal Post-Tract
Auditory function is dependent on nourishment from the essential qi of the kidney. If that essence is sufficient, then keen hearing will result. Otherwise, there will be deafness and a ringing in the ears. The perineal ante-tract (including urethra and vagina) and post-tract (anus) have the action of reproduction and excretion through the function of kidney qi. Insufficiency will manifest as frequent urination, enuresis or scanty urine; for the reproductive organs there will be spermatorrhea, impotence, premature ejaculation, or infertility; bowel movements will be subject to either morning diarrhea or constipation.