The ancient Chinese used water and fire to symbolize yin and yang; anything moving, hot, bright and hyperactive is yang, and anything quiescent, cold, dim and hypoactive is yin.

The yin and yang properties of things are not absolute but relative. As an object or person changes so the yin and yang components change at a gradual rate. Each of the yin and yang properties of the object is a condition for the existence of the other; neither can exist in isolation.

These two opposites are not stationary but in constant motion. If we imagine the circadian rhythm, night is yin and day is yang; as night (yin) fades it becomes day (yang), and as yang fades it becomes yin. Yin and yang are therefore changing into each other as well as balancing each other.

The Application of Yin and Yang to Chinese Medicine
Each organ has an element of yin and yang within it. The histological structures and nutrients are yin, and the functional activities are yang. Some organs are predominantly yang in their functions, such as the gan-liver, while others are predominantly yin, such as the shen-kidney. Even though one organ may be predominantly yin (or yang) in nature, the balance of yin and yang is maintained in the whole healthy body because the sum total of the yin and yang will be in a fluctuating balance.

If a condition of prolonged excess or deficiency of either yin or yang occurs then disease results. In an excess of yin the yang qi would be damaged, and a disease of cold of shi nature would develop. Excess of yang will consume yin and a disease of heat of shi nature would develop. In a deficiency of yin, diseases of heat of xu nature develop, while a deficiency of yang causes diseases of cold of xu nature.

II. The Channels and Collaterals
The channels and collaterals are the representation of the organs of the body. They are also a functional system in their own right and they are responsible for conducting the flow of qi and blood through the body. The flow of qi can be disrupted by direct damage to the channels, such as trauma, or by an internal imbalance of yin and yang within the body.

The central principle of traditional Chinese medicine is to diagnose the cause of the internal disease, or yin yang imbalance within the body, and, by using the relevant acupuncture points, to correct the flow of qi in the channels and thus correct the internal disease. The acupuncture points that are on the channels have a direct influence on the flow of qi through the channels, and also on the internal organs. The zang channels are yin in nature and the fu channels are yang in nature.

Qi circulates through the channels of the body in a well defined circadian rhythm.

III. Zang and Fu Organs
The zang and fu organs are the internal visible organs of the body. The xin-heart, gan-liver, pi-spleen, fei-lung, shen-kidney and pericardium are the zang organs. The small intestine, large intestine, stomach, gall-bladder, urinary bladder and sanjiao are the fu organs.

The zang organs have a Chinese prefix because a direct translation from the Chinese might be misleading. The Chinese xin has functions rather different from the concept of the heart in Western medicine, so if we call the heart 'xin-heart', or the liver 'gan-liver', we are able to understand that we are referring to the organ of the heart or the liver, but it is really rather different from our concept of those organs.

The zang organs are of paramount importance in the body. They co-ordinate with the fu organs and connect with the five tissues (channels, jin¹ muscles, skin-hair, bones), and the nine openings (eyes, nose, ears, mouth, tongue, anus and external genitalia), to form the system of the Five Zang. The pericardium is not considered to be an important zang organ.

The Functions of the Zang Organs
The xin-heart
The xin-heart dominates the circulation of blood. When it functions properly the tissues and organs are well perfused and nourished, but when it malfunctions there is precordial pain, cyanosis and ischaemia. This disease is due to 'stagnation of the blood of xin-heart'.

The xin-heart 'keeps' the mind. Normally there is a clear mind, normal mentality, normal sleep and a good memory. When this fails there is coma, insomnia or somnolence, amnesia and mental derangement, because the xin-heart is failing to 'keep' the mind.

The xin-heart takes the tongue as its orifice and opens through it. Normally the tongue is reddish, moist, and moves freely. When the tongue has ulcers, is swollen or becomes purplish-red, there is 'upward blazing of the fire in xin-heart'. When the tongue is rigid and curled up (this may be accompanied by mental symptoms) 'phlegm and heat are covering the orifice of the xin-heart'.

The gan-liver
The gan-liver is the main yang organ of the body.

The gan-liver stores blood. Normally there is sufficient blood supply to all tissues. When this fails there is ischaemia, dizziness, malaise, abnormal menstruation and hemorrhage.

The gan-liver takes charge of freeing. Freeing really means the free flow of blood and qi through the body, especially digestion and the discharge of bile. When this is impaired there is irritability, mental depression, anorexia, abdominal distension and jaundice.

The gan-liver controls the jin which governs the muscle tone. When this function is disturbed there is muscle spasm, twitching, opisthotonos and convulsions. This is due to an 'insufficiency of yin and blood of the gan-liver, resulting in the malnutrition of the jin'.

The gan-liver takes the eye as its orifice and opens through it. Usually there is normal vision and normal eye movement. When this function is disturbed there is poor vision, night blindness, nystagmus and abnormal eye movements. This is due to an 'insufficiency of yin and blood in the gan-liver causing malnutrition of the eyes and stirring of the inner wind of the gan-liver.'

The pi-spleen
The pi-spleen governs the transportation and transformation of food, i.e. digestion. When digestion is abnormal there is anorexia, distension of the abdomen, diarrhea, emaciation, lassitude and oedema. This is due to 'a deficiency of the qi of pi-spleen'.

The pi-spleen commands the blood. Normally the blood circulates within the blood vessels but when this function fails there is extravasation of blood, chronic recurrent hemorrhage and bruising.

The pi-spleen dominates the muscles. This really means controlling the muscle bulk. Normally there is no muscle wasting, but when there is malnutrition of the muscles they are weak and wasted.

The pi-spleen takes the mouth as its orifice and opens through it. Normal people have a good appetite, a sense of smell and taste and red and moist lips. Abnormally there is anorexia, tastelessness or a sweetish, greasy taste, and pale sore lips. This is due to 'heat and damp in the pi-spleen'.

In addition the qi of pi-spleen lifts and fixes the internal organs in their normal position.

The fei-lung
The fei-lung takes charge of respiration. Normally respiration is even and the tissues are well oxygenated. When this function fails breathing is uneven, there is a cough, dyspnoea, shallow respiration and anoxia. This is due to 'a deficiency of qi of fei-lung which causes an impairment of dissipation and descent of clean qi (oxygen).

The fei-lung frees and regulates the water passage. This function covers the transportation and distribution of nutrients and water, the secretion of sweat and the excretion of urine. Abnormally there will be hyperhydrosis or hypohydrosis, oedema and difficulty in urination due to 'obstruction of the water passage'.

The fei-lung dominates the hair and skin. Normally the skin is lubricious, the hair lustrous, and sweating is normal. Abnormally the skin is rough, the hair dry and withered and the skin is 'loose'. This looseness opens the pores and increases the susceptibility to invasion by pathogenic factors.

The fei-lung takes the nose as its orifice and opens through it. Normally the nose is open and there is an acute sense of smell. Abnormally it may be obstructed, there may be anosmia, epistaxis and flaring of the alae nasi (usually accompanied by fever). This is due to 'invasion of the fei-lung by wind and cold or wind and heat'.

The shen-kidney
The shen-kidney is the main yin organ of the body. The shen-kidney dominates growth, reproduction and development. When this function fails there is a loss of reproductive function, retardation of growth, failure to thrive, and premature senility due to 'an insufficiency of the qi of shen-kidney'.

The shen-kidney produces marrow, filling the brain with marrow, dominating the bones and producing blood. Normally the spinal cord and the brain are fully developed, the bones are strong and the blood sufficient. Abnormally there will be dizziness, tinnitus, insomnia, poor memory and lassitude. The bones will be weak and brittle and the blood will be insufficient. This is due to 'an insufficiency of the essence of shen-kidney'.

The shen-kidney controls body water. This entails normal urine production and micturition. Abnormally there will be oliguria or anuria, oedema, difficult or dribbling micturition, polyuria, enuresis and incontinence. This is due to 'an insufficiency of yang of the shen-kidney failing to control body water'.

The shen-kidney controls the intake of clean qi (air). Abnormally there will be wheezing due to 'the failure of the shen-kidney to control the intake of clean air'.

The shen-kidney takes the ear as its orifice, opening through it. Normally there is sharp hearing, abnormally there is tinnitus, hearing loss, and even total deafness.

The pericardium
This may be called the organ of circulation in some texts. It is the least important of the zang organs.

It encloses and protects the xin-heart and the diseases of the pericardium result in dysfunction of the xin-heart.

The Functions of the Fu Organs
In general the traditional functions of the fu organs are very similar to their functions in Western medicine. Each fu organ channel connects internally and externally with a zang organ channel. This can have therapeutic importance in that a point on the fu channel may be used to treat a problem on its connected zang channel, and vice versa.

The small intestine
The small intestine connects with the xin-heart. The small intestine receives and digests food from the stomach. It absorbs the pure part and distributes it to the whole body, the impure part going on to the large intestine. This function of the small intestine belongs to the transforming and transporting function of the pi-spleen.

The gall-bladder
The gall-bladder connects with the gan-liver. It stores and discharges bile. The expulsion of bile from the gall-bladder is closely related to the freeing function of the gan-liver. The gan-liver and the gall-bladder take charge of freeing together, and jaundice results when this function is deranged.

The stomach
The stomach connects with the pi-spleen. The stomach stores and digests food, passing it on to the small intestine. A deficiency of qi of the stomach causes indigestion, epigastric pain and sour regurgitation When the qi of the stomach ascends then nausea, heartburn, vomiting, hiccoughs and flatulence occur.