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.