1. Yin-yang and organic structure
When the theory of yin-yang is applied to explain the organic structure of the human body, the underlying premise is that the human body should be viewed as an organic whole. As the whole body is involved, the exterior portion pertains to yang, while the interior to yin; the back or dorsal aspect pertains to yang, while the front or anterior aspect to yin; the upper portion pertains to yang, while the lower portion to yin. Among the zang-fu organs, zang-organs pertain to yin, while fu-organs to yang. Among the zang-organs, the Heart and Lungs, situated in the upper portion of the body (thoracic cavity), pertain to yang, while the Liver, Spleen and Kidneys, situated in the lower portion of the body (abdominal cavity), to yin. In terms of each organ, it can be further divided into yin and yang, e.g., the Heart can be divided into the Heart yin and the Heart yang, the Kidneys can be divided into the Kidney yin and the Kidney yang. (Yin and yang of a zang-fu organ denote the material and functional aspects of that organ, respectively. In this context, yin refers to the essential materials such as vital essence, nutrients, fluids an blood, and yang includes various functional activities as well as heat energy.)
2. Yin-yang and physiological functions
All the normal life activities are based on the dynamic balance between yin and yang, which oppose each other and at the same time unite in coordination. As far as function and matter are concerned, function pertains to yang, while matter pertains to yin. All physiological activities have their material basis; without material movements, there would be no physiological functions. On the other hand, physiological activities, in turn, promote material metabolism. Therefore, the interrelationship of unction and matter is precisely the interdependent and inter-consuming supporting relationship of yin and yang.
3. Yin-yang and pathological changes
The dynamic balance of yin and yang guarantees health. Once the balance is impaired, disease ensues. Breakdown of the normal balance of yin and yang is related to both pathogenic and anti-pathogenic factors. There are two types of pathogenic factors: yin pathogenic factors and yang pathogenic factors. For example, cold and damp are yin pathogenic factors, while wind, dryness, heat and fire are yang pathogenic factors. Anti-pathogenic factors also include yin and yang, i.e., the material aspect and the functional aspect. When yin pathogenic factors cause disease, this may lead to preponderance of yin, which damages yang and gives rise to cold syndromes. When yang pathogenic factors cause disease, this may lead to an excess of yang, which gives rise to heat syndromes and consumes yin. (As stated in The Medical Classic of the Yellow Emperor, "Yin in excess consumes yang, and yang in excess consumes yin." "Yin in excess causes cold syndromes, and yang in excess caused heat syndromes.")
If the disease is due to deficiency of the anti-pathogenic factors, there are deficiency syndromes. According to the principle of equilibrium between yin and yang, deficiency of yin leads to relative preponderance of yang, giving rise to internal heat, while deficiency of yang leads to relative preponderance of yin, giving rise to external cold.
In advanced cases, however, deficiency of either yin or yang may lead to the consumption of the other, known respectively as "impairment of yin affecting yang" and "impairment of yang affecting yin."
4. Yin-yang and diagnosis
Since diseases or syndromes can be classified into yin and yang according to their nature, and the root cause of the occurrence and development of disease is imbalance between yin and yang, the key to clinical diagnosis is correct differentiation of the yin and yang nature of the disease as well as determination of the imbalanced condition of yin and yang. In this way, complicated clinical conditions can be simplified, and a correct diagnosis can be made.
5. Yin-yang and treatment
Treatment in Chinese medicine is always aimed at restoring the normal balance of yin-yang. The following therapeutic principles derived from the theory of yin-yang are of the utmost significance.
"Replenish what is in deficiency," "reduce what is in excess," "treat cold with warming measures" and "treat heat with cooling measures" are the general rules of treatment in order to restore the normal balance of yin-yang.
An acute disease due to invasion of exogenous pathogenic factors is usually excess in nature, if the patient is normally healthy. In this case, the treatment is to eliminate or reduce the pathogenic factors. For pathogenic factors of a yin nature (such as cold), yang measures (e.g., warming therapy) should be used, and for pathogenic factors of a yang nature (such as heat), yin measures (e.g., cooling therapy) are appropriate.
In deficiency syndromes, replenishing or reinforcing measures are indicated. Deficiency of yin is usually accompanied by relative preponderance of yang, and deficiency of yang by relative preponderance of yin, and the treatment should be to replenish yin and reinforce yang, respectively. When the deficiency is put right, the relative preponderance will naturally disappear. In complicated cases, however, the treatment may also be complex. Sometimes it is necessary to "treat the yang aspect for diseases of a yin nature," and "treat the yin aspect for diseases of a yang nature," for example, by reducing yang to relieve dizziness and headaches in hypertensive patients suffering from yin deficiency with relative exuberance of yang, and replenish yin-fluid in cases of high fever (yang-heat) which consumes the body fluids. In acupuncture, it is not unusual to needle the points of the yang meridians for disorders of the yin meridians, and vice versa.