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Explanation of the histological structure of the human body
TCM believes that the human body is an organic whole and that all the tissues and organs in the body depend on each other in function. Such an interdependence between these tissues and organs can also be explained according to the theory of yin and yang. That is why it is said in Huangdi Neijing that "man has a physical shape which is inseparable from yin and yang".
Generally speaking, the upper part of the body pertains to yang while the lower part to yin; the exterior pertains to yang while the interior to yin the back pertains to yang while the chest and the abdomen to yin the chest pertains to yang because it is located in the upper part of the body while the abdomen to yin because it is located in the lower part of the body; the lateral sides of the four limbs pertain to yang while the medial sides to yin. As to the zang-organs and the fu-organs, the five zang-organs pertain to yin because they store essence, but never discharge it; the six fu-organs pertain to yang because they transport and transform food, but never store it. Among the five zang-organs, the heart and the lung are located in the chest, so they pertain to yang; but the liver, the spleen and the kidney are located in the abdomen, so they pertain to yin.
Each organ itself can be further divided into yin and yang aspects, such as heart-yin and heart-yang, kidney-yin and kidney-yang, etc.
Explanation of the relationship between structure and function
The theory of yin and yang holds that the normal activities of life result from the balance between yin and yang and that the close relationship between histological structure and physiological functions signifies the opposition and the unity between yin and yang. The histological structure of the body, including the viscera, the meridians, qi, blood and body fluid, all pertains to yin because they are all substantial. However, their functions all pertain to yang. Thus there is an intrinsical relationship between substances that pertain to yin and functions that pertain to yang, i.e. the mutual opposition, interdependence, wane and wax as well as transformation of yin and yang. In terms of the interdependence between yin and yang, the human body relies on the viscera, the meridians, qi, blood and body fluid to perform and maintain its physiological functions. On the other hand, the metabolism of qi, blood and body fluid depends on the functional activities of the related viscera. Similarly, the performance of various functional activities will inevitably consume qi, blood and body fluid. This process is marked by wax of yang and wane of yin. Besides, constant transformation and generation of nutrients must rely on the functional activities of the viscera and consume certain amount of energy. This process is characterized by wax of yin and wane of yang. This process of metabolism is also accompanied by mutual transformation of yin and yang between substances and functions. Only when the balance between yin and yang is ensured in this series of physiological process can normal state of life be maintained.
Explanation of pathogenesis
When the balance between yin and yang in the body is damaged, it leads to various diseases known as "imbalance between yin and yang". Though complicated, pathological changes generally fall into two categories, relative predominance of yin or yang and relative decline of yin or yang, according to the .analysis of pathogenesis with the theory of yin and yang.
Relative predominance of yin or yang
Relative predominance of yin and yang, a pathological change due to excessive increase of yin and yang, includes two aspects: relative predominance of yin and relative predominance of yang. Yin or yang that becomes predominant inevitably turns into a pathogenic factor. So predominant yin becomes a pathogenic factor of yin nature and predominant yang a pathogenic factor of yang nature.
Relative predominance of yang, usually caused by invasion of pathogenic factors of yang nature or exuberant heat transforming from the pathogenic factors of yin nature activated by yang, is a manifestation of excess-heat syndrome known as "predominance of yang leading to heat" in Huangdi Neijing. Pathogenic factors of different nature impair the corresponding healthy qi in the body. So pathogenic factors of yang nature tends to consumption of yin-fluid in the course of pathological changes. That is why it is said in Huangdi Neijing that "predominance of yang leading to disease of yin." "Disease" here means impairment. For example, invasion of pathogenic heat into the body will make yang-qi in the body hyperactive, leading to high fever, sweating, reddish complexion and rapid pulse. With the progress of the morbid condition, pathogenic factors of yang nature consume yin-fluid in the body, bringing on thirst, scanty urine and constipation.
Relative predominance of yin is usually caused by invasion of pathogenic factors of yin nature into the body, leading to exuberance of yin-cold and bringing on excess cold syndrome known as "predominance of yin leading to cold" in Huangdi Neijing. If pathogenic factors of yin nature further impair yang-qi in the body, a pathogenesis of "predominance of yin leading to disease of yang" will arise, as is recorded in Huangdi Neijing. Take pathogenic cold for example. When it invades the body, it leads to cold symptoms like aversion to cold, cold limbs and cold abdominal pain on the one hand, and symptoms due to yang-deficiency like floating whitish complexion, lassitude, frequent lying on bed, slow and weak pulse on the other hand.
Besides, when relative predominance of yin or yang develops to the extreme point, mutual transformation will occur under certain condition according to the principle of inter-transformation between yin and yang. That is to say that yin-cold syndrome will turn into yang-heat syndrome and vice versa.
Relative decline of yin or yang