The human body is an integrated whole. All its tissues and structures are organically connected and may be classified as two opposite aspects -- yin and yang. That is why Su wen states, "Man, having a form, can not deviate from yin and yang." In terms of the anatomical locations, the upper part of the body is yang and the lower part is yin; the exterior is yang and the interior, yin; the back is yang and the abdomen, yin; the lateral aspects of the extremities are yang and the medial aspects, yin.

Concerning the zang-fu organs, the zang-organs store but not discharge essence-qi and, therefore, they are yin; while the fu-organs transmit and transform food into essence-qi but not store it, and, for this reason, they are yang. Furthermore, each of the zang- or fu-organs can be redivided into yin and yang. For example, heart-yin and heart-yang, kidney-yin and kidney- yang, stomach-yin and stomach-yang, etc. As concerns yin and yang of the meridian-collateral system, there are two categories: yin meridians and yang meridians; yin collaterals and yang collaterals. All of them are opposite pairs. Thus, in line with the yin-yang theory, the unity of opposites between yin and yang exist in the upper and lower, internal and external, front and back parts of the human body, and within all the internal organs as well.

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