The quintessence of the theory of yin-yang is the law of unity of opposites, which means keeping a dynamic balance. In fact, this law can also be applied to modern physiology at various levels -- at the levels of systems, organs, cells, and even molecules. For example, in the nervous system, excitation and inhibition, the sympathetic and parasympathetic functions, the actions of acetylcholine and norepinephrine are all in opposition, and between each pair of opposites a dynamic balance should be maintained for normal nervous activities. In the endocrine system, there are estrogen and testosterone in opposition; in the kidneys there are diuresis and anti-diuresis, renal tubular secretion and reabsorption; for muscles there are contraction and relaxation; for body temperature there are thermogenesis and thermolysis; for glucose metabolism there are insulin and glucagons; for blood there are coagulation and anticoagulation; in the body fluids there are acid and base; and so on. All these opposites should be kept in dynamic balance. Therefore, it can be concluded that the maintenance of dynamic balance by uniting various opposites is a general law of life activities.
Another philosophical view in traditional Chinese medicine is dialectics. It is expressed as the theory of yin-yang. Yin and Yang were and still are two topographical terms designating the shady and sunny sides of a hill, respectively. Since everything under the sun has two sides, the shady and the sunny, by extension, yin and yang represent two opposites of an object or phenomenon. According to this theory, all things and phenomena in the world contain two opposite aspects: yin and yang, which are in conflict and at the same time mutually dependent. Therefore, this theory can be taken as a law of unity of opposites.
The theory of yin-yang as a dialectical way of thinking can be applied to any field of medicine. There is no conflict between the theory of qi and that of yin-yang. In fact, qi, as the basic element that constitutes the world, can also be divided into two, i.e., yin qi and yang qi. Yin qi chiefly refers to the material aspect of the element, and yang qi, the dynamic aspect. The Medical Classic of the Yellow Emperor states, "The clear yang qi ascends to form Heaven, while the turbid yin qi descends to form Earth."
Modern physiology has shown that homeostasis, metabolism and adaptability are the three characteristic features of life activities. In Chinese medicine, these three features are all well discussed, but expressed in different ways: the dynamic balance of yin-yang within the body for homeostasis, waxing and waning of yin-yang or transformation of yin-yang for metabolism, and adaptable conformity between the human being and the external environment for adaptability. If the yin-yang theory is compared with modern physiology in this way, it will be easy to find the rationale of this ancient theory from a modern perspective. Of course, there are great differences. The main difference is that the Chinese theory is macrocosmic, while the modern Western theory is microcosmic. The latter has many advantages. The deeper it goes, the more detail and precision attaches to the knowledge. However, it may have the tendency to pay more attention to the local part than to the whole body. The knowledge gained by macrocosmic observation in Chinese medicine is often general and vague, but because of thousands of years of accumulation of experience, there is a wealth of useful knowledge, which is neglected by Western medicine and its microcosmic point of view. That is why integration of the advantages of Chinese medicine with Western medicine may promote the development of world medicine as a whole.