Yin–Yang’s dynamic balance is normal physiology; imbalance is disease. Treatment aims at restoring the balance. In 1624, Zhang Jingyue stated in The Complete Works of Jingyue, “Although medicine is complicated, it can be summarized in one word, namely, Yin–Yang.”4 Yin–Yang theory is part of traditional Chinese medicine’s approach to physiology, pathology, diagnosis, and treatment. It also applies to modern conventional medicine.
1.Describing the Structure of the Human Body
The human body is composed of tissues, organs, and meridians of Yin–Yang. As Yin–Yang explains the body’s organic structure, the underlying premise is that the body is an organic whole. In general, Yang body structures are superior, exterior, posterior-lateral surface, and back. Yin body structures are inferior, interior, front-medial surface, and front. The body’s exterior is Yang, the interior Yin; the back Yang, the front Yin; the upper Yang, the lower Yin.
Among the Zang–Fu organs, Zang are Yin, and Fu are Yang. But among the Zang organs, the Heart and Lungs in the upper body (thoracic cavity) are Yang, and the Liver, Spleen, and Kidneys in the lower body (abdominal cavity) are Yin. This reflects the relativity of Yin and Yang. Further, each organ can be divided into Yin and Yang, e.g. the Heart has Heart-Yin and Heart-Yang. lists specific body structures, organs, and energies of a Yin–Yang nature.
Specific body structures, organs, and energies of a Yin–Yang nature.
| Yin | Yang |
|---|---|
| Front: chest-abdomen | Back |
| Lower: body | Higher: head |
| Interior: organs | Exterior: skin, muscle |
| Interior-medial surface of limbs — Yin meridian flow | Posterior-lateral surfaced of limbs — Yang meridian flow |
| Yin organs | Yang organs |
| Yin meridians | Yang meridians |
| Structures of organs | Functions of organs |
| Blood, body fluids | Qi |
| Nutritive Qi | Defensive Qi |
Most of the Yin meridians flow in front and carry Yin’s nourishing energies. Most of the Yang meridians flow in back and carry Yang’s energies to protect the body from exterior pathogens. Yin nourishes the interior, while Yang protects the exterior.
Internal organs are classified into Yin organs and Yang organs according to their anatomical structures and physiological functions. Yin organs, with more substantial contents, store lucid essences and substances. Yang organs, with less or sometimes without fine substantial contents, transform, digest, and excrete turbid substances and metabolites. For instance, the Heart, Liver, Spleen, Lungs, and Kidneys are Yin organs and the Small Intestine, Gall Bladder, Stomach, Large Intestines, and Urinary Bladder are Yang organs.
2.Illustrating the Physiology
The normal process of life results from a dynamic balance between Yin and Yang, in opposition, and at the same time coordination. Each physiological object has a substantial structure (Yin), without which there would be no physiological function (Yang). Physiological activities (Yang), in turn, promote material (Yin) metabolism. Normal life activities are maintained by the body’s Yin–Yang balance. The relation between body materials/structures and their functions/activities is both opposite and interdependent between Yin and Yang. Maintenance of physiological functions relies on the movement of Qi, ascending, descending, entering, and exiting, while Qi movements rely on the dynamic of Yin and Yang. For example:
- Fire–Water balance: The body’s Fire–Water balance is an important factor in maintaining the body’s physiology and is essential in TCM. Fire affects all metabolic processes:
(a)Assists the Heart in housing the Mind (Shen, 
);
(b)Provides Yang Qi (energy), which the Spleen needs to transform raw food into fine food essence and Qi (nutrients) and transport them to the Lungs;
(c)Stimulates the Small Intestine in separating lucidity and turbidity;
(d)Provides energy that the Bladder and the lower Energizer (lower Jiao) need to transform and excrete body fluids;
(e)Supplies energy that the uterus needs to facilitate the blood circulation.
If physiological Fire decreases, depression, edema, infertility, and other diseases may occur. Physiological Fire is the Fire of the Gate of Life (Ming Men, 
) and originates in the Kidneys. Water coming from the Kidneys moistens and cools the body during the metabolic and physiological processes and inhibits Fire’s warming action. The Fire–Water balance maintains the normal process of all physiological activities. When Fire is excessive, it tends to ascend to the upper body and head, and may cause red eyes, thirst, and headaches. When Water is excessive, it tends to descend, and may cause ankle edema and frequent urination.
- Yin–Yang organ balance: Yin–Yang organs are both opposite and interdependent, and work in pairs to maintain the organ system balance. The Heart pairs with the Small Intestine, the Spleen with the Stomach, the Liver with the Gall Bladder, the Lungs with the Large Intestine, the Kidneys with the Urinary Bladder, and the Pericardium with the Triple Energizer (San Jiao). Yin (Zang) organs store and Yang (Fu) organs transform, digest, and excrete. This balance keeps each pair of Yin–Yang organs in check and maintains physiological functions.
- Qi and Blood Balance: Qi is the commander of Blood, and Blood is the mother of Qi. The dynamic interaction and balance between Qi and Blood is essential in maintaining health.
In summary, as The Inner Canon of the Yellow Emperor states, “The essence of the life originates from Yin and Yang”.
3.Explaining Pathology
Yin–Yang dynamic balance leads to health of the body; Yin–Yang imbalance results in diseases. Yin–Yang imbalance is related to not only pathogens but also anti-pathogens. In a human body there are both Yin pathogens (e.g. coldness and dampness) and Yang pathogens (wind, dryness, heat, fire). There are Yin substantial anti-pathogens and Yang functional antipathogens. When Yin pathogens cause disease, Yin may predominate, damage Yang, and cause cold syndromes. When Yang pathogens cause disease, Yang may predominate, cause heat syndromes, and consume Yin. As stated in The Inner Canon 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 causes heat syndromes” . Yin–Yang imbalance presents pathological changes and clinical manifestations.
- Preponderance of Yin or Yang (excess pattern): Excess Yin consumes Yang, and causes Yang deficiency and cold. Excess Yang consumes body fluid, and causes Yin deficiency and heat.
- Weakness of Yin or Yang (deficiency pattern): When Yin or Yang is below the normal physiological level, illness arises. “If Yang deficiency, outer coldness will result; if Yin deficiency, inner heat will result”.
- Yin–Yang transformation: Yang syndrome can be transformed into Yin syndrome, and vice versa. “Coldness of the extremity will lead to hotness; hotness of the extremity will result in coldness”.
- Yang energy tends to ascend, and manifests as red face and/or eyes. The Governor Vessel (GV) goes on the back and oversees the functions of all Yang meridians. The GV 20 point in the head can raise Yang energy.
- Yin of the chest and abdomen is easily affected by Yin pathogenic factors such as Cold and Dampness. The Conception Vessel (CV) goes on the front of the body and nourishes all Yin meridians. The CV 12 point can nourish Yin energy.
- Yang Qi protects the body from the cold invasion — there will be no cold invasion when Qi is intact and strong.