Understanding Yin, Yang and Qi 
By Alan Keith Tillotson, LAc., Registered Herbalist
It may help you remember the triad by using the following analogy found in ancient Chinese medical texts. Think of the stomach as a pot of soup. The Yin represents the nutrients in the soup, and the Yang is the fire under the soup. The Qi is the nutrient-filled steam that rises up from the pot when Yin and Yang work together, This nutrient steam travels through tiny pathways (meridians), carrying its warmth and nutrients to the organs. If the pathways are blocked, the restrictive area becomes painful, and the organs beyond it wither from lack of energy and nutrition. If the blockage occurs in the larger channels flowing up and down between the trunk of the body and the brain, the person becomes depressed, constricted, and less creative. Because "the mind directs the Qi," one major goal of meditation is to strengthen the Qi energy and use the mind to feel and direct its flow throughout the body.
[from The One Earth Herbal Sourcebook, pg. 71.]
The triad of Yin, Yang and Qi (pronounced chee) serves as the basis for the medical theory of Traditional Chinese Medicine (TCM). Entire textbooks have been written about this subject in China, and one could argue that similar mechanisms are found everywhere in the universe, even at the molecular level. For our purposes we are going to simplify this philosophy as much as possible, while describing the terms as understood by TCM doctors. The most essential thing to know is that TCM doctors use these broad general medical terms to orient their medical thinking, diagnose disease and choose herbs. The purpose is to bring these three into balance. In Chinese theory this process is conceived as bringing Yin and Yang into balance with each other, which results in the production of Qi.
Yin represents the nutritive processes and substances of the body. When the Yin is strong, the body is strong, moist, well nourished and fertile. When the Yin is in excess, the body, or the individual organ, becomes sluggish and damp. When the Yin is weak, the body is weak, dry, deficient, and can flare up with heat. There can be sensitivity to heat, weight loss, insomnia, hot flashes, dryness and sometimes dizziness and heart palpitations. This presentation of symptoms is known as Yin deficiency, a very important TCM medical concept. To treat Yin deficiency, TCM herbalists use Yin tonic herbs. These herbs generally nourish and moisten the tissues and increase nutritive forces.
Some of the most commonly used Yin tonics are raw rehmannia root (sheng di huang/Rehmannia glutinosa), glehnia root (sha shen/Adenophora tetraphylla), scrophularia root (xuan shen/Scrophularia ningpoensis), ligustum berry (nu shen zi/Ligustrum lucidum) American ginseng root (xi yang shen/Panax quinquifolium), ophiopogon root (mai men dong/Ophiopogon japonicus) and wild asparagus root (tian men dong/Asparagus lucidis).
Notice that many of the Yin tonics are roots, used by plants to absorb nutrients from the soil. Interestingly, the Chinese use seeds, such as sesame seeds, as Yin tonics. At our clinic I often use nutritive oils such as fish oils, flaxseed oil (Linum usitatissimum) or evening primrose oil (Oenothera biennis) along with the other herbs mentioned above to moisturize and reduce inflammation.
Yang represents the heat and metabolic processes of the body. When the Yang is strong the body is energetic, warm and powerful. When the yang is in excess, the body becomes inflamed. When the yang is weak the body is fatigued, cold and weak, often to the point of exhaustion. There can be symptoms of low back pain, impotence, diarrhea, and weakness in the four extremities. These symptoms are known as Yang deficiency. TCM doctors use Yang tonic herbs to treat Yang deficiency. These herbs are generally warming and drying. Chinese research has shown that many of these herbs benefit the endocrine system (Bensky & Gamble 2004).
Some of the most commonly used Yang tonic herbs are prepared aconite (fu zi/Aconitum palmatum), dried ginger root (gan jiang/Zingiber officinalis), cinnamon bark (rou gui/Cinnamon zeylanicum), deer antler (lu rong (Cervus nipon) and morinda root (ba ji tian/Morinda officinalis).
Qi (pronounced "chee," and sometimes written as "chi") represents the vital energy of the body flowing along invisible energy channels. The balance of Qi is dependent upon the functional relationship between Yin and Yang. When the Qi is strong, the digestion is strong, the organs are well regulated, and nourishment and energy flow through and vitalize the organs. When the Qi is weak or blocked, the digestion weakens, dampness accumulates, and the corresponding organs exhibit pain, spasm or irregular functioning. There can be extreme fatigue, poor digestion, diarrhea, muscle atrophy, compromised immunity, or weakness in the lungs. This is called Qi deficiency. When Qi is weak, TCM doctors use herbs that supply Qi, known as Qi tonics.
The most common Qi tonic herbs are ginseng root, astragalus root, codonopsis root (dang shen/Codonopsis pilosula), licorice root, and white atractylodes rhizome.
The easiest way to understand the mechanisms of this triad as you learn is to substitute the word "nutrient" when you hear the word Yin, "metabolism/heat" when you hear the word Yang, and "vital force" when you hear the word Qi.
 ^Except as noted, this entry originally appear as a section of "The Language of Herbs: Essential Concepts and Vocabulary." The Tillotson Institute Of Natural Health. Copyright 2001 by Alan Keith Tillotson.