Acupuncture treats the human body as a whole that involves several "systems of function" that are in some cases loosely associated with (but not identified on a one-to-one basis with) physical organs. Some systems of function, such as the "triple heater" (San Jiao, also called the "triple burner") have no corresponding physical organ, rather, represents the various jiaos or levels of the ventral body cavity (upper, middle and lower). Disease is understood as a loss of balance between the yin and yang energies, which bears some resemblance to homeostasis among the several systems of function, and treatment of disease is attempted by modifying the activity of one or more systems of function through the activity of needles, pressure, heat, etc. on sensitive parts of the body of small volume traditionally called "acupuncture points" in English, or "xue" (穴, cavities) in Chinese. This is referred to in TCM as treating "patterns of disharmony".
Treatment of acupuncture points may be performed along several layers of pathways, most commonly the twelve primary pathways meridians, located throughout the body. Other pathways include the Eight Extraordinary Pathways Qi Jing Ba Mai, the Luo Vessels, the Divergents and the Sinew Channels. Unaffiliated, or tender points, called "ah shi" (signifying "that's it", "ouch", or "oh yes") are generally used for treatment of local pain. Of the eight extraordinary pathways, only two have acupuncture points of their own. The other six meridians are "activated" by using a master and couple point technique which involves needling the acupuncture points located on the twelve main meridians that correspond to the particular extraordinary pathway. Ten of the primary pathways are named after organs of the body (Heart, Liver, etc.), one is named for the serous membrane that wraps the heart (Heart Protector or Pericardium), the last is the 'three spaces' (San Jiao). The pathways are capitalized to avoid confusion with a physical organ (for example, we write the "Heart meridian" as opposed to the "heart meridian"). The two independent extraordinary pathways Ren Mai and Du Mai are situated on the midline of the anterior and posterior aspects of the trunk and head respectively. The twelve primary pathways run vertically, bilaterally, and symmetrically and every channel corresponds to and connects internally with one of the twelve Zang Fu ("organs"). This means that there are six yin and six yang channels. There are three yin and three yang channels on each arm, and three yin and three yang on each leg.
The three yin channels of the hand (Lung, Pericardium, and Heart) begin on the chest and travel along the inner surface (mostly the anterior portion) of the arm to the hand.
The three yang channels of the hand (Large intestine, San Jiao, and Small intestine) begin on the hand and travel along the outer surface (mostly the posterior portion) of the arm to the head.
The three yin channels of the foot (Spleen, Liver, and Kidney) begin on the foot and travel along the inner surface (mostly posterior and medial portion) of the leg to the chest or flank.
The three yang channels of the foot (Stomach, Gallbladder, and Bladder) begin on the face, in the region of the eye, and travel down the body and along the outer surface (mostly the anterior and lateral portion) of the leg to the foot.
The movement of qi through each of the twelve channels is comprised of an internal and an external pathway. The external pathway is what is normally shown on an acupuncture chart and it is relatively superficial. All the acupuncture points of a channel lie on its external pathway. The internal pathways are the deep course of the channel where it enters the body cavities and related Zang-Fu organs. The superficial pathways of the twelve channels describe three complete circuits of the body, chest to hands, hands to head, head to feet, feet to chest, etc.
