According to the theory of zang-fu organ meridian and collateral, although pain can be caused by various reasons, it can generally be attributed to disorders of qi and blood. Nan Jing‧Chapter 30 states when discussing qi and blood: "Blood is the nutrient, qi is the defense, they follow each other up and down, this is called nutrient-defense. They circulate through the meridians and collaterals, nourishing the exterior." Therefore, disorders of qi and blood are nothing more than "blockage" or "deficiency."
Firstly, "blockage causes pain." Suwen-Jutong Lun says: "The meridians flow continuously, circulating without end. When cold qi enters the meridians and lingers, it stagnates and does not move. If it lodges outside the vessels, it causes blood deficiency; if it lodges within the vessels, it causes qi blockage, hence sudden pain," "When cold qi lodges between the stomach and intestines, beneath the membrane source, blood cannot circulate, and the small collaterals are urgently pulled, hence pain," "When heat qi lingers in the small intestine, there is pain in the intestines, with scorching heat and thirst, then the stool becomes hard and dry and cannot be expelled, hence pain and blockage." These discussions succinctly describe the basic principle of pathogenic qi obstructing the collaterals, causing qi and blood to congeal and stagnate, leading to pain. During the Jin-Yuan period, Li Dong-yuan, based on Neijing, further clearly proposed the concept of "pain due to blockage."
Another cause of pain is "deficiency causes pain." Suwen-Jutong Lun says: "When yin qi is exhausted and yang qi has not yet entered, there is sudden pain." Lingshu‧Chapter on Body Fluids says: "When the marrow and fluids are reduced and descend, excessive descent leads to deficiency, hence pain in the lower back and legs." These discussions refer to external pathogens causing zang-fu organ dysfunction and qi-blood deficiency, leading to pain.
In summary, all pain can be attributed to disorders of qi and blood. Modern medicine explains pain caused by inflammation, vascular or muscular rigidity leading to local tissue congestion or ischemia as qi-blood blockage or deficiency; as for pain caused by trauma, it is due to the direct effect of external injury on zang-fu organ meridians and collaterals, causing qi-blood circulation disorders.Although pain can be attributed to qi-blood disorders, it can only occur with the participation of "spirit." The spirit is most closely related to the heart and kidney among the zang-fu organs, especially under the governance of the heart. The heart holds the primary position among the zang-fu organs, being "the great ruler of the five zang and six fu organs, the dwelling place of the spirit" (Lingshu-Xieke), "Hence, if the ruler is clear, the subordinates are at peace, and nurturing life in this way leads to longevity... If the ruler is unclear, the twelve officials are in danger, the pathways are blocked and do not circulate, and the body is greatly harmed, nurturing life in this way leads to disaster" (Suwen-Linglan Midian Lun). In summary, "the zang-fu organ skeleton, is commanded by it, intelligence and wisdom all arise from it" (Zhang Jing-yue: Leijing‧Visceral Manifestation Class One). If the spirit, under the governance of the heart, undergoes some change, the body's sensitivity and tolerance to pain stimuli also change accordingly. For example, when the body is in a state of "unconsciousness" or "loss of spirit," that is, when the spirit is unconscious and awareness is lost, pain cannot be perceived. Therefore, Suwen-Zhizhenyao Da Lun concludes: "All pain, itching, and sores belong to the heart."
As for the analgesic effect of acupuncture points, it is achieved through the actions of "regulating qi" and "treating the spirit." Regulating qi involves adjusting the excess or deficiency of qi and blood in the zang-fu organ meridians and collaterals; it also involves coordinating the circulation of qi and blood to enable normal nutrient-defense functions. Treating the spirit involves mobilizing the governing function of the heart to shift the spirit, and also facilitating the movement of qi, thereby regulating the activities of the zang-fu organ meridians and collaterals. In summary, "the use of needles is to regulate qi," "the true essence of acupuncture must first treat the spirit." This is the basic principle of clinical acupuncture treatment and the fundamental principle of acupuncture analgesia.