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Yin Qiao Mai (Yin Heel Vessel) is a divergent branch of the Foot-Shaoyin Kidney Meridian. Its vessel originates from the middle of the heel, behind the Ran Gu point (Rangu, KI2) of the Foot-Shaoyin Kidney Meridian (Rangu is located in the depression one cun anterior and inferior to the medial malleolus). It travels with the Foot-Shaoyin Kidney Meridian along the lower aspect of the medial malleolus through the Zhaohai point (Zhaohai, KI6) (located five fen below the medial malleolus), ascends two cun above the medial malleolus, passing through the Jiaoxin point (Jiaoxin, KI8) (located above the medial malleolus bone, between the tendons and bones, anterior to the Foot-Shaoyin and posterior to the Foot-Taiyin). It then goes straight up along the medial aspect of the thigh into the genital region, ascends along the interior of the chest entering the Clavicular fossa (Quepen), ascends anterior to the Yingchang point (Yingchang, ST9), reaches the throat, intersects and penetrates the Chong Mai (Chong Vessel), enters the medial aspect of the thigh, ascends and attaches to the inner canthus of the eye. At the Jingming point (Jingming, BL1) (located in the depression one fen lateral to the inner canthus), it meets with the Hand-Taiyang Small Intestine Meridian, Foot-Taiyang Bladder Meridian, Foot-Yangming Stomach Meridian, and Yang Qiao Mai (Yang Heel Vessel)—five vessels in total—and ascends after intersection. There are eight acupoints in total.

Zhang Ziyang states in the *Classic of the Eight Vessels*: Among the Eight Vessels, the Chong Mai is below the Fengfu point (Fengfu, GV16); the Du Mai (Du Vessel) is posterior to the umbilicus; the Ren Mai (Ren Vessel) is anterior to the umbilicus; the Dai Mai (Dai Vessel) encircles the waist; the Yin Qiao Mai is anterior to the Weilü area and inferior to the scrotum; the Yang Qiao Mai is two segments posterior to the Weilü area; the Yin Wei Mai (Yin Linking Vessel) is one cun three fen anterior to the vertex; and the Yang Wei Mai is one cun three fen posterior to the vertex. Ordinary humans possess these eight vessels, but they belong to the "Yin Spirit" and remain closed and unpatent. Only immortals can open them with Yang Qi, thereby attaining the Dao. The Eight Vessels are the root of the primordial Great Dao and the ancestor of the One Qi. In gathering Qi, priority is given to the Yin Qiao Mai. Once this vessel moves, all other vessels become patent. Subsequently, the Du, Ren, and Chong vessels serve as the source of creation for the meridians. The Yin Qiao Mai appears scattered in various alchemical classics under many names: Tian Gen (Heavenly Root), Si Hu (Gate of Death), Fu Ming Guan (Return-to-Life Pass), Feng Du Gui Hu (Ghost Gate of Fengdu), and Si Sheng Gen (Root of Dead Birth). A deity governs it, named Tao Kang. It connects upward to the Niwan Palace and downward through the Yongquan point (Yongquan, KI1). If one understands this and allows the True Qi to gather and disperse through this passageway, the Heavenly Gate remains open, the Earthly Gate remains closed, the Kao Mai circulates throughout the body, penetrating upper and lower levels, and Harmonious Qi naturally ascends. Yang grows while Yin diminishes; Fire arises from Water, and flowers bloom in snow. This is described as "Freely visiting the Heavenly Root and Lunar Cavern, making all thirty-six palaces spring-like." Those who attain this experience lightness and vigor in their bodies, reversing aging and restoring youth. Their minds are tranquil and obscure, like being drunk or in a trance; this is the verification. One must know that the Southwest direction corresponds to Kun Earth, specifically the area anterior to the Weilü, posterior to the bladder, inferior to the small intestine, and superior to Linggui. This is where the Qi generated daily by Heaven and Earth resides, the place of producing Lead. Physicians are unaware of this.

Binhu (Li Shizhen's pseudonym) says: Alchemical texts discussing Yang Essence and the River Cart often refer to the Ren, Chong, and Du vessels, Mingmen (Gate of Life), and Sanjiao (Triple Burner), but rarely specifically designate the Yin Qiao Mai. The meridians described in Zhang Ziyang's *Classic of the Eight Vessels* differ slightly from medical theories. However, the internal pathways of the collaterals can only be observed clearly through introspective vision (Fan Guan Nei Shi); thus, his words must not be erroneous.