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The pulse is the vessel of blood, the precursor of qi and blood, and the tunnel through which blood flows; its pulsation corresponds to the rhythm of respiration.

Water flowing from its source in branches is called a river channel. The convergence, divergence, rising, and falling of water follow distinct and orderly paths. Blood in the human body is like water, and the vessels within the blood are like these channels. When the channels are unobstructed, the water source remains active; when the vessels are unobstructed, qi and blood circulate. Qi and blood, existing within the human body, are both a unified whole and distinctly organized; they are neither absolutely one nor absolutely two. If the meridians do not first undergo circulation, the mechanism of hidden movement and silent operation would be obstructed. How could the functions of nourishment and guarding then fulfill their respective duties? (Chapter 22 of *Nan Jing* states: "Qi primarily warms [Xu], while blood primarily nourishes [Ru]." *Su Wen · Yin Yang Ying Xiang Da Lun* states: "Yin resides within, serving as the guard for Yang; Yang resides outside, serving as the envoy for Yin.") The "tunnel" refers to the meridians. This describes how, within the blood, there are precise and concealed pathways that form their own distinct routes. (*Ling Shu · Ying Wei Hui Hui* chapter states: "The path of Ying Qi treasures the internal grain." Grain enters the stomach, is transmitted to the lungs, overflows internally, and spreads externally. The pure and concentrated portion travels through the meridian tunnels. [The meaning that Ying Qi originates from the Middle Jiao becomes clear.] *Ling Shu · Jue Qi* chapter states: "The Middle Jiao extracts juices, transforms them into redness, and this is called blood. It constrains Ying Qi, preventing it from deviating, and this is called the vessel." [To constrain here means to press or urge, forcing entry into the tunnel so that no other path is available for avoidance.] *Ling Shu · Wei Qi* chapter states: "Its essential qi circulating within the meridians is called Ying Qi." *Ling Shu · Ying Wei Hui Hui* chapter states: "The Middle Jiao also lies within the stomach, posterior to the Upper Jiao. The qi received here separates dregs, steams fluids, transforms essence and subtle substances, and directs them upward to the lung vessels, thereby transforming into blood. To nourish the living body, nothing is more precious than this; thus, it alone can travel through the meridian tunnels." Synthesizing these discussions, we understand that 'blood' is the general term, while 'vessel' refers specifically to the unceasing pathway of Ying Qi circulation. If one were to separate the vessel from the blood, in reality, there would be no distinction. However, without respiration, circulation cannot occur; hence it is said that it corresponds to breath. (Zhang Zhongjing's *Ping Mai Pian* states: "Respiration is the head of the pulse." Furthermore, *Ling Shu · Dong Shu* chapter states: "Its movement occurs with the coming and going of breath.")