The channels are older than the texts that describe them completely. The earliest surviving accounts are the cauterisation manuscripts recovered from the Han tomb at Mawangdui, sealed in 168 BCE, which describe eleven vessels rather than twelve, give them no points, and do not yet connect them into a circuit. The scheme the atlas records, twelve channels forming a closed loop with named and numbered points, is set out in the Huangdi Neijing, the Inner Canon, assembled around the first century BCE and expanded for centuries afterwards.
The Lung channel is placed first in that circuit, and its position is doctrinal rather than accidental. In the account given by the tradition, qi enters the body with the breath, so the circulation begins where breathing begins, and the Lung is described as governing qi and as the uppermost of the organs, the canopy over the rest.
The same placement drives the horary clock, the scheme that assigns each of the twelve channels a two hour period of the day in which its qi is held to be at its height. The Lung takes the first watch, from three to five in the morning. Twelve channels at two hours each closes the day exactly, which is the kind of fit that tells a reader they are looking at a designed system rather than an observed one.
The atlas records what Chinese medicine holds about these channels. It does not assert that the channels correspond to any anatomical structure, and nothing here is medical guidance.