Yogic and tantric texts describe a body made of channels through which a vital breath is said to move. The description functions as a practitioner's map rather than as anatomy.
The system is described in its own terms
These texts speak of nadis, meaning channels or conduits, and of prana, a vital breath understood to move through them and to divide into functions within the body.
Numbers of channels vary enormously between texts, running into thousands in some accounts, with a small number treated as principal and given individual names.
The variation suggests these are not attempts at a fixed inventory. The named channels matter because practices are attached to them, and the rest fill out the scheme.
Three channels carry the main weight
Most accounts single out a central channel running along the axis of the body, flanked by two others that are associated with alternating currents or tendencies.
Breath practices in these traditions are frequently described as balancing the two flanking channels so that activity can move into the central one.
Alternate-nostril breathing and similar exercises are explained in exactly these terms, which is why the technique makes little sense when the framework is dropped.
Prana is divided by function
The texts subdivide vital breath into several functional aspects, associated with upward movement, downward movement, distribution, digestion and pervasion through the body.
Each is located in a region and connected with processes occurring there, giving the scheme a practical vocabulary for describing where a practitioner's attention is directed.
The divisions are used diagnostically within the tradition, with imbalance in one aspect described as producing characteristic difficulties in practice.
The map is instructional rather than anatomical
These structures are not claims about tissue that could be confirmed by dissection, and the traditions themselves generally describe them as subtle rather than gross.
What the map does is give practitioners a shared vocabulary for internal sensation and a set of locations to direct attention toward during practice.
Read that way, the descriptions are coherent and detailed, and treating them as failed physiology misses the work they were composed to do.
Why the scheme persisted
A shared map allows a teacher and a student to discuss experience that is otherwise private, which is a substantial practical advantage in transmitting practice.
It also organises a large body of technique into an order, indicating which practices are preparatory and which depend on the earlier ones having been established.
Contemporary teaching in these lineages still uses the vocabulary, and understanding it as a map of practice rather than a description of tissue makes that usage intelligible.