The Body Yoga Talks About Is Not Only the Physical One
Most people meet yoga as a set of shapes to get into. The system it comes from is describing something else entirely.
Koshas, prana and nadi
In yoga the physical body is the outermost of several layers, the koshas, and inside it is an energetic body through which prana, life force, is held to move. It moves along channels called nadi, which are not anatomical structures and are described much the way the meridians of Chinese medicine are. Yoga puts the number of them at 72,000, which is a traditional figure rather than a measurement.
Chakras
Where those channels cross and bind up, the tradition describes energy centers, the chakras. The useful way to think of them is as constrictions rather than as organs: places where movement can get stuck. Three main channels run along the spine and meet at the seven principal ones.

