Interoception
The sense of the body's own internal state. It is the channel through which deliberately changing a physiological variable can change how a person feels, and breathing is the one such variable that is both automatic and available to voluntary control.
What it is
Heart rate, breathing, gut, temperature. Most of it never reaches awareness, and the part that does arrives as mood rather than as information, which is why people are so bad at saying why they feel as they do.
The practical consequence is that breathing occupies an unusual position. It runs without attention and it can be taken over at will, which makes it the only obvious handle on an otherwise automatic system.