Four words, one image: lungs that feel "nicely mitred." A mitre is the angled joint a carpenter cuts so two pieces of wood meet cleanly at a corner. Grenier applies that woodworking precision to the inside of the body, which is a surprising and slightly funny move. Lungs aren't something we normally notice, let alone picture as neatly joined.
The speaker seems to be describing a moment of physical ease. Breathing feels smooth, fitted, with no catch or strain. The word "nicely" keeps the tone modest and casual, as if this were a small private satisfaction rather than a grand statement.
The short lines, one word each, slow you down. You almost breathe in rhythm with them, pausing at every line break. The mood is calm, quiet and attentive. It asks you to notice something you usually ignore, and to find a little pleasure in simply being able to breathe well.