The title gives away the central image: the branching bronchial tubes inside the lungs, which look just like an upside-down tree. The speaker addresses this "blood tree" directly, as if it were a strange creature that has appeared from inside the body, "fully formed like a god." Then come questions, and no answers. Will it decay into a "travesty of spring," a false, rotten version of new growth? What birds could nest in branches that are going out of tune, and what would they sing?
The tone is uneasy and a little tender. The speaker seems to be imagining the body's fragility, and the poem hints at illness or mortality, though it never says so outright. The last questions turn darker: does the tree long to be free of oxygen, the very thing that keeps us alive, and would it burn? The poem ends on that single, stark line, leaving a quiet, eerie feeling of breath, decay and fire sitting side by side.