Dave Bonta's "Small bladder blues" is a funny, affectionate complaint addressed straight to the speaker's own body. The title borrows the language of the blues, a song of lament, and applies it to something wonderfully undignified: the need to pee. The speaker pleads with the bladder to hold on, asking it to "reserve one / spoonful of beer," which suggests a drinker who knows exactly how this will end.
The images get sharper in the middle. Calling the bladder "a new-drawn knife" in the morning and evening suggests an urgent, almost painful sensitivity at those times. Then comes the canary, a bird known for its song, who here "never sing[s] till night" and then lets out "a great discharging." The joke is that the release is treated like music.
The mood is playful and a little rueful. The speaker sounds tired of the body's demands but fond of it too, laughing at the whole situation.