As arresting claims go, "a river of piss runs through art history" is a good one. The New Yorker's Dan Piepenbring substantiates it with reference to Claude Lebensztejn's new book Pissing Figures, 1280-2014, which details the recurrent depictions of urination in fine art. In so doing, it traces a transition from the innocent, cherubic pissing boy of Renaissance frescoes to the eroticised pissers of twentieth-century art, and the actual use of urine in the creation of artworks. Nevertheless, it just goes to show that an obsession with bodily functions and bodily fluids isn't only a contemporary one, or that it is confined to so-called "low art".
(Thanks to Adam for the link.)