Large proteins begin passing through the nephrons en masse and damaging them because they’re too big.

Just in case this isn't enough warning, note that this can permanently reduce your kidney function and harm your body in other ways as well.

I know it's very unlikely for anyone here to do this, but if this happens you aren't just having a "crazy workout" you are giving yourself an injury and possibly permanently affecting your future health. So please don't exercise until you piss blood.