(1) how do the kidneys clean the blood? . a) by breaking down nutrients. b) kidneys clean the blood by filtering it to remove wastes. c) by absorbing nutrients. d) by destroying microbes and preventing preventing infection. (2) what role does skin play in the excretory system? . a) secretes excess water as sweat. b) protects against pathogens. c) absorbs water through osmosis. d) facilitates gas exchange