What does the urinary system filter?
The filtration takes place in the kidneys. For the diagram of
the urinary (or renal) system, search either of them in the google
images.
As for the exact area where this happens, search for the
"bowman's capsule" or "glomerulus". This is a bed of capillaries
where the 'junk' is filtered out from the blood. The blood flowing
in the glomerulus (which happens to be in the bowman's capsule) is
under a large pressure, which forces the impurities out.