If glomerulonephritis increases capsular permiability (leading to haematuria) why does it also result in oligouria and decreased GFR?

I've never seen a good explanation for this myself. The best explanation I've ever seen came from a colleague of mine, who thought of it like this: normally, there are stacks of nephrons in the kidney, doing their thing, filtering the blood, etc.

In diseases like glomerulonephritis, a lot of these become damaged and stop filtering, which is why GFR goes down. The remaining ones, though, are often damaged so don't filter everything like they're supposed to, so they start to let blood and protein through.

So that's how we saw it. There are fewer functioning units, so reduced GFR, but they are letting anything through, so haematuria and proteinuria.