In order to understand glomerular filtration, its necessary to see that the glomerular Starlings Forces are only half of the story. The glomerulus is surrounded by the Bowmans capsule, and filtrate is forced into the Bowmans capsule.  Fluid exchange in the Bowmans capsule is governed by the same Starlings forces operating in the glomerulus. That is, the fluid there also exerts a hydrostatic pressure and it, in effect, opposes the hydrostatic pressure coming from the glomerulus.

In theory there should be virtually no plasma proteins in the Bowmans capsule, since blood cells and plasma proteins should not be filtered from the glomerulus.