Plasma protein cannot come into filtrate across the Glomerular Membrane - QS Study
QS Study

The quantity of glomerular filtrate formed per minute in all the functioning nephrons of both kidneys is called glomerular filtration rate (GFR). It is a test used to check how well the kidneys are working. Specifically, it estimates how much blood passes through the glomeruli each minute.

Size of albumin = 6 nm (M.W = 69,000)

Iso-electric pH = less than body pH (< 7.4)

So when albumin comes into the body fluid, it remains in the anionic form due to its less iso-electric pH. Glomerular membrane repels negatively charged albumin as the membrane (sialoprotein) itself is negatively charged. That’s why it cannot pass through the glomerular membrane inspite of its standard size (< 8 nm).

Fig: Glomerular Membrane

The glomerular membrane consists of mesangial cells, modified pericytes that in other parts of the body divide capillaries from each other. The podocytes bordering them have filtration slits of diameter 25 nm that is formed by the pseudopodia arising from them. The filtration slits are covered by a diaphragm that includes the transmembrane protein nephrin. The glomerular filtration barrier between the vasculature and urinary space is tailored to allow the proficient flow of water and small solutes while retarding the passage of plasma proteins, particularly albumin and immunoglobulins

Other plasma proteins are larger than albumin. They also cannot pass through the glomerular membrane. For these reasons, plasma proteins (especially albumins) are absent in ultrafiltrate.