Why does protein concentration increase from the arterial blood to the postglomerular blood

Proteins have a very high molecular weight and cannot be present as macro molecules in the glomerular blood. Before even reaching the blood, they are well masticated and processed to simple amino acids and other nitrogenous compounds like urea, uric acid etc. (in the digestive system itself). They are eliminated from the body while glomerular filtration through urine. The glomerular blood thus becomes free from such compounds before leaving the kidney.