The normal glomerular filtration membrane allows the smooth passage of proteins with molecular weights <20,000 to 40,000, so the glomerularly filtered primary urine contains mainly small-molecule proteins (such as lysozyme, β2 microglobulin, light chain proteins, etc.), and albumin (molecular weight 69,000) and immunoglobulins with larger molecular weights are less abundant. More than 95% of the protein in the primary urine filtered by the glomerulus is reabsorbed by the proximal tubule, so the protein content in the final urine of normal people is extremely low (<150mg/d), of which about half of the protein content comes from Tamm-Horsfall protein and other urogenital proteins secreted by the ascending branches of the distal tubule and medullary collaterals, and the other half is albumin, immunoglobulin, light chain, β2 microglobulin and various enzymes. The other half of the protein composition is albumin, immunoglobulin, light chain, β2 microglobulin and various enzymes. Normal human urine is not clinically detectable by qualitative urine tests because of the low protein content. When urine protein exceeds 150 mg/d, the urine is positive for protein and is called proteinuria. The glomerular filtration membrane is composed of glomerular capillary endothelial cells, basement membrane and dirty layer epithelial cells. The role of the filtration membrane barrier includes: 1, molecular barrier; 2, charge barrier. Damage to any of these barriers can cause proteinuria, and glomerular proteinuria is often dominated by albumin.