Proteinuria is a typical symptom of chronic kidney disease. If your urine contains a higher amount of foam, you need to be alert if it is proteinuria, so how exactly is proteinuria formed? Healthy urine protein Glomerular capillaries are composed of endothelial cell layer, basement membrane layer and epithelial cell layer, all three layers of cells are distributed with filter pores of different sizes. Under normal circumstances, these pores can only allow small molecules to pass through, while large molecules of protein cannot pass through these pores. In addition, these filter membranes are negatively charged and will repel protein molecules that are also negatively charged. Therefore, the urine excreted by a healthy person is protein-free. How proteinuria is formed However, when the glomerular filtration membrane is damaged by inflammation and other factors, the pore size of the filter pores increases and breaks, and the electrostatic barrier is weakened, then plasma proteins are filtered through the filter pores, and once the reabsorption capacity of the proximal tubule is exceeded, proteinuria is formed. If the glomerular damage is severe, even globular protein will be filtered out. Proteinuria can also occur in kidney tubular diseases such as infection and poisoning, because of reduced reabsorption. Other types of proteinuria are mixed proteinuria, overflow proteinuria, and tissue proteinuria. It is worth mentioning that the amount of proteinuria does not reflect the pathological damage of nephropathy and must be judged by a nephrologist after a thorough understanding of your condition.