As we all know, normal people do not have protein in urine because there is filtering barrier inside the kidneys which can keep the protein inside blood vessels and prevent leakage of protein. This filtering barrier includes mechanic barrier and electric charge barrier. And both of the two kinds of barrier is composed of the glomerular endothelial cells, glomerular basement membrane, glomerular epithelial cells. And there are pores between those cells, which micro-molecule protein can be leaked out from those pores but will be reabsorbed by tubular epithelial cells. This is how mechanic barrier works. And besides, there are some substances secreted by those glomerular epithelial and endothelial cells which carry negative charges, and we all know that protein also carries negative charges, so, those substances on the surface of those cells can prevent leakage of negative protein.
However, when there is high blood pressure, infection, inflammation, etc. that change the hemodynamics or damage the mechanic barrier or charge barrier, then protein will go into urine.
But different causes can lead to leakage of different type of protein in urine, so, figuring out which type of protein it is, analyzing the exact factors causing this pathological change and treating accordingly is the basic way to treat protein in urine.
