For normal adults, urinary protein excretion should not exceed 150mg per day. A large number of proteinuria is caused by abnormal glomerular filtration membrane. The normal glomerular filtration membrane has a selective filtration effect on plasma proteins, which can effectively prevent most plasma proteins from glomerular filtration, and only a small amount of plasma proteins enter the glomerular filtrate. Factors affecting protein filtration may be:
Molecular size of a protein: the removal of a substance by a glomerular capillary is inversely proportional to the effective molecular radius of the substance, and the greater the molecular weight of the protein, the less or no more filtration. Under normal circumstances, the molecular weight in 60 thousand ~ 70 thousand Dalton plasma protein filtration is less, the molecular weight is greater than 200 thousand, Dalton can not filter, and the smaller molecular weight plasma protein can be free filtration. The filtration effects vary depending on the molecular weight of the protein. The barrier is determined by the ultrastructure of the glomerular filtration membrane.
Take charge of protein: the inner layer and the outer layer of the glomerular basement membrane, glomerular capillary loops, endothelial surface and epithelial cell membrane matrix contains rich amino polysaccharide and sialic acid, both the glomerular filtration membrane with negative charge, constitute the electrostatic barrier. The principle of isotropic charge repulsion, negatively charged protein clearance rate is the lowest, while having a positive charge removal rate is the highest. Studies have shown that the glomerular basement membrane sialic acid content is markedly reduced in glomerular diseases, which results in the filtration of albumin with negative albumin. In addition to the electrostatic barrier, the glomerular negative charge field also has the function of maintaining cell morphology and capillary structure.
The morphology and variability of proteins: because of the mechanical barrier of the glomerulus, molecules with loose, threadlike morphology are more likely to pass through the glomerular filtration membrane than molecules that are closely aligned and spherical.
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