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. In general, the molecular weight of 60 thousand to 70 thousand less plasma protein filtration Dalton, molecular weight of more than 200 thousand Dalton to filter out, the plasma proteins with low molecular weight, such as lysozyme, beta 2-mg and immunoglobulin light chain, can be freely filtered. The filtration effect is due to the different molecular weight of proteins, which is called the molecular barrier. The barrier is determined by the ultrastructure of the glomerular filtration membrane. The glomerular filtration membrane consists of the endothelium, the glomerulus, the basement membrane, and the epithelium. The gap between the endothelial cells of 40 ~ 100nm, plasma soluble substance can be through GBM; from the loose layer, dense layer and loose layer, a filtering GBM, the radius of the hole is 3.5 ~ 4.2nm, forming a layer of coarse filter, allowing part of albumin and transferrin by. Epithelium: fissures in the epithelial processes of the epithelial cell, with a membrane on it, a small pore, and a pore diameter of 4 * 14nm, forming a fine filter that prevents the molecules larger than albumin.
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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