Why Is There Protein in Urine

In normal condition, for a normal person, the total amount of protein in urine is less than 150mg in 24 hours. Even for young men, the maximum is 300mg 1n 24 hours. If the amount of urine is more than the maximum, we will say that the person has proteinuria.
From the mechanism perspective, proteinuria can be caused by mainly four factors such as the damage of renal glomerular filtration barrier, the damage of renal mesangial cells, the reabsorption of renal tubules, the secretion and discharge of renal tubules. In this passage, we mainly introduce the first factor: the damage of renal glomerular filtration barrier.
From the above, we can get a conclusion that many factors influence the leaking out of protein in urine such as the size of the protein, the charge amount they take, the structure of molecules, and so on. The filtration membrane is made up of three layers, epithelial layers, basilar membrane, and endothelial layers. From the microscopy, we can see that there are numerous pores in the epithelials, the diameter of the pore is 600～1000A. In endothelial cell layers, there is crack, 100～400A in width and 200～900 in length.
In addition, haemodynamics is also a factor which can influence the protein passing renal filtration membrane. If the blood flow in renal glomerular capillary plasma decreases, or the inner pressure in renal glomerulus increases, both of them can increase the permeability of renal glomerular filtration membrane.