The process of glomerular filtration is very selective because of the way the filtration membrane is set up.
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#.......#The filtration membrane is comprised of three things:#..........#
#color(white)(------)color(blue)"The endothelium"#
#color(white)(------)color(green)"Glomerular basement membrane"#
#color(white)(------)color(orange)"The filtration slits"#
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Below is a wonderful image showcasing you on exactly how the filtration membrane of the glomerulus is set up.
Now don't be confused or intimidated. The filtration membrane is like a 3-point check, where at every point some molecules are able to pass by and some have to stay behind. The direction of filtration starts from the capillary endothelium (on the bottom of the image) and ends after going through the filtration slits (at the top of the image).
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#color(white)(-------)color(blue)barul"| The endothelium |"#
This layer is lined with endothelial cells which have pores, or in other words, are fenestrated. The pores are on average #60-100" nm"# wide. This means that the pores are big enough for pretty much everything, including plasma proteins, to pass through. ALL EXCEPT RED BLOOD CELLS (RBC).
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#color(white)(-----)color(green)barul"| Glomerular basement membrane |"#
This is the #2^(nd)# checkpoint. Here, solutes such as ions, organic molecules, water (along with other molecules not mentioned), and ONLY SMALL PROTEINS are able to cross through. THIS MEANS, THIS LAYER PREVENTS THE CROSSING OF LARGER PROTEINS.
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#color(white)(-------)color(orange)barul"| The filtration slits |"#
And here we have the #3^(rd)# and final checkpoint. It is here where nearly all small plasma proteins stay behind, not able to cross through (the slits are around #~7" nm"# wide.) So basically, the filtrate is finalized and is composed of ions, water, vitamins, amino acids, urea, #H^+#, nitrogen waste, glucose, bicarbonate and NEARLY NO PLASMA PROTEINS.
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If you find plasma proteins or RBC in your urine, there is some damage at the level of the glomerular filtration membrane.
