Proteinuria

Proteinuria means protein in the urine. It is not one single protein: the term covers albumin, globulins and others. And what matters is persistence: when proteinuria persists, it is a marker of kidney damage.

It mostly starts in the filter. Inside the kidney, each glomerulus has a filtration barrier made of three layers stacked one on top of another: the fenestrated endothelium, the vessel wall with pores; the glomerular basement membrane, the supporting sheet in the middle; and the podocyte, a cell with long feet that wraps around the vessel from the outside. That is where blood is filtered, and whatever crosses those three layers is what ends up in the urine.

And that filter does not sort by size alone. The wall of the glomerular capillary carries a negative charge, and that charge repels proteins that are also negative, such as albumin, so even the ones small enough to fit do not get through. It is a three-mesh strainer that also uses charge to turn away what would otherwise pass on size alone. When the permeability of that barrier is altered, albumin begins to leak into the urine.