Useful substances, such as protein and red blood cells, are too big to pass through the holes in the filter. Diabetes can damage this system. High levels of glucose make the kidneys filter too much blood. All this extra work is hard on the filters. After many years, they start to leak.
Useful protein is lost in the urine.
Having small amounts of protein in the urine is called microalbuminuria. Having larger amounts is called proteinuria or macroalbuminuria. In time, the stress of overwork causes some filters to collapse. This collapse makes more work for the remaining filters and they, too, begin collapsing. As the capillaries lose their filtering ability, waste products start to build up in the blood.