As a result of the non-selectivity of the kidney's filtration of small molecules, see concept 44.4 (page 986)

As a result of the non-selectivity of kidney's filtration of small molecules, useful substances must be selectively reabsorbed to the blood.  Filtration is the mass movement of water and solutes from plasma to the renal tubule that occurs in the renal corpuscle. About 20% of the plasma volume passing through the glomerulus at any given time is filtered. Glomerulus lets fluid and waste products pass through it, however, it prevents blood cells and large molecules, mostly proteins, from passing. 

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