A nephron is a microscopic functional unit of a kidney, combining a filtration apparatus with a tubule that modifies the resulting fluid. The glomerulus filters fluid from the blood into the initial urinary space; the tubule selectively returns substances toward the circulation and secretes others into its lumen.[ref]
Your Kidneys and How They Work by NIDDK. How do my kidneys work?: glomerular filtration and tubular processing
For a specified substance, an accounting relation iswhen all terms refer to the same interval and other gains or losses are absent. Filtering 100 arbitrary units, reabsorbing 95 and secreting another 2 gives excretion of 7 units. Filtration alone therefore does not determine urine composition. The anatomical separation of blood and tubular lumen supplies the geometry for this controlled exchange.
New to topics? Read the docs here!