Nephron (source code)
= Nephron
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.<Your Kidneys and How They Work by NIDDK>[How do my kidneys work?: glomerular filtration and tubular processing]{ref}
For a specified substance, an accounting relation is
$$
\text{excreted amount}=\text{filtered amount}-\text{reabsorbed amount}+\text{secreted amount},
$$
when 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.
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