Filtration, Reabsorption, Secretion: the Three-Step Mechanism
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The nephron processes fluid in three steps: filtration forces fluid out of the blood, reabsorption returns needed substances to blood, and secretion adds waste directly into the forming urine.
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Glucose is normally fully reabsorbed after filtration, so healthy urine contains none.
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Filtration is nonselective for small molecules — selectivity happens afterward, through reabsorption and secretion.