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Q.
In the presence of vasopressin, the greatest fraction of filtered water is absorbed in
Excretory Products and their Elimination
Solution:
Vasopressin also known as antidiuretic hormone from the posterior pituitary gland increases the permeability of the collecting ducts to water by causing the rapid insertion of aquaporin-$2$ water channels into the luminal membrane of principal cells. In the presence of enough vasopressin to produce maximal antidiuresis, water moves out of the hypotonic fluid entering the cortical collecting ducts into the interstitium of the cortex, and the tubular fluid becomes isotonic. In this fashion, as much as $10 \%$ of the filtered water is removed. An additional $4.7 \%$ or more of the filtrate is reabsorbed into the hypertonic interstitium of the medulla, producing a concentrated urine.