Resting (basal) human saliva is markedly hypotonic compared with plasma, despite being secreted by acinar cells as an isotonic fluid. Which property of the salivary duct epithelium accounts for the final hypotonicity?

• A The ducts actively secrete sodium and chloride into the lumen
• B Aquaporin channels are completely absent from both acinar and ductal membranes
• C The ducts absorb large quantities of urea, creating an osmotic gradient
• D The ducts are relatively impermeable to water while reabsorbing NaCl and secreting K+ and HCO3- ✓

Explanation

Acini secrete an isotonic primary fluid resembling plasma in electrolytes. As it passes down the striated ducts, Na+ and Cl- are reabsorbed and K+ and HCO3- are secreted, but this duct epithelium is tight and nearly water impermeable, so water cannot follow the solute and the final saliva remains hypotonic. At high flow rates there is less time for ductal modification, so saliva approaches isotonicity and its bicarbonate content rises.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.