Resting (unstimulated) saliva collected from the parotid duct is markedly hypotonic compared with plasma. The mechanism responsible for the low osmolality of final saliva is:

• A Active transport of water out of the acinar cells
• B Reabsorption of Na+ and Cl- by striated ducts that are relatively impermeable to water ✓
• C Secretion of a protein-rich primary fluid that dilutes the electrolytes
• D Aldosterone-mediated exchange of sodium for potassium in the ducts

Explanation

Acini produce an initial secretion isotonic with plasma. As it passes along the striated ducts, Na+ is actively reabsorbed and K+ secreted, with Cl- following passively, but these ducts are tight to water, so the final saliva becomes hypotonic. Aldosterone does modulate ductal transport but is not the reason saliva is hypotonic at rest.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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