Salivary acinar cells secrete a primary fluid that is isotonic to plasma. As this fluid passes through the salivary ducts, it becomes hypotonic because the ductal epithelium:

• A Secretes NaCl actively while allowing free movement of water along the osmotic gradient
• B Reabsorbs Na+ and Cl- faster than it secretes K+ and HCO3-, and is relatively impermeable to water ✓
• C Reabsorbs K+ and HCO3- while secreting Na+ and Cl- into the lumen
• D Adds a protein-rich exudate that dilutes the electrolyte content of the fluid

Explanation

Ductal cells reabsorb sodium and chloride and secrete potassium and bicarbonate into the lumen. Because the ducts are tight epithelia with very low water permeability, solute leaves without water following, so the final saliva is hypotonic. Option C reverses the direction of ion exchange, which is the classic trap. At high flow rates there is less time for ductal modification, so saliva becomes closer to plasma tonicity.

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

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Written and medically reviewed by the StethoPrep medical team.