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Saliva Hypotonicity Mechanism — MFDS Part 1 MCQ

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HardPhysiologySaliva Hypotonicity MechanismMFDS Part 1

An isolated major salivary gland produces a sodium chloride-rich primary acinar fluid that is approximately isotonic with plasma. After this fluid passes through the ductal system, its volume changes little but its osmolality falls. Which epithelial transport mechanism best explains these findings?

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Reveal the answer and explanation

Correct answer: D — Net ductal reabsorption of sodium and chloride exceeds secretion of potassium and bicarbonate, while little water follows

Explanation lettering: E = shown as A · C = shown as B · D = shown as C · A = shown as D · B = shown as E

A is correct. Acinar cells initially produce a sodium chloride-rich secretion that is approximately isotonic with plasma. During ductal passage, sodium and chloride are reabsorbed and potassium and bicarbonate are secreted, with net removal of solute. Because the ductal epithelium has limited water permeability, little water follows the reabsorbed ions, so final saliva becomes hypotonic without a comparable fall in volume. Option B incorrectly describes the primary acinar fluid as hypotonic. Option C incorrectly attributes dilution to active ductal water secretion. Salivary proteins do not generate the hypotonicity described in option D, and calcium phosphate precipitation is not the physiological mechanism responsible, excluding option E.

Reference: Abd El-Aziz TM, Singh BB. Ion Channel Integration and Functional Coupling in Salivary Gland Fluid Secretion. Cells. 2026;15(4):369. https://pubmed.ncbi.nlm.nih.gov/41744812/