GI physiology: salivary secretion: practice question

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Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Sodium and bicarbonate concentrations rise and potassium falls, with saliva still hypotonic to plasma

Acinar cells secrete an isotonic fluid resembling plasma, which is then modified in the ducts by reabsorption of sodium and chloride and secretion of potassium and bicarbonate. At high flow rates there is less time for ductal modification, so saliva approaches plasma composition: sodium and chloride rise and potassium falls. Saliva remains hypotonic because the duct is relatively impermeable to water.

Option by option

• A. All electrolyte concentrations fall in proportion to the greater volume

  Concentrations do not fall in proportion; they change differently according to ductal handling.

• B. Sodium and bicarbonate concentrations rise and potassium falls, with saliva still hypotonic to plasma (correct)

  Shorter ductal transit raises sodium and bicarbonate and lowers potassium, although saliva stays hypotonic.

• C. Sodium falls and potassium rises, with saliva becoming hypertonic to plasma

  The direction is reversed; sodium rises and potassium falls with faster flow.

• D. The composition remains the same because acinar secretion is isotonic

  The initial secretion is isotonic but the ducts alter it, and flow rate determines how much.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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