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NBME 20 Answers

nbme20/Block 4/Question#16

An investigator wishes to determine the ...

Ductal ion reabsorption is more efficient at low flow rates

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Overview of Saliva production:

  1. Acinar cells secrete initial saliva, which is isotonic to plasma.
  2. Ductal cells modify initial saliva, causing reabsorption of Na+, Cl- and secretion of K+ and HCO-. -> higher K+ and HCO- concentrations, lower Na+ Cl- concentrations in saliva. In this process, MORE NaCl is reabsorbed, meaning that saliva ends up being hypotonic to plasma.

Ductal reabsorption process is hindered in high-flow states, which causes isotonic saliva

Salivary secretion 1. At low flow = High concentration of potassium; low concentrations of sodium, bicarb, & chloride 2. at high flow = low concentration of potassium; high concentrations of sodium, bicarb, & chloride

sherry  That's exactly what I was thinking when I was taking the test. But I was sidetracked by same HCO3 level. Can somebody explain this part to me?? +  
charcot_bouchard  Because salivary duct removes Na & Cl while secrete K & Hco3 in lumen. In low flow rate HCO3 & K inc because duct is doing its thing for more time. At high flow rate K slightly dec (as cant be secrted as much) but HCO3 stays almost same. the reason is high flow indicates higher metabolism & higher bicarb production. +  
cienfuegos  Regarding the bicarb (via BRS Physiology, which explains flow rate as coming down to "contact time" where slow flow allows more reabsorption of NaCl): The only ion that does not β€œfit” this contact time explanation is HCO3βˆ’; HCO3βˆ’ secretion is selectively stimulated when saliva secretion is stimulated. +2  

Pt 1 (takes citric acid)

Pt 2 (takes candy)

Pt 1 (2x saliva = high flow rate, 2xNaCl [irrelevant - ignore])

Pt 2 (1x saliva = low flow rate, 1xNaCL [ignore not imp])

Pt1 = citric acid stim* ductal cell to secrete intracellular ductal storage of bicarb. bicarb neutralizes citric acid. thus low bicarb.

Pt2 = low flow rate, thus more bicarb reabsorbed out of duct. thus low bicarb

why do they have the same bicarb, because low flow rate.

*via Ach signal transduction to increase intracellular ductal calcium release --> HCO3 stored inside ductal cell