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TBR Bio: ADH and Plasma Osmolarity

Discussion in 'MCAT Study Question Q&A' started by nj484, Jan 13, 2014.

  1. nj484

    nj484 7+ Year Member

    Apr 15, 2009
    From Bio Chapter 3 on the Kidney, the question asks to find the graph that best represents the plasma osmolarity (X-axis) of the blood and the plasma concentration of antidiuretic hormone (Y-axis).
    The answers include the following curves (try to get the best general shape example):

    A. sigmoid function with flat vertical region like the titration of a strong acid with a strong base: at one point there is a huge increase in ADH before plasma osmolarity is really impacted

    B. flat horizontal region like a titration curve (see general curve below going through the origin: the idea that ADH increases in 2 phases as ADH concentration stays the same before increasing again with increased plasma osmolarity? the shape is similar to this: [​IMG]

    C. increasing log function like saturation kinetics. graph shape similar to this:
    D. graph where X increases and Y stays the same until 1 point where there is a linear positive increase in the graph (see graph below)

    The answer given by TBR is that the best graph is the sigmoid function (A) with a flat vertical region in that there is a quick response from ADH and as ADH is being released the osmolarity doesn't change much.

    Why is it not choice D where there is a threshold at which plasma osmolarity is sensed by osmoreceptors to release ADH?
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  3. tom_bombadil


    Feb 16, 2014
    Did you ever figure out the answer to this? I have the same question
  4. dyspareunia


    Mar 6, 2014
    Initially as ADH is released it has tons of receptors to bind -> lots of AQP2 inserted. Eventually there aren't anymore places for AQP2s to be inserted and the graph will flatten out. The concept is saturation as it relates to enzyme kinetics.
  5. kingofmedicine

    kingofmedicine That is how daddy eat. WOOF. 2+ Year Member

    You may be right. No one is perfect...even a test writer.

    From what I read and understand, the concept is that all sigmoid shaped curves have allosteric activators and allosteric inhibitors (aka something that turns it on and something that turns it off, respectively of course).
    Last edited: Apr 29, 2014
  6. Lunasly

    Lunasly 5+ Year Member

    May 17, 2010
    Great explanation. I suppose that what you are describing is not physiologically realistic, while the last graph that the OP showed is from an actual study that represents true conditions. I suppose that this question was questioning our ability to reason what would happen no more aquaporins could be placed.

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