Describe the mechanisms by which water and electrolytes are reabsorbed across the renal tubules.
The intent of the question was to have the candidate describe in the context of a
classification the mechanisms by which water and electrolytes may cross a cell membrane
and use the renal tubule to provide an example of each mechanism.
It was expected the answer would talk about transport “mechanisms” across membranes.
These would include processes such as reabsorption, diffusion, facilitated diffusion, primary
and secondary active transport, endocytosis, osmosis and solvent drag. Many candidates
used colloquial and vague language to describe precise concepts.
Some candidates structured their answer as an outline of the principal mechanisms at each
segment of the tubule. Thus there was repetitive reference to mechanisms without a
description as requested in the question. This approach also resulted in some candidates
omitting some mechanisms altogether.
Some might say that if "intent of the question was to have the candidate describe in the context of a classification the mechanisms by which water and electrolytes may cross a cell membrane", then perhaps the question itself should be worded, "Describe in the context of a classification the mechanisms by which water and electrolytes may cross a cell membrane; use the renal tubule as an example of each mechanism". This infuriatingly poor assessment design has led to 67% of the candidates failing this question purely because they read the question carefully and tried to answer it as it was phrased.
So... What did they want? Perhaps something like this:
So, they asked "describe the mechanisms by which water and electrolytes are reabsorbed across the renal tubules", where they actually mean was "describe the mechanism by which molecules can cross membranes, using the tubule as an example". Most people seem to have interpreted this SAQ quite literally, and described the handling of water and solutes in the different segments of the nephron, which turned out to be a disastrous move. Only 33% of them passed. The following answer therefore addresses the remaining 67%, and transports them to a parallel universe where CICM exam answers have some direct relationship with the phrasing of CICM exam questions.
|Segment||What happens to water||Regulatory mechanisms|
Main mechanism is to influence glomerular blood flow:
|Proximal convoluted tubule||
|Descending thin limb||
|Ascending thin and thick limbs||
|Distal convoluted tubule||
|Connecting tubule and collecting duct||
Wilson, David B. "Cellular transport mechanisms." Annual review of biochemistry 47.1 (1978): 933-965.
Yang, Nicole J., and Marlon J. Hinner. "Getting across the cell membrane: an overview for small molecules, peptides, and proteins." Site-Specific Protein Labeling. Humana Press, New York, NY, 2015. 29-53.
Stein, Wilfred. Transport and diffusion across cell membranes. Elsevier, 2012.
Cussler, E. L., Rutherford Aris, and Abhoyjit Bhown. "On the limits of facilitated diffusion." Journal of membrane science43.2-3 (1989): 149-164.
Wu, Ling-Gang, et al. "Exocytosis and endocytosis: modes, functions, and coupling mechanisms." Annual review of physiology 76 (2014): 301-331.