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NBME 24 Answers

nbme24/Block 3/Question#34

An otherwise healthy 35-year-old man sustains a ...

Decreased blood volume

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submitted by colonelred_(48),

Looked it up and found that because you’re in a supine position for a long time you’re going to have increased venous return which leads to increased CO. This negatively feedsback on RAAS, leading to decreased aldosterone. As a result, you’re going to have increased diuresis which leads to decreased blood and plasma volume.

medstruggle  Doesn’t supine position compress IVC leading to decreased venous return? (This is the pathophys of supine hypotension syndrome.) There was a UWorld questions about this ... +2  
tea-cats-biscuits  @medstruggle *Supine position* decreases blood pooling in the legs and decreases the effect of gravity. *Supine hypotension syndrome*, on the other hand, seems specific to a pregnant female, since the gravid uterus will compress the IVC; in an average pt, there wouldn’t be the same postural compression. +1  
welpdedelp  this was the exact same reasoning I used, but I thought the RAAS would inactivate which would lead to less aldosterone and less sodium retention +1  
yotsubato  You gotta be preggers to compress your IVC +1  
nwinkelmann  Could you also think of it in a purely "rest/digest" vs "fight/fright/flight" response, i.e. you're PNS is active, so your HR and subsequently your CO is less? But the explanation given above does make sense. Also because I think just saying someone is one bed rest leaves a lot up for interpretation, maybe not with this patient because his pelvis is broken, but lots of people on bed rest aren't lying flat.... ? +1  
urachus  wouldnt low aldosterone cause low plasma sodium? choice B +  

Because you are supine, there’s increased preload going back to your heart (no need to work against gravity, your blood isn’t pooling in your legs as much either). As a result, ANP is secreted due to RA stretch, leading to diuresis and a reduction of blood volume.

urachus  When the person is lying down (supine position), gravitational forces are similar on the thorax, abdomen and legs because these compartments lie in the same horizontal plane. In this position, venous blood volumes and pressures are distributed evenly throughout the body. https://www.cvphysiology.com/Cardiac%20Function/CF017 +  

Why is the answer decreased blood volume as opposed to decreased plasma sodium concentration?

tea-cats-biscuits  I think it might just be what NBME feels “decreased plasma sodium concentration” means, since through the mechanism that BV is lowered in bedrest, you would definitely have a decreased plasma sodium concentration compared to not-bedrest. However you won’t be presenting with any pathologic signs of hyponatremia because the Na+ would still be maintained in normal limits. Low blood volume is the cause of one of the main pathologic states associated w/bedrest -- cardiac deconditioning and postural hypotension once out of bedrest. Seems like poor wording though. +2  
mnemonia  Remember that changes in sodium concentration over a long period time need to be due to a water dysregulation problem (like SIADH, polydipsia, HF, etc.). Here we just have physiologically increased effective circulating volume, and the body will compensate by diuresing, and since Na+ (and K+) are regulated ions, their plasma levels will not fluctuate. +2