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NBME 24 Answers

nbme24/Block 3/Question#34 (78.7 difficulty score)
An otherwise healthy 35-year-old man sustains ...
Decreased blood volume🔍,📺
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 +23 
submitted by tea-cats-biscuits(251),
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eBaucse you era s,nuipe e’tsehr areidnesc ralpedo noigg akbc to your ertah (no ende to krwo aatsngi g,rtaiyv ryuo bolod s’nti onopgli in uryo elgs sa hcmu e.)itehr sA a ultre,s APN si esetdecr deu to RA esct,rth ldeigna to dsieiusr nad a ntroducei of obodl o.lmvue

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 +  
thelupuswolf  RAAS trumps ANP though, so the dec. in RAAS as said by colonelred_ is likely going to have more of an effect than ANP. +1  
lola915  If you have an increase in ANP wouldn't natriuresis occur and cause a decrease in blood sodium? +2  
mynamejeff  Because you are suprine, 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). As a result, ANP is secreted due to RA stretch, leading to diuresis and a reduction of blood volume. +1  
sars  This is a "read my mind" question and complete it in 1.5 minutes. Thanks +4  
faus305  @sars that is every question +  



 +18 
submitted by colonelred_(111),
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Loodek ti pu and ofnud tath busecae oury’e in a usienp ponsiito for a ognl itme ouy’re oigng to vahe renisecda vunoes nreurt hwcih sdeal ot inedscare .OC ihTs ieygaventl fcbdeaesk on A,SRA gialdne ot ceersdeda etloednr.sao sA a erslu,t oreu’y ggnio ot ehav sencedira uisersdi wchhi sdael ot ededaersc bldoo and aplsam olmuve.

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 ... +4  
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. +10  
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 +3  
yotsubato  You gotta be preggers to compress your IVC +5  
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 +5  
kpjk  could it be that, while low aldosterone levels decrease plasma sodium levels- there is also decrease in blood volume(plasma),so there wont be a decrease in the "concentration" of sodium +4  
almondbreeze  FA 2019 pg 306 on Lt heart failure induced orthopnea - Shortness of breath when supine: increased venous return from redistribution of blood +  
almondbreeze  if there was no HF, it would lead to increased CO --> decreased aldosterone +  
theunscrambler  @peqmd thanks for sharing that. According to the presentation, the diuresis via ANP occurs (along with sodium), which is followed up by an increased in RAAS --> maintains sodium levels. This cycle can then continue. Slide 13. +  
jj375  @urachus - Either BB or Sattar taught me this but I feel like it is often forgotten. "RAAS/Aldosterone affects blood volume, and ADH affects Na level". So Increasing aldosterone will increase blood levels however water follows the sodium so you will not get a change in sodium levels. ADH however does affect Na since aquaporins are bringing in water without affecting sodium levels. +  



moxomonkey  ain't nobody got time for that +6  



 +0 
submitted by zelderonmorningstar(83),
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Wyh is het esnwar esdrdceea dolob uvelmo as osppdeo ot decdeares almspa idmuos anoeitntconcr?

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. +4  
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. +5