NBME 24 Decreased blood volume (NBME Answers)

NBME 24 Answers

nbme24/Block 3/Question#34 (reveal difficulty score)
An otherwise healthy 35-year-old man sustains ...
Decreased blood volume 🔍 / 📺 / 🌳 / 📖
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+27 upvote downvote
submitted by ∗tea-cats-biscuits(273)

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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.

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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? +3

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 +5

faus305 @sars that is every question +

srmtn figure 1 of this article: https://www.nursingtimes.net/clinical-archive/cardiovascular-clinical-archive/effects-of-bedrest-1-introduction-and-the-cardiovascular-system-26-11-2018/ +

victorlt14 @lola915 the question refers to sodium concentration though, thus increased natriuresis wouldn't change it as the patient is losing both salt and water +1

+21 upvote downvote
submitted by ∗colonelred_(124)

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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.

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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. +12

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 +5

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 +

peqmd Pretty good image here https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.slideshare.net%2Fskshess%2Fapp-e-deleterious-effects-of-bedrest&psig=AOvVaw3bd9-XKHL505vECzCTJJMM&ust=1582938661456000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCNiWk5eI8-cCFQAAAAAdAAAAABAJ +

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. +

+1 upvote downvote
submitted by ∗zelderonmorningstar(96)

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

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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. +5

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. +6

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submitted by ∗an1(114)

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I use parasympathetic/ sympathetic principles to figure this out. If you're laying down, you're most likely resting and digesting. That rules out E, C (RAAS is due to high Sympathetic tone) and also D (high plasma volume is a result of RAAS). This left A and B. Decreased plasma sodium volume would have been the case if he was drinking a lot of water, or had SIADH possibly. I couldn't really find the connection between this option and his PNS activity. So I went for A, especially because it was directly opposite of one option I KNEW was wrong.

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submitted by ∗cocoxaurus(59)

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Found this link pretty helpful: https://www.nursingtimes.net/clinical-archive/cardiovascular/effects-of-bedrest-1-cardiovascular-respiratory-and-haematological-systems/5002005.article

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moxomonkey ain't nobody got time for that +7

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submitted by ∗unknown001(9)

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FROM THE ANSWER CHOICES, ITS A RENAL POHYSIOLOGY QUESTION, SO THE GUY THAT MENTIONED ABOUT THE ANP STUFF MAKES THE MOST SENSE.

GRACIAS TEASCATSBISCUITS

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drdoom hi there. please limit your use of all-caps. thanks! +1

unknown001 noted. i just found this website so amazing so i guess coudnt put a leash on my excitement +

drdoom lol fair enough +

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