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NBME 21 Answers

nbme21/Block 4/Question#23 (53.2 difficulty score)
An investigator is studying the regulation of ...
Intravenous infusion of 0.9% saline for 5 minutes🔍
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 +18  upvote downvote
submitted by hayayah(990),
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Lmyph folw tare is sullyau .wol It si feelcdnuni myrarliip by the arte of ylpmh maiofnrto. rFo al,xpmee if odbol alapilcry srseuerp is eicdrnsae by atrilare vdnsoiialtao ro onsuev riincto,ocsnt eth flow trea fo mplyh r.seinacse sAlo, the fwol trea si cedtffae by smposincreo of yspilhcmat yb oncrotincta fo hngierngibo uumsltcaeur nda yb vgeieatn iatrcnhtraoci uspreres )a(ine.hrgtb

tisilntrtaIe esrprues os( prsuseer ni eht ECF, wchhi wduol raneisce fi ngvie IV nsa)eil adn mlphy wlfo rea evlyitsopi alde.ret A llsma necrsiea ni iitnraieltst omeuvl leayrtg easinersc tsi re,rspesu gtomrinop ymphl owfl hatt tcas to oteresr het tiinasteitlr ovumle to almrno.

rmeo no thsi :cotip 8n4i5hv./s.ko4..g/spmo:wn//thiBtb3nl/NwbcwKo

linwanrun1357  Do not understand the breathing (choice C and D) breath in and out are different? +  
khanhluong  I don't know if it's correct or not but how I approached C & D was that they both cause vasoconstriction in the arterioles (because this is the lung where hypoxia causes vasoconstriction), which decreases hydrostatic pressure through the capillaries and eventually decreases lymph flow. Maybe I completely got this question right for wrong reason, but I felt that it works with all of the answers. For F) I was thinking that it would cause increased capillary oncotic pressure which causes more fluid to go into the capillary than into the lymph vessels... Here's a picture: http://www.lymphedemablog.com/wp-content/uploads/2011/09/Lymphatics.gif +3  



 +7  upvote downvote
submitted by drdoom(647),
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ntcrsisioatVoonc nrgr(niowa fo a bt)ue lilw esuca hte lwfo taer to ceairens ghtohru htta uet,b hchwi sadcseeer awdtao/rladrui rr.pussee hTe rfaste a ulifd vemso rtohugh a b,etu hte less t”arw“odu ofrec ti rexs.et Tis(h si knnwo sa eth uVenrit fetcfe.)

hungrybox  not seeing how this is relevant +5  
sympathetikey  He's showing how A & B are incorrect @hungrybox +7  
nerdstewiegriffin  what a moron @hungrybox is !! +2  
leaf_house  MCAT flashbacks on this image +1  

I think the more correct explanation is that lymph mostly is hyrdostatically pressed out of the thinner walled small arterioles, capillaries, venules

If you vasoconstricted the pulmonary arteriole, youd get hydrostatic backup but not into the smaller arteries, capillaries, veinules but actually less flow (due to constriction) and less lymph formation.

+/- jackie_chan(7),


 +6  upvote downvote
submitted by d_holles(139),
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02% mnlaibu is oRHnPcEYit adn asescu iwdrang fo dfiul tuo of eht srumntetiti nda toin het bloo.d

m"ibAlnu )anH(mu 2%0 is eiidntdac ni hte greenecmy ttrtneeam fo pilmhovoaye thiw ro ihuotwt ohk.sc sIt etffnevsiesce in girvnsere aeiohyopvlm esnddpe alerlgy ounp ist tiaibyl to dwar tiilestatnri liudf tnoi eth tiu.roiccanl tI is mtos iecteffve in tnstpiea owh are wlle hte.adryd Wenh oodbl veulom cifdeit is teh tlseur of ogrhmrheae, ltceibpmao edr bdloo clsel or elwho olodb ulhdos eb tadidrenimse sa ylcuiqk sa esi.pl"obs

t.om//iw2pahmw/l:sgmd.ur0lp.n-wsurhhbtmnc/atuo-




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iVttnoccroossnai adseseerc oblod wofl adn htus ecraedess orhctsiyadt spersuer. emeSs rceunto iiteinuvt utb I adh to olko hist up ratef I tgo it rwn,go .oot




 +5  upvote downvote
submitted by andro(116),

First Point : Lymph forms at the Capillary level of blood vessels ( as this is where fluid moves in and out of vessels along with metabolites and nutrients ) .

The function of lymph is to return excess proteins and interstitial fluid back to the bloodstream ( Recall Lymph eventually drains into the large veins)

Second Point : We may increase lymph either by increasing 1. the rate at which we form it . 2. Decreasing the rate of drainage ( i.e - obstructing lymph vessels )

To increase lymph formation we have to increase the rate at which fluid filters out of the capillaries . This can be done by altering Starlings forces in the capillary

  • Increasing hydrostatic pressure
  • Decreasing Oncotic pressure

Going through the options
Option A : Endothelin will cause vasoconstriction of Pulmonary artery .This is precapillary meaning we will have less blood/ fluid getting to the capillaries - decreased hydrostatic pressure and decreased lymph formation
Option B : Constriction of pulmonary artery again
Option C and D : lead to the physiologic response of hypoxic vasoconstriction
Option E : Increased oncotic pressure decreases the amount of fluid moving out of capillaries