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NBME 21 Answers

nbme21/Block 4/Question#23 (58.9 difficulty score)
An investigator is studying the regulation of ...
Intravenous infusion of 0.9% saline for 5 minutes🔍
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 +18 
submitted by hayayah(1057),
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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 +5  



 +10 
submitted by andro(173),

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

lovebug  now I understand~~!! THX :) +1  
limberry  I believe you mean "Option F: Increased oncotic pressure decreases the amount of fluid moving out of capillaries" instead of Option E, right? +  



 +8 
submitted by drdoom(819),
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iatotVncsicosorn rorannig(w of a e)utb iwll cuesa het flwo taer to enrcesai otrghhu htta b,ute ichwh escrsadee ddlaoatar/wiur s.ruserpe eTh rtesaf a idulf esmov trhhuog a e,btu hte ssel twru”o“da feorc it t.rxese si(hT is nknwo sa teh netiurV eftfec.)

hungrybox  not seeing how this is relevant +8  
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(16),


 +6 
submitted by d_holles(173),
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 +5 
submitted by assoplasty(91),
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