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Retired NBME 21 Answers

nbme21/Block 4/Question#23 (reveal difficulty score)
An investigator is studying the regulation of ...
Intravenous infusion of 0.9% saline for 5 minutes ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
tags: immuno Saline Lymphatic

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 +23  upvote downvote
submitted by โˆ—andro(269)
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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

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lovebug  now I understand~~!! THX :) +2
limberry  I believe you mean "Option F: Increased oncotic pressure decreases the amount of fluid moving out of capillaries" instead of Option E, right? +3
weirdmed51  Yes . Albumin aka colloid. +



 +18  upvote downvote
submitted by โˆ—hayayah(1212)
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Lymph flow rate is usually low. It is influenced primarily by the rate of lymph formation. For example, if blood capillary pressure is increased by arterial vasodilation or venous constriction, the flow rate of lymph increases. Also, the flow rate is affected by compression of lymphatics by contraction of neighboring musculature and by negative intrathoracic pressure (breathing).

Interstitial pressure (so pressure in the ECF, which would increase if given IV saline) and lymph flow are positively related. A small increase in interstitial volume greatly increases its pressure, promoting lymph flow that acts to restore the interstitial volume to normal.

more on this topic: https://www.ncbi.nlm.nih.gov/books/NBK53448/

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linwanrun1357  Do not understand the breathing ๏ผˆchoice C and D๏ผ‰ 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



 +8  upvote downvote
submitted by โˆ—drdoom(1206)
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Vasoconstriction (narrowing of a tube) will cause the flow rate to increase through that tube, which decreases radial/outward pressure. The faster a fluid moves through a tube, the less โ€œoutwardโ€ force it exerts. (This is known as the Venturi effect.)

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hungrybox  not seeing how this is relevant +9
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.



 +6  upvote downvote
submitted by โˆ—d_holles(218)
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20% albumin is HYPERtonic and causes drawing of fluid out of the interstitum and into the blood.

"Albumin (Human) 20% is indicated in the emergency treatment of hypovolemia with or without shock. Its effectiveness in reversing hypovolemia depends largely upon its ability to draw interstitial fluid into the circulation. It is most effective in patients who are well hydrated. When blood volume deficit is the result of hemorrhage, compatible red blood cells or whole blood should be administered as quickly as possible."

https://www.drugs.com/pro/albumin-human-20.html

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 +5  upvote downvote
submitted by โˆ—assoplasty(108)
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Vasoconstriction decreases blood flow and thus decreases hydrostatic pressure. Seems counter intuitive but I had to look this up after I got it wrong, too.

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 +0  upvote downvote
submitted by โˆ—bgreen27(6)
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Can someone help a brother out? My thinking was that increased fluid in the interstitial leads to increased lymph, so I thought they were looking for something to simulates high altitude pulmonary edema or arterial pulmonary HTN, which I am pretty sure both lead to pulmonary edema. Where did I go wrong?

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ali_hassan  this might be stupid but i got it right with this rationale: increase "normal" fluid (0.9% saline) in body = increase lymph flow. absolutely no medical concept to it, just logic lol +



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