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NBME 21 Answers

nbme21/Block 4/Question#23

An investigator is studying the regulation of ...

Intravenous infusion of 0.9% saline for 5 minutes

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 +10  upvote downvote
submitted by hayayah(345),

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.

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 +4  upvote downvote
submitted by drdoom(166),

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 “outward” force it exerts. (This is known as the Venturi effect.)

hungrybox  not seeing how this is relevant +1  
sympathetikey  He's showing how A & B are incorrect @hungrybox +1  

 +3  upvote downvote
submitted by d_holles(31),

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."

 +1  upvote downvote
submitted by assoplasty(35),

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.