NBME 21

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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submitted by hayayah(54),

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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submitted by drdoom(15),

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.)


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submitted by assoplasty(3),

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.