Flashcards in Water Balance and Handling Deck (13)
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1
In the loops of Henle, what's the maximum osmotic gradient that can be achieved at any time?
200 mOsm / kg water
2
3 things you need to have a countercurrent multiplier?
A loop.
H2O permeability on the descending limb.
Na+ pumps on the ascending limb.
3
Why is the countercurrent multiplier model for water removal / urine dilution in the loop of Henle inadequate?
The thin ascending limb doesn't actually have those Na+ pumps. We don't know exactly how things work... probably involves how the tubes are arranged... and urea.
(it won't be on the exam)
4
How permeable are the vasa recta to water and (many) osmolytes? Why is this important?
Very permeable to both.
This, when in a loop, allows the blood vessels to carry 300mOsm blood through the medulla without washing out the hypertonic areas.
5
How hypertonic, in mOsms does the medulla get?
It ranges from 300mOsm to about 1000mOsm at the deepest level.
6
Why can the collecting duct passively reabsorb water if ADH is present?
The medullary interstitium is very hypertonic, so the water easily goes out. (via aquaporin-2)
7
3 stimuli that induce water retention via ADH?
Plasma osmolarity greater than 280mOsm.
Decreased atrial stretch due to low blood volume.
Decreased blood pressure.
8
You all known that ADH = vasopressin = arginine vasopressin (AVP), right?
Right.
9
In more molecular detail, how does vasopressin get more aquaporin-2 to be on the collecting duct apical membrane?
AVP -> vasopressin receptor (a GPCR) -> increased cAMP -> exocytosis of vesicles with Aquaporin-2 imbedded in the membrane.
10
What's the plasma osmolality threshold for AVP release?
How about for thirst?
About 285mOsm.
It's slightly higher for thirst - about 290-295mOsm.
11
Why don't you really have to measure AVP?
Urine osmolality is tightly correlated with plasma AVP. (dilute urine = low AVP, concentrated urine = high AVP)
12
Does the ability to concentrate urine increase with time spent with low fluid intake?
Yes - one can produce more concentrated urine after several days of low fluid intake.
13
