Water Balance and Handling Flashcards
In the loops of Henle, what’s the maximum osmotic gradient that can be achieved at any time?
200 mOsm / kg water
3 things you need to have a countercurrent multiplier?
A loop.
H2O permeability on the descending limb.
Na+ pumps on the ascending limb.
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)
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.
How hypertonic, in mOsms does the medulla get?
It ranges from 300mOsm to about 1000mOsm at the deepest level.
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)
3 stimuli that induce water retention via ADH?
Plasma osmolarity greater than 280mOsm.
Decreased atrial stretch due to low blood volume.
Decreased blood pressure.
You all known that ADH = vasopressin = arginine vasopressin (AVP), right?
Right.
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.
What’s the plasma osmolality threshold for AVP release?
How about for thirst?
About 285mOsm.
It’s slightly higher for thirst - about 290-295mOsm.
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)
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.
What’s the concept of free water excretion?
You can split the amount of urine a person makes into 2 parts: a volume with osmolality = 280mOsm (same as plasma), and osmolyte free (or electrolyte-free) water.
(more on this in the hypo/hypernatremia lectures)
