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Flashcards in Water Balance and Handling Deck (13):
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

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)