Control of Water Balance

Question 1

What is the range of total urine osmolality depending on how much water is being excreted?

Answer 1

can range from 50 to 1300 mOsms

Question 2

Positive water balance causes a ____ in extracellular and intracellular osmolality and compensation for positive water balance includes ____ urine volume and a ______ in urine osmolarity.

Answer 2

decreased extracellular and intracellular osmolality
increased urine volume
decreased urine osmolality

you’re just getting the water out

Question 3

Water balance is associated with an extracellular and intracellular normal osmolality of what?

Answer 3

285-295 (about 300) mOsms

Question 4

Negative water balances causes a ____ in etracellular an dintracellular osmolality and compensation for negative water balance includes _____ urine volume and a ______ in urine osmolality.

Answer 4

increase in extracellular and intracellular osmolality
low urine volume
increased in urine osmolality

Question 5

What percentage of water is reabsorbed in the proximal tubule?

Answer 5

65%

Question 6

What percentage of water is reabsorbed in the descending limb of LOH?

Answer 6

about 10%

Question 7

What percentage of water is reabsorbed in the ascending limb of LOH and the distal convoluted tubule?

Answer 7

0 - no aquaporins

Question 8

What percentage of water is reabsorbed in the cortical and medullary collecting ducts?

Answer 8

it varies between 5 to 24.5%

Question 9

What hormone controls whether you have 5% or 24.5% reabsorbed?

Answer 9

antidiuretic hormone

more ADH = more reabsorbtion and less excretion

Question 10

Without ADH, what is the maximum osmolaity of the medulla? How about with ADH?

Answer 10

600 mOsms without

1200-1400 with

Question 11

Why is the extracellular osmolality maximum 600 without ADH? Why 1200 with?

Answer 11

You have about 300 mM NaCl that gets pumped into the extracellular space, so 300 Na + 300 Cl = 600

with ADH, you have that 600 for the same reason, plus an extra 600 mM of urea = 1200

Question 12

Why is the intracellular osmolality without ADH 600? Why 1200 with?

Answer 12

WIthout: you have 300 mOsms of the usual cell solutes + 300 mOsms of osmolytes made by the cell = 600 mOsms

with ADH = you have 300 of the normal solutes, 300 osmolytes and 600 mM of urea = 12000

Question 13

How does the cell make osmolytes to balance osmolality?

Answer 13

in part with a transcription factor called tonicity-responsive enhancer binding protein which promotes intracellular accumulation of organic osmolytes

Question 14

Why is the extracellular space in the medulla loaded with NaCl?

Answer 14

countercurrent multiplication - whatever that means

Question 15

Urea equals what percentage of the osmolality in the medullary extracellular space? How about the osmolality of the urine?

Answer 15

about 50% for each!

About 50% of the filteredload is excreted and the rest is reabsorbed

Question 16

Why does urea increase in the extracellular space in response to ADH?

Answer 16

ADH upregatees secondary active urea transporters, so you get more urea reabsorbed into the extracellular space

Question 17

What happens to SPECIFIC aquaporins with high levels of ADH?

Answer 17

you greatly increase the insertion of luminal aquaporin 2 channels and basolateral aquaporin 3 and aquaporin 4 channels into the last third of the distal convoluted tubule and all of the collecting duct

this means you get a lot of water reabsorbtion

Question 18

The water that leave ste collectin duct via aquaporin 2 after ADH secretion goes into what blood vessels?

Answer 18

the peritubular capillaries called the vasa recta

Question 19

Although the vasa recta reabsorb water just like any peritubular capillaries, they don’t disturb the medullary osmolality gradients. How can this be?

Answer 19

They are also in a countercurrent arrangement, allowing for countercurrent exchange

Question 20

Describe the signalling pathway for ADH to aquaporin insertion

Answer 20

ADH binds to the V2 receptor, which is a GPCR
the receptor activates adenylyl cyclase
you use ATP to form cAMP
the cAMP activates protein kinase A
PKA activates the cAMP response element binding protein, which is a transcription factor that promotes mRNA transcription and protein translation of aquaporins
the aquaporins are then inserted into vesicles that fuse with the cell membrane in response to ADH

Question 21

When you don’t have aquaporins, you don’t reabsorbe water thorugh the collecting duct. What is the osmolality of very dilute urine and why is it not 300 mOsms?

Answer 21

It’s 50 mOsms - not 300 because 5% of the Na+ is reabsorbed in the medullary collecting duct

the goal is basically to excrete as close to plain water as you can

Question 22

What are the triggers for ADH release?

Answer 22

1. Low water concentration/high osmolality (sensed by osmoreceptors)
2. Decreased plasma volume (sensed by baroreceptors)

Question 23

Neurons from the osmoreceptors and baroreceptors terminate where to trigger ADH release?

Answer 23

on the neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus

Question 24

Where is the termination fo the neurosecretory cells? aka…where is ADH released from?

Answer 24

the posterior pituitary

Question 25

What are the 5 actions of ADH?

Answer 25

1. increased water permeability cuz of AQP2
2. urea transporters upregulated ina scending limb and collecting duct to cause urea recycling and high medullary osmolality
3. vasoconstriction
4. increased thick ascending limb Na/K/2Cl pumping - to increase medullary osmolality
5. possible third mediation

Question 26

If you don’t get ADH release from the posterior pituitary when you need it, what syndrome do you have? symptoms?

Answer 26

central diabetes insipidus

peeing and thirsty all the time

Question 27

If you can release ADH fine, but the receptor or signalling pathway is busted, what syndrome do you have? symptoms?

Answer 27

nephrogenic diabetes insipidus.

same thing - peeing and thirsty

Question 28

What effect does ethyl alcohol have on ADH action?

Answer 28

at a blood alcohol level of 0.1%, you have 17 mOsms of ethyl alcohol, which means you go from 300 mOsms to 317 mOsms

It completely shuts down ADH secretion, so you’re basically left with a temporary central diabetes insipidus

Question 29

If you secrete ADH all the time, what syndrome do you have? symptoms?

Answer 29

syndrome of inappropriate ADH secretion

hypervolemia and hyponatremic