Renal Water Balance

Question 1

What is most important in regulating plasma tonicity?

Answer 1

Water

Question 2

How much water is absorbed by the proximal convoluted tubule?

Answer 2

65%

Question 3

How much water is absorbed by the descending loop of henle?

Answer 3

10%

Question 4

How much water is absorbed by the collecting duct?

Answer 4

5-25%

Question 5

How is water reabsorbed by the nephron?

Answer 5

Osmosis through aquaporins

Question 6

Which aquaporin is regulated by ADH?

Answer 6

AQP2 of principal cells in the collecting duct

Question 7

How is water reabsorbed in the proximal tubule?

Answer 7

AQP1 (80%) and paracellular route (20%)

Question 8

How is water reabsorbed in the descending thin segment of the loop of henle?

Answer 8

AQP1

Question 9

What three things regulate water reabsorption from tubular lumen?

Answer 9

1. Osmotic gradient
2. Functional water channels
3. Anti-diuretic hormone

Question 10

What are three features of the medullary osmotic gradient?

Answer 10

1. Loop of henle
2. Vasa recta
3. Counter current mechanism

Question 11

What is a counter current system?

Answer 11

A system in which the inflow runs parallel to, counter to, and in close proximity to the out-flow for some reason

Question 12

Why do we need a counter current mechanism within the medulla?

Answer 12

To generate the medullary osmotic gradient (which will favour the reabsorption of water from the collecting duct)

Question 13

What structure releases antidiuretic hormone?

Answer 13

Posterior Pituitary

Question 14

When is ADH released?

Answer 14

1. When osmoreceptors sense a higher than normal osmolality

2. When baroreceptors (in carotid bodies and aortic arch) sense a lower than normal blood pressure/volume

Question 15

Is osmoregulation or volume regulation more sensitive to changes?

Answer 15

Osmoregulation (as little as 1% change, coupled to thirst)

Question 16

What are four renal effects of ADH?

Answer 16

1. Increase water permeability of collecting duct via V2 receptor to input more AQP2
2. Increase urea permeability of collecting duct
3. Increase Na absorption and K secretion
4. Increase renal prostoglandin production (vasodilation)

Question 17

What are three extra renal effects of ADH?

Answer 17

1. Systemic vasoconstriction via V1 receptor
2. Increase release of von Willebrand factor
3. Stimulate ACTH release via V3 receptor

Question 18

What happens when plasma osmolality increases?

Answer 18

Increase thirst and increase ADH release to cause water retention

Question 19

When osmolality and volume do not correspond, what takes precedence?

Answer 19

Volume
(activation of RAAS and ADH despite low plasma osmolality and hyponatremia –> ongoing water retention despite low plasma osmolality)

Question 20

Describe SIADH

Answer 20

Syndrome of Inappropriate ADH

Chronic activation of ADH leading to excessive water retention (but NOT sodium); osmolality changes not volume changes

Question 21

Describe Diabetes Insipidus

Answer 21

Disease in which ADH is not effective; polyuria, polydipsia, dehydration, hypernatremia

Question 22

What is the difference between central and nephrogenic diabetes insipidus?

Answer 22

Central - problem with ADH secretion, responsive to exogenous ADH
Nephrogenic - problem with V2 receptor on AQP2, unresponsive to exogenous ADH

Question 23

Impermeability to water is essential to the medullary interstitial gradient in which segment of the nephron?

Answer 23

Thick ascending limb of the loop of henle

Question 24

What molecules are important in generating an interstitial concentration gradient within the medulla?

Answer 24

Urea, sodium, chloride