5 Water Homeostasis

Question 1

What is the major function of ADH on kidney tubules?

Answer 1

Increase H2O reabsorption from the cortical and medullary collecting ducts and increase urea reabsorption from the medullary collecting duct

Question 2

Where is the receptor for ADH located?

Answer 2

Basolateral membrane of principal cells in collecting duct

Question 3

Explain the mechanism of action for ADH.

Answer 3

1) Receptor activates adenylate cyclase
2) Second messenger cAMP activates PKA
3) PKA phosphorylates Aqp2, which allows it to move from an intracellular vesicle to the cell surface and insert in the apical membrane
4) Aquaporins remain in apical membrane with a half life of 15-20 min
5) Aquaporins reinternalize in the absence of ADH

Question 4

What causes diabetes insipidus, and what are its symptoms?

Answer 4

Diabetes insipidus is an inability to concentrate urine caused by an absence of ADH; symptoms include polyuria, polydipsia, and hypernatremia/hyperosmolality

Question 5

What causes Syndrome of Inappropriate ADH Secretion (SIADH), and what are its symptoms?

Answer 5

SIADH is caused by excessive secretion of ADH and results in an inability to dilute urine; symptoms include positive water balance, excessive water retention, dilution of body fluid in ICF and ECF, and hyponatremia/hypoosmolality

Question 6

What are dipsogenic centers and where are they located?

Answer 6

Dipsogenic centers are thirst generating centers that work in concert with ADH to restore water balance; they are located next to osmoreceptors in the CNS

Question 7

The kidney’s ability to dilute and concentrate urine is dependent on what two factors?

Answer 7

1) The presence or absence of ADH

2) The ability to separate solute and water and develop an osmotic gradient

Question 8

What are the key features that allow countercurrent multiplication in the loop of Henle?

Answer 8

1) Differing tubular properties of the descending and ascending limbs
2) Two parallel loops with tubular fluid flowing in opposite directions (countercurrent flow)

Question 9

Where and how does the process of countercurrent multiplication originate?

Answer 9

Originates in TAL; active transport of Na+ and Cl- from tubular lumen into interstitium leads to dilution of tubular fluid

Question 10

Which segment of the nephron is known as the diluting segment?

Answer 10

TAL

Question 11

Which segment of the nephron is known as the concentrating segment?

Answer 11

Collecting duct

Question 12

What is the function of urea in the nephron?

Answer 12

It is an important component of the medullary interstitial gradient

Question 13

What is the effect of ADH on urea reabsorption?

Answer 13

ADH makes the inner medullary collecting duct more permeable to urea (and water)

Question 14

What are the steps of urea recycling?

Answer 14

1) Urea is reabsorbed from IMCD to interstitium
2) Urea is recycled into loop of Henle by diffusing into thin limbs
3) Urea is carried downstream from loop of Henle to IMCD
4) Small percentage of urea is removed by vasa recta

Question 15

How do anatomic factors alter concentration of urine?

Answer 15

Longer nephrons have more concentrating ability

Question 16

How does urea balance alter concentration of urine?

Answer 16

Reduced urea (low protein intake) reduces concentrating ability

Question 17

What are the functions of the vasa recta?

Answer 17

1) Provision of nutrients and oxygen to medullary tubules
2) Maintenance of mass balance and medullary interstitial gradient
3) Functions as a countercurrent exchanger

Question 18

How do the permeability characteristics of the vasa recta contribute to countercurrent exchange?

Answer 18

1) Trap solutes (Na+, Cl-, and urea) in medullary interstitium
2) Prevents removal of ions which make up medullary interstitial gradient