Countercurrent Multiplication + Urea Recycling

Question 1

What is the cortiopapillary gradient established by?

Answer 1

Urea recycling
Countercurrent multiplication

Question 2

What is the corticopapillary gradient maintained by?

Answer 2

Vasa recta

Question 3

What is the water permeability of the loop of henle?

Answer 3

Descending limb: permeable to water
Ascending limb: impermeable to water

Question 4

What is the Na+ and Cl- transport in the loop of henle?

Answer 4

Descending limb: permeable to Na+ and Cl-
Thin ascending limb: minimal Na+ and Cl- transport
Thick ascending limb: NKCC co transporter in apical membrane

Question 5

How does the osmolality change as you descend the loop of henle?

Answer 5

Increases
300mOsm at top
1200mOSm at bottom

Question 6

What is the function of countercurrent multiplication?

Answer 6

Preserves the osmolality/concentration gradient

Question 7

Outline countercurrent multiplication

Answer 7

• fluid osmolality is 300mOsm
• H2O leaves descending limb
• osmolality increases as fluid descends into medulla to 1200mOsm
• Na+, K+ and Cl- leave thick ascending limb via NKCC co transporter
• fluid osmolality 100mOsm at top of thick ascending limb
• vasa recta runs countercurrently to preserve the osmotic gradient
• solutes diffuse into vasa recta from interstitium until osmolality increases to 1200mOsm at hair pin
• water enters as vasa recta ascends
• aquaporins in collecting duct allows from water to move into interstitium + into vasa recta

Question 8

Outline urea recycling

Answer 8

• urea is freely filtered into Bowman’s capsule (100%)
• 50% reabsorbed in PCT
• there is a high conc of urea in the interstitium as you descend the medulla > urea diffuses into LoH from interstitium
• urea conc is now higher (110%) than at the start (100%)
• ADH increased number of UT1 > urea diffuses out of CD
• urea acts as an osmole + increases reabsorption of H2O

Question 9

What does urea recycling help to maintain?

Answer 9

Medullary hypertonicity

Question 10

How does ADH increase urea reabsorption?

Answer 10

Increases number of urea transporters UT1 on apical surface of medullary collecting tubules

Question 11

How much urea is excreted from the kidney?

Answer 11

40%

Question 12

How much urea is reabsorbed by UT1?

Answer 12

70%

Question 13

What happens to urea in the PCT?

Answer 13

50% reabsorbed

Question 14

What happens to urea down the loop of Henle?

Answer 14

• urea conc is higher in interstitium than LoH
• urea diffuses into LoH
• urea conc is greater than at start 110%

Question 15

What happens to urea in the collecting duct?

Answer 15

• ADH increase number of UT1
• urea diffuses out into interstitium
• urea acts as an osmole + increases reabsorption of water