Lecture 5 - Countercurrent Multiplication & Urea Recycling

Question 1

In which type of nephrons does countercurrent multiplication occur?

Answer 1

Juxtamedullary nephrons - long LoH

Question 2

What is the corticopapillary gradient established by? (2)

What is it maintained by?

Answer 2

• Established by countercurrent multiplication & urea recycling
• Maintained by vasa recta

Question 3

How does countercurrent multiplication lead to the production of concentrated urine?

Answer 3

• Transporters within the TAL can establish a difference of 200 mOsm between between the tubular fluid and the outside (interstitium) at any point along its path.
• TAL transports Na+ out into interstitium but is impermeable to H2O. This creates osmotic pressure for H2O to move out of the thin descending limb into the interstitium.
• This causes concentration (osmolality) in descending limb to increase. This high osmolality filtrate moves down the LoH, leading to a gradient of 300 to 1200 mOsm from top to bottom (as process is repeated/multiplied)

Question 4

What is the role of urea recycling & how does this process work?

Answer 4

• Maintains medullary hypertonicity & adds to the functioning of ADH
• 50% of urea reabsorbed in PCT
• Tubular urea increases in LoH as it moves down concentration gradient from medulla into the lumen
• Ascending limb & DCT impermeable to urea, concentration rises as solutes and water are reabsorbed
• ADH causes increase in UT1 expression on apical surface of CD tubules, enabling for reabsorption of urea into interstitium, drawing water with it.
• From 110% in LoH, 70% reabsorbed via UT1, 40% excreted in urine