Countercurrent multiplication and urea recycling

Question 1

What is the vasa recta?

Answer 1

• A very specific network of blood vessels descending into the medulla alongside the loop of Henle

Question 2

What establishes the corticopapillary gradient?

Answer 2

• Urea recycling
• Countercurrent multiplication
• Maintained by Vasa Recta

Question 3

Describe the structure of the loop of Henle

Answer 3

• Responsible for 25% Na+ reabsorption
• Thin descending limb is permeable to WATER, Na+ and Cl-
• Thin ascending limb is impermeable to WATER
• Thick ascending limb is impermeable to WATER but has Na+/K+/2Cl- transporters on apical membrane

Question 4

Outline how the counter-current multiplication mechanism works

Answer 4

• Fluid entering descending limb of loop of Henle is iso-osmotic with interstitium
• Descending limb is permeable to water, so water is reabsorbed into blood
• Concentration of filtrate increases due to loss of water
• This allows solutes to move out of filtrate at ascending limb
• Blood moves in Vasa Recta in opposite direction to filtrate next to loop of Henle
• Solute moves out of ascending limb and into blood, making it hyperosmotic
• As blood moves past descending limb of loop of Henle, it picks up water and becomes iso-osmotic with interstitium again

Question 5

What allows water to be absorbed from collecting duct?

Answer 5

• The Na+/K+/2Cl- transporter
• Draws water into blood by osmosis

Question 6

What is the normal osmolality of the plasma as it enters the loop of Henle?

Answer 6

• 300 mOsm/kg
• Iso-osmotic with interstitium

Question 7

What is the concentration of the fluid leaving the Loop of Henle like?

Answer 7

• Hypotonic (100 mOsm/kg)

Question 8

How is the Vasa recta adapted to its function?

Answer 8

• Hairpin arrangement - permeable to solutes and water
• As Vasa Recta descends, it absorbs lots of solutes (Na+, Cl-, urea)
• As Vasa Recta ascends, it reabsorbs water
• Slow flow enables vasa recta to equilibrate at each stratification level
• Absorbs water released from collecting duct in presence of ADH - this maintains high osmolality of interstitium

Question 9

Outline the process of urea recycling

Answer 9

• 50% of filtered urea moves into nephron from blood
• Descending limb of loop of Henle is permeable to urea so urea diffuses into loop of Henle - reaches concentration of 110%
• Bottom of loop of Henle and ascending limb are impermeable to urea
• So urea remains in nephron until collecting duct
• Concentration of urea increases as water and other solutes are reabsorbed
• UT1 transporter moves urea out of collecting duct and into interstitium
• 70% of urea is reabsorbed by UT1 and 40% is excreted
• Urea is then recycled

Question 10

Why does urea need to be recycled?

Answer 10

• So that it can be used to maintain the corticopapillary gradient
• Corticopapillary gradient necessary to produce concentrated urine

Question 11

How does ADH affect urea reabsorption?

Answer 11

• ADH increases number of UT1 transporters in wall of collecting duct
• This means that more urea is reabsorbed, so more H2O is absorbed
• H2O is then absorbed into vasa recta