12. Countercurrent multiplication and urea recycling

Question 1

describe urea recycling

Answer 1

• Helps to maintain the medullary hypertonicity • 50% filtered urea is reabsorbed in the PCT with Na+
• Tubular concentration of urea ↑ as it diffuses down
concentration gradient from medulla into lumen
(110% at base of LoH )
• Ascending limb and early DCT impermeable to urea. Concentration ↑as solutes and water reabsorbed
• ADH causes ↑urea transporter (UT1) on apical
surface of medullary collecting tubules. Enables
urea to flow down concentration gradient.
• 70% filtered urea is reabsorbed by UT1 • 40% excreted
• Urea secreted back into LoH and goes back up to DCT/CD and is recycled

Question 2

How is CORTICOPAPILLARY GRADIENT established and maintained?

Answer 2

Established by:
• Urea recycling (urea)
• Countercurrent multiplication (solutes)
Maintained by:
• Vasa recta

Question 3

Describe the permeability of LoH

Answer 3

Thin descending limb
• Permeable to water, Na+ and Cl-

Thin ascending limb
• Impermeable to water
• Minimal Na+ and Cl- transport

Thick ascending limb
• Impermeable to water
• Na+K+2Cl- co transporter apical membrane (Na+K+ATPase basolateral membrane)

Question 4

WHat are the transporters of loh

Answer 4

gradient created by sodium potassium ATPase on basal surface allowing transport of solutes into cell from Na+K+2Cl- cotransporter on apical surface.
Cl-K+ channel moves chloride and potassium back into tubule

Question 5

describe role of vasa recta in countercurrent multiplication

Answer 5

• Descend – absorb solutes such as Na+, Cl- and urea, water lost
• Ascend – reabsorb water & loss of solutes
• Slow flow enables vasa recta to equilibrate at each stratification level – minimises washout
• Absorbs water released from CD in presence of ADH – maintains high osmolality of intersitium

Question 6

Which type of nephrons are responsible for making concentrated urine?

Answer 6

Juxtamedullary nephrons which make up 10-15% of total nephrons

Question 7

What does the efferent glomerullar capillary turn into in juxtamedullary nephrons?

Answer 7

Vasa recta

Question 8

How is vertical concentration gradient in the kindey interstitum generated?

Answer 8

Juxtamedullary nephron - long loop of Henle establish (create) the vertical osmotic gradient
• Vasa recta help to maintain (preserve) this osmotic gradient

Question 9

What is the mechanism that sets up the gradient called?

Answer 9

Medullary counter current mechanism

Question 10

Where in the insterstitium does the osmolality increase?

Answer 10

Increases after the corticomedullary border (300 at the border) and increases up 1200 at the papilla

Question 11

WHat is the function of the thick ascending limb?

Answer 11

Removes solute without water and therefore increases osmolarity of the interstitium, (ultrafiltrate becomes more diluted)

Question 12

What gradient are the NaKCC2 transporters able to maintain between the TAL and the interstitium?

Answer 12

200 mOsm

Question 13

Describe the counter current mechanism.

Answer 13

1. the active salt pump in the TAL establishes a 200 mOsm gradient at each horizontal level.
2. as fluid flows several “frames”, a mass of 200 mOsm fluid exits into the DCT, and a new mass of 300 mOsm fluid enters from the PCT
3. ascending limb pump and descending limb passove flux establish 200 mOSm gradient
4. once again fluid flows
5. 200 mOsm gradient established
6. final vertical osmotic gradient is established and maintained by the ongoing countercurrent multiplication of the long loop of henle

Question 14

How do substances enter/leave the vasa recta?

Answer 14

Passive diffusion only, no active transport

Question 15

Describe direction of flow of blood in vasa recta compared to in loop of henle?

Answer 15

Opposite to tubular fluid flow

Question 16

What happens in the descending limb of the vasa racta?

Answer 16

Isosmotic blood in vasa recta enters hyperosmotic milieu of the medulla, Na+, Cl- + urea diffuse into the lumen of vasa recta, slow flow so it equilibrates at each stratified level

Question 17

What happens in the ascending limb of the vasa racta?

Answer 17

• Blood ascending towards cortex will have higher solute content than surrounding interstitium
• Water moves in from the descending limb of the loop of Henle

Question 18

How do the vasa recta prevent ‘washing out’ of the gradients set up by the loop of henle?

Answer 18

Slow blood flow (5-10% of total RPF)

Question 19

What are the 2 response pathways of osmoreceptors?

Answer 19

Concentration of urine and Thirst

Question 20

Where is ADH produced?

Answer 20

Supraoptic nucleus of hypothalamus