Mechanisms of Urine conc. and dilution

Question 1

Why does water diffuse passively out of the nephron?

Answer 1

Increasing gradient of osmolality from cortex to papillary regions of kidney

Question 2

What feature of juxtamedullary nephrons allow them to concentrate or dilute urine?

Answer 2

LoHs that penetrate deep into renal medulla

Question 3

How is hypoosmotic urine produced?

Answer 3

Nephron must absorb solutes from tubular fluid and not reabsorb water

Question 4

How is hyperosmotic urine produced?

Answer 4

Nephrons must remove water from tubular fluid without solutes

Question 5

What must the kidney generate to reabsorb water osmotically?

Answer 5

hyperosmotic compartment

Question 6

What is this compartment?

Answer 6

Interstitium of renal medulla

Question 7

What is critical for generating the hyperosmotic medullary interstitium?

Answer 7

TAL

Question 8

How is the hyperosmolar gradient produced?

Answer 8

LoHs act as counter current multipliers
Urea recycling in renal medulla

Question 9

How is the hyperosmolar gradient maintained?

Answer 9

Vasa recta act as counter current exchangers

Question 10

What generates the small trans-epithelial osmotic gradient in the interstitium?

Answer 10

Active and passive transport systems in the TAL removing and adding solutes

Question 11

What happens when tubular fluid flows towards the hairpin of the LoH?

Answer 11

Extends the zone of high osmolality deeper into the medulla

Question 12

What happens when the hyperosmotic fluid enters the ascending limb?

Answer 12

Actively transports more solutes against a gradient of 200mOsm Kg-1
Interstitium osmolality increases

Question 13

What is the type of gradient present in the medulla?

Answer 13

Longitudinal gradient

Question 14

What creates the large longitudinal osmotic gradient?

Answer 14

Counter-current arrangement of LoH

Question 15

Where in the nephron is urea impermeable?

Answer 15

TAL
Distal convoluted tubule
Cortex and outer medullary collecting ducts

Question 16

What is the clearance of urea?

Answer 16

Freely filterable
50% is reabsorbed in proximal tubule

Question 17

How does reabsorption of urea occur in the IMCD?

Answer 17

Expresses apical UT-A1 and basolateral UT-A3

Question 18

What happens at the IMCDF?

Answer 18

Urea diffuses down its conc. gradient into interstitium and is recycled back into nephron

Question 19

How is urea secreted back into the nephron at the IMCD?

Answer 19

Thin LoH expresses UT-A2 transporters, descending vasa recta expresses UT-B transporters

Question 20

How does urea contribute to hypertonic medullary interstitium?

Answer 20

Urea recycling traps urea in inner medulla increasing osmolality

Question 21

What does the hair-pin loop of vasa recta ensure?

Answer 21

That very little solutes are washed out of the renal medulla -> hyperosmolar gradient maintained

Question 22

What is the osmolality of the fluid entering the cortical compared to the plasma?

Answer 22

Hyperoosmotic

Question 23

What 3 ways does ADH act?

Answer 23

1. Increasing permeability of collecting duct
2. Increasing permeability of medullary portion of collecting duct to urea
3. Stimulating reabsorption of NaCl by the TAL, DCT and collecting ducts