L7 Mechanisms of Urine Concentration and Dilution

Question 1

What is normal blood plasma osmolality?

Answer 1

~290mOsm/kg H2O

Question 2

Why does Water diffuse out of the descending limb?

Answer 2

Hyperosmolar gradient in the medullary interstitium

Question 3

Function of Juxtamedullary Nephrons?

Answer 3

Juxtamedullary nephrons (15% of nephrons) have LoHs that penetrate deep in to the renal medulla, and function to concentrate or dilute urine

Question 3

How is the Hyperosmolar gradient in medullary interstitium produced? How is it maintained

Answer 3

Produced by: (50% contribution of each)

• Counter-current arrangement of the LoHs multiplies a relatively small trans-epithelial osmotic gradient (200mOsm/kg) into a large longitudinal osmotic gradient
• Urea recycling in the renal medulla

Maintained by: Vasa recta (Counter current exchangers)

Question 4

Osmolality of Urine under normal Condiutions?

Answer 4

Under normal conditions, the osmolality of urine is ~300-800mOsm/kg

Question 5

Describe the process of Urea Cycling?

Urea is freely filterable at the glomerulus and approximately 50% of the filtered load is __________ reabsorbed in the proximal tubule.

From the ________ onwards the nephron is impermeable to urea but reabsorption occurs in the IMCD (Apical ______ and basolateral _______.)

At IMCD, urea diffuses down its concentration gradient into the interstitium and is recycled (secreted) back into the nephron (Thin Loop of Henle: _________ transporters; Descending (arterial) vasa recta: ______ transporters).

Urea recycling “traps” urea in the inner medulla, increasing osmolality and contributing to the __________________.

Answer 5

Urea is freely filterable at the glomerulus and approximately 50% of the filtered load is paracellularly reabsorbed in the proximal tubule.

From the TAL onwards the nephron is impermeable to urea but reabsorption occurs in the IMCD (Apical UT-A1 and basolateral UT-A3)

At IMCD, urea diffuses down its concentration gradient into the interstitium and is recycled (secreted) back into the nephron (Thin Loop of Henle: UT-A2 transporters; descending (arterial) vasa recta: UT-B transporters).

Urea recycling “traps” urea in the inner medulla, increasing osmolality and contributing to the hypertonic medullary interstitium

Question 6

Sections of the Nephron that are Urea Impermeable?

Answer 6

Thick Ascending Loop of Henle

Distal Convoluted Tubule

Cortex and Outer Medullary Collecting Duct

Question 7

Sections of the Nephron that are urea permeable?

Answer 7

Freely Filtered at Glomerulus

Proximal Convoluted Tubule (50% Paracellular Reabsorption)

Thin Descending/Ascending Limbs (Water leaves, Urea Concentrated back to 100%- UT-A2 transporters)

Reabsorption occurs in the Inner Medullary Collecting Duct (apical UT-A1and basolateral UT-A3)

Question 8

Significance of reduction in blood flow from the cortex to inner medulla?

Answer 8

nsures that the interstitial medullary gradient is not “washed away”

• Cortex HIGHLY vasularized
• Only blood supply to inner medula is the Vasa Recta

Question 9

Significance of Hair-Pin Looped Structure of Vasa Recta?

Answer 9

Hair-pin looped structure of vasa recta maintain hypertonic interstitial medulla and ensure very little solutes ‘wash out’ of the renal medulla

Question 10

The osmolality of fluid in the collecting duct depends on whether there are high or low levels of __________

Answer 10

The osmolality of fluid in the collecting duct depends on whether there are high or low levels of circulating ADH

Question 11

Descending Limb of Vasa Recta: Water ______, solute ______

Ascending Limb of Vasa Recta: Water ______, Solute _____

Answer 11

Descending Limb of Vasa Recta: Water losing, solute gaining

Ascending Limb of Vasa Recta: Water gaining, Solute losing

Question 12

The fluid entering the cortical collecting duct is _________ relative to plasma. From there on ______ is crucial for water reabsorption

Answer 12

The fluid entering the cortical collecting duct is hypoosmotic relative to plasma. From there on ADH is crucial for water reabsorption

Question 13

Normally, there is some ADH present in the circulation and urine volume is ~________Lday (~_______mOsm/kg H2O)

In the absence of ADH, urine volume can be ~______L/day, with an osmolality as low as ________mOsm/kg H2O

Answer 13

Normally, there is some ADH present in the circulation and urine volume is ~1.5Lday (~ 300-800mOsm/kg H2O)

In the absence of ADH, urine volume can be ~25L/day, with an osmolality as low as 60mOsm/kg H2O

Question 14

Renal Actions of ADH?

Answer 14

1. Increases the permeability of the collecting duct to water
2. Increases permeability of the medullary portion of the collecting duct to urea (Incorporates Urea Transporter UT1)
3. Stimulates reabsorption of NaCl by the TAL, the DCT and the collecting duct

Question 15

ADH enhances the countercurrent multiplier system by enhancing __________ and _________

Answer 15

ADH enhances the countercurrent multiplier system by enhancing urea recycling and NaCl reabsorption