Concentration and Dilution of Urine

Question 1

Production of Concentrated Urine

Answer 1

• Hyperosmotic urine: urine osmolarity>blood osmolarity
• Produced when circulating ADH levels are high
  
  • Water deprivation
  • Hemorrhage
  • SIADH

Question 2

Corticopapillary osmotic gradient in High ADH

Answer 2

• Gradient of osmolarity from cortex to papilla
  
  • 300 mOsm/L to 1200 mOsm/L
  • Composed mainly of NaCl and urea
• Established by countercurrent multiplication and urea recycling
• Maintained by countercurrent exchange in vasa recta

Question 3

Countercurrent multiplication in the loop of henle

Answer 3

• Depends on NaCl reabsorption in TAL
• and countercurrent flow in descending and ascending limbs
• Augmented by ADH that stimulates NaCl reabsorption in TAL
  
  • Increases size of coricopapillary osmotic gradient

Question 4

Urea Recycling

Answer 4

• From inner medullary collecting ducts into medullary interstitial fluid
• Also augmented by ADH

Question 5

Vasa Recta

Answer 5

• Capillaries that supply the loop of henle
• Maintain corticopapillary gradient
• Serve as osmotic exchangers
• Blood equilibrates osmotically w/ interstitial fluid of medulla and papilla

Question 6

Proximal Tubule in presence of high ADH

Answer 6

• Osmolarity of glomerular filtrate identical to plasma (300mOsm/L)
• 2/3 filtered H2O reabsorbed isosmotically
  
  • w/ Na+, Cl-, HCO3-, Glc, AA, etc
• TF/Posm=1.0 thru out proximal tubule b/c H2O reabsorbed isosmotically w/ solute

Question 7

Thick ascending limb in presence of high ADH

Answer 7

• Diluting segment
• Reabsorbs NaCl by Na/K/2Cl- cotransporter
• Impermeable to H2O and TF becomes more dilute
• Fluid leaving TAL has osmolarity of 100 mOsm/L
• TF/Posm<1.0 due to dilution

Question 8

Early distal tubule in high ADH

Answer 8

• Cortical diluting segment
• Reabsorbs NaCl
• Impermeable to H2O
• Tubular fluid further diluted

Question 9

Late Distal Tubule in Presence of High ADH

Answer 9

-Increases H2O permeability of principal cells
-HO reabsorbed from tubule until osmolarity of TF equal to surrounding interstitial fluid
300 mOsm/L
-TF/Posm=1.0 b/c equilibration

Question 10

Collecting Duct in presence of high ADH

Answer 10

• ADH increases H2O permeability of principal cells
• As TF flows thru collecting duct, it passes thru the corticopapillary gradient (increasingly higher osmolarities)
• H2O reabsorbed from collecting ducts until osmolarity of TF equals that of surrounding interstitial fluid
• Osmolarity of final urine -that of bend in loop (1200mOsm/L)
• TF/Posm>1.0 b/c osmotic equilibration occurs w/ gradient

Question 11

Production of Dilute Urine

Answer 11

• Hyposmotic urine
• Urine osmolarity<blood osmolarity
• Produced when ADH levels are low
  
  • Water intake
  • central diabetes insipidus
• Produced when ADH is ineffective
  
  • Nephrogenic diabetes insipidus

Question 12

Corticopapillary osmotic gradient with No ADH

Answer 12

• Smaller than when ADH present

- B/c ADH stimulates countercurrent multiplication and urea cycling

Question 13

Proximal Tubule if No ADH

Answer 13

• As w/ ADH present, 2/3 filtered H2O reabsorbed isosmotically
• TF/Posm=1.0

Question 14

TAL of Loop of Henle w/ No ADH

Answer 14

• As w/ ADH present, NaCl reabsorbed w/o H2O
• Tubular fluid further diluted
• Not quite as dilute as in the presence of ADH
• TF/Posm<1.0

Question 15

Early Distal Tubule w/ No ADH

Answer 15

• As w/ ADH present, NaCl reabsorbed w/o H2O
• Tubular fluid further diluted
• TF/Posm<1.0

Question 16

Late distal tubule and collecting ducts w/ no ADH

Answer 16

• In the absence of ADH, impermeable to H2O
• Even though TF flows thru corticopapillary osmotic gradient, equilibration doesn’t occur
• Osmolarity of final urine will be dilute, as low as 50 mOsm/L
• TF/Posm<1.0

Question 17

Free Water Clearance

Answer 17

-Used to estimate ability to concentrate or dilute urine
-Free water (solute free water) produced in diluting segments
-Absence of ADH: free water excretion (C-H2O is positive)
-Presence of ADH: free water not excreted (C-H2O is negative)
C-H2O=V-Cosm
C-H2O is free water clearance
V is urine flow rate
Cosm is osmolar clearance

Question 18

Urine that is isosmotic to plasma

Answer 18

• Isosthenuric
• C-H2O is zero
• Produced during tx w/ loop diuretic
  
  • Inhibits NaCl reabsorption in TAL
  • Inhibits TAL dilution and gradient formation
• Urine cannot be diluted during high water intake
• Cannot be concentrated during water deprivation

Question 19

Urine that is hyposmotic to plasma

Answer 19

• Low ADH
• C-H2O is positive
• Produced w/ high water intake (ADH release suppressed)
  
  • Central diabetes insipidus (insufficient ADH)
  • Nephrogenic diabetes insipidus (collecting ducts unresponsive to ADH)

Question 20

Urine that is hyperosmotic to plasma

Answer 20

• High ADH
• C-H2O is negative
• Produced in water deprivation (ADH release)
  
  • Also SIADH