Concentration and Dilution of Urine
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Transcript of Concentration and Dilution of Urine
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Concentration and Dilution of Urine
Study Question 1
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Regulation of Plasma OsmolarityBy varying the amount of water excreted relative to the amount of solute
excreted (responses to water deprivation and water intake).
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Concentrated Urine
• Also called hyperosmotic urine• Urine osmolarity > blood osmolarity• Produced when circulating ADH levels are high– Water deprivation– Volume depletion
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PRODUCTION OF CONCENTRATED URINE
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1. CORTICOPAPILLARY OSMOTIC GRADIENT
• Cortex (300mOsm/L) –> Papilla (1200 mOsm/L)
• Composed of NaCl and urea
• Established by:– Countercurrent
multiplier– Urea recycling
• Maintained by:– Vasa recta
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1. CORTICOPAPILLARY OSMOTIC GRADIENT
• Countercurrent multiplier (loop of Henle)– Depends on NaCl reabsorption in
thick AL and countercurrent flow in loop of Henle
– Augmented by ADH
• Urea recycling– IMCD -> Medullary interstitial
fluid– Augmented by ADH
• Vasa recta– Osmotic exchangers– Blood equilibrates osmotically
with the ISF of medulla and papilla
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2. Proximal Tubule
• Osmolarity of glomerular filtrate is identical to plasma (300 mOsm/L)
• 2/3 of filtered water is reabsorbed isosmotically in PT
• TF/P = 1.0 in PT because water is reabsorbed isosmotically with solute.
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3. Thick Ascending Limb
• Diluting segment• Reabsorbs NaCl via the
1Na – 1K – 2Cl contransporter
• Impermeable to water• TF that leaves TAL
becomes dilute– 100mOsm/L– TF/P: <1.0
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4. Early Distal Tubule
• Cortical diluting segment
• Reabsorbs NaCl• Impermeable to water• Further dilution of TF
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5. Late Distal Tubule
• ADH increases water permeability of the principal cells
• H20 is reabsorbed until osm of distal TF = osm of surrounding ISF in renal cortex (300 mOsm/L)
• TF/P = 1.0 at end of distal tubule (osmotic equilibration occurs in the presence of ADH)
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6. Collecting Ducts• ADH increases water
permeability of the principal cells• TF passes through
corticopapillary gradient• H20 is reabsorbed until osm of TF
= osm of surrounding ISF• Final Urine OSM = osm of TF at
bend of loop of Henle and at the tip of papilla (1200mOsm/L)
• TF/P > 1.0 (osmotic equilibration occurs with the corticopapillary gradient in the presence of ADH
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Dilute Urine
• Also called hypoosmotic urine• Urine osmolarity < blood osmolarity• Produced when circulating levels of ADH is
ineffective
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PRODUCTION OF DILUTE URINE
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1. CORTICOPAPILLARY OSMOTIC GRADIENT
• Smaller than in presence of ADH– ADH stimulates
countercurrent multiplication and urea recycling
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2. Proximal Tubule
• 2/3 of filtered water is reabsorbed isosmotically
• TF/P = 1.0
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3. Thick Ascending Limb
• Reabsorbs NaCl via the 1Na – 1K – 2Cl contransporter
• Impermeable to water• TF that leaves TAL
becomes dilute (although not as dilute as that in the presence of ADH)– TF/P: <1.0
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4. Early Distal Tubule
• Reabsorbs NaCl• Impermeable to water• Further dilution of TF• TF/P <1.0
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5. Late Distal Tubule and Collecting Ducts
• Cells are impermeable to H20
• Osmotic equilibration does not occur even if TF flows through corticopapillary osmotic gradient
• Osm of final urine is dilute (~50mOsm/L)
• TF/P < 1.0
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