Concentration and Dilution of Urine

Study Question 1

Regulation of Plasma OsmolarityBy varying the amount of water excreted relative to the amount of solute

excreted (responses to water deprivation and water intake).

Concentrated Urine

• Also called hyperosmotic urine• Urine osmolarity > blood osmolarity• Produced when circulating ADH levels are high– Water deprivation– Volume depletion

PRODUCTION OF CONCENTRATED URINE

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

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

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.

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

4. Early Distal Tubule

• Cortical diluting segment

• Reabsorbs NaCl• Impermeable to water• Further dilution of TF

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)

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

Dilute Urine

• Also called hypoosmotic urine• Urine osmolarity < blood osmolarity• Produced when circulating levels of ADH is

ineffective

PRODUCTION OF DILUTE URINE

1. CORTICOPAPILLARY OSMOTIC GRADIENT

• Smaller than in presence of ADH– ADH stimulates

countercurrent multiplication and urea recycling

2. Proximal Tubule

• 2/3 of filtered water is reabsorbed isosmotically

• TF/P = 1.0

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

4. Early Distal Tubule

• Reabsorbs NaCl• Impermeable to water• Further dilution of TF• TF/P <1.0

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