Urine Concentration and Dilution

Question 1

explain how loop of Henle generates osmolality gradient along renal medulla. how can this gradient be used to concentrate or dilute final urine

Answer 1

increased osmolality from corticomedullary junction to the apex
countercurrent multiplication- establishes osmotic gradient. involves renal tubule and interstitium
countercurrent exchange- maintains gradient (vasa recta and interstitium)

this gradient allows for production of hyperosmotic urine, important bc allows for excretion of solute without losing H2O

Question 2

role urea transport in establishing medullary osmotic gradient

Answer 2

urea handling contributes to generation in long-loop nephrons (no active transport)
TAL, DT, and CD are impermeable to urea

• H2O (and Na+) reabsorption in cortical/outer medullary CD lead to increased [urea]tubule
• urea transporters in inner medullary CD allow for urea to leave the CD passively as a result the [urea]interstitium increases
• increased [urea]inter provides gradient for passive H2O reabsorption in DTL and CD

Question 3

describe countercurrent exchange, how it contributes to medullary osmotic gradient

Answer 3

urine [] involves coordinated function of all segments from loop of Henle and CD
transport and permeability properties along nephron allow for generation of medullary osmotic gradient, regulation of urine osmolality

TAL is motor that powers generation of gradient. Active Na+ reabsorption, separates H2O from solutes

then passive H2O reabsorption from DTL results in osmotic equilibrium btw DTL and interstitium

Question 4

countercurrent exchange

Answer 4

descending vasa recta: plasma osmolality interstitium
gradient for solutes to move from plasma to interstitium. H2o to move interstitium to plasma. both contribute to diluting the plasma

= prevents washout of gradient