Urine Concentration and Dilution

What makes up the countercurrent mechanism

countercurrent multiplier in JG nephron, countercurrent exchanger and urea recycling

What is the characteristic of the descending limb (concentrating segment)

Permeable to water, impermeable to solutes

What is the characteristic of the TAL (diluting segment)

Impermeable to water, Na/K/2Cl absorbs electrolytes

Collecting duct function in maintaining medullary osmotic gradient

Has ADH channels to allow for water reabsorption

What happens in the DL to establish the the Medullary Osmotic Gradient

Water moves OUT of the descending limb → Tubular fluid is more concentrated

What happens in the TAL to establish the the Medullary Osmotic Gradient

Na, Cl and K move OUT of TAL → Tubular fluid is less concentrated

What does the vasa recta do

Permeable to both solute and water → Slow blood flow allows for more exchange and minimizes loss of solute

What is the process of countercurrent exchange

Blood descending to medulla is hypo-osmotic → Water leaves tubule in DL → Na, K and Cl leave tubule in TAL → vasa recta pick up NaCl (hyperosmotic) → NaCl moves out vss → Water moves into vss

How much water is absorbed into each nephron segment

70% to PCT, 20% to DT, 10% to CD

Function of ADH

Acts on plasma membrane of principal cells → release of cAMP → aquaporin 2 is inserted to apical membrane and more is synthesized

How does the Countercurrent Mechanism Produce Concentrated Urine

When there is aquaporin-2 the tubules are permeable to water → Draws water from CD until the fluid is iso-osmotic

What causes dilute urine

Water is reabsorbed in descending limb but not TAL, solute reabsorption does not change → Low ADH makes DCT and CD impermeable to water

How does DI cause dilute urine

Low ADH release → CD and DCT is impermeable to water → Water is not reabsorbed and released in urine

What is used to transport urea

ADH has a urea transporter → UT-A1 and UT-A3