urine concentration

role of loop of henle

• to lower water potential in the medulla.

1) ascending limb pump

• Na+/Cl- are pumped out using active transport in ascending limb.

• lowers water potential in interstitial region..

2) osmosis down descending limb

PERMEABLE TO H2O

• start: fluid wp=blood wp.

• as it moves down, H2O diffuses out due to lower conc in medulla.

• makes fluid more concentrated.

3) ions diffusion

• due to more conc fluid, Na, Cl are able to diffuse out.

• taken over by active transport at ascending limb.

4) collecting duct

• H2O diffuses out due to lower water potential.

• small vol of conc urine produced

counter current multiplier

• descending limb flows down

• ascending limb flows up (opposite)

• efficiency of ion transfer out of fluid in ascending limb is amplified by transfer of water out of descending limb.