countercurrent exchange

where does countercurrent exchange occur

at the vasa recta

descending segment of vasa recta

-permeable to NaCI & urea

• enter

-limited H20 permeability

• leaves

makes it more concentrated the more u go down

NaCI entering and H2O leaving descending v.r

creates a high osmality at the bottom of the vasa recta

ascending segment of v.r

high osmality blood from the bottom of vasa recta is moved into a less hypertonic region of the medulla

-permeable to NaCI & H2O

• NaCI leaves

• and H2O enters (reabsorbed/omosis)

because blood is more hypotonic than medulla

3 sources of water for the ascending vasa recta

desc v.r, desc. LH, Collecting Duct (all leave structures passively)

purpose of countercurrent exchange

remove h20 from medulla and have NaCI in medulla to maintain hypertonicity and gradient