Medullary Conc Gradient

to maintain water balance or homeostasis, kidneys need to be able to secrete urine that is sometimes? How when water only moves by osmosis?

• less concentrated (dilute, <300_)

• more concentrated than plasma (conc. > 300)

• countercurrent multiplication

goals of countercurrent multiplication

compared to plasma:

• dilute tubular fluid

• To make medullary intersitial fluid to be concentrated

because of the way loop of henle is designed:

• 1st end result: exrete dilute:

  • at the turn, filtrate is concentrated

  • more NaCl leaves than H2O

• 2nd end result: excrete concentrated urine

Body can excrete either dilute or concentrated urine, because of the ?

• countercurrent mechanism: Loop of henle

  • dilutes tubular fluid

  • concentrates medullary intersitiam (concentrated urine)

• Variable permeability of collecting duct to water

__ controls the water permeability of collecting duct: how?

antidiuretic hormone

• collecting duct is impermeable to water: dilute urine

  • NO ADH → no channels on luminal membrane (#2)

• Collecting duct is permeable → concentrated urine:

  • ADH: Aquaporin 2 = aquaporinin channel on luminal membrane

ADH activation

• adh binds to V2 receptor on basolateral side of cell

• Aquaporin 2 on luminal side is activated

because of the countercurrent mechanism:

medullary intersitium is increasingly concentrated

With ADH and water channels, water continues to be reabsorbed as ?

filtrate goes down collecting duct, due to the medullary interstitium increasinly concentrated

With no ADH: osmolairty is ?

max ADH + max osmolarity is ?

50, no aquaporins, very hydrated

1200, dehydrated, many aquaporins

in presence of ADH, what will happen to urine?

• smaller volume of more concentrated uirne

normal 24 hour urine osmolarity average

500-800