April 14 Lecture

Kanagy

plasma = 300 mOsm    maximum concentration = 4 fold = 1200 mOsm    maximum dilution = 10 fold = 30 mOsm

tip of loop of Henle in tissue with 1200 mOsm in humans.   longest loops in desert rats (never drink water - get all water from food metabolism).
"shortest loops in marine mammals";   not really - marine mammals can concentrate their urine by 8 fold (2400 mOsm) allowing them to drink sea water (2000 mOsm) and get some net water gain from sea water.  freshwater mammals; e.g., Beavers, do have shorter loops of Henle

ADH  osmoreceptors in hypothalamus  ADH secreted in posterior pituitary

ADH secretion also stimulated by decreased blood pressure.  carotid sinuses most important (not cardiac baroreceptors)

ADH=AVP    vasoconstriction via V1 receptor Gq via phospholipase C.    Antidiuresis via V2 receptor cAMP - PKA - phosphorylates aquaporins - inserts aquaporins in luminal membrane (modulated by PGE2 - fine tuning of water reabsorption),  also inserts urea channels adding urea to interstitial fluid. 

Free water clearance

total clearance = water clearance + solute clearance

urine volume = Cwater + Cosm

or Cwater = urine volume - Cosm

example for dehydrated person:  urine volume = 1 ml/min   Uosm =1000 mOsm  Posm = 333 mOsm

Cwater = 1 ml/min  -   (1000 x 1)/333 = 1 - 3 = - 2 ml/min

for overhydrated person: urine volume = 10 ml/min   Uosm = 30 mOsm   Posm = 300 mOsm

Cwater = 10 ml/min - (30 x 10)/300  = 10 - 1 = 9 ml/min