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3 WATER BALANCE (Maintained by kidney (3 main components: ((1) generation…
3
WATER BALANCE
Maintained by kidney
water deprived
produce urine that has 7-8 times the osmolality of plasma
water overload
hypotonic urine is produced
3 main components:
(1) generation of hypertonic medullary interstitium
allow excretion of concentrated urine
produced & maintained primarily by
reabsorption of osmotically active substances by tubules in medulla
removal of water from medulla interstitium by vasa recta
(2) dilution of tubule fluid by thick ascending limb & dct
allows excretion of dilute urine
(3) variability in water permeability of collecting duct in response to ADH
determines final urine concentration
nephrons are subdivided into:
superficial
short loops of Henle
extend only into inner stripe of outer medulla
juxtamedullary
long loops of Henle
extend deep into inner medulla
particularly responsible for kidney's ability to concentrate urine at much higher level than osmolality of plasma
Proximal tubule
reabsorbs majority of glomerular filtrate
active transport
Na+, K+ -ATPase pump in basolateral plasma membrane
transports Na+ from cell into interstitial fluid
drives carrier-mediated secondary active transport & passive uptake of solutes & water
passive diffusion
Na+ & other solutes are removed from tubule fluid
creates slight gradient favoring movement of water into cells & intercellular spaces
apical brush border & complex basolateral plasma membrane in proximal tubule
create large surface area for reabsorption
highly permeable to water
abundant water channels, aquaporin-1 & aquaporin-7
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