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Regulation of water balance in kidney

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Western University
Physiology 3120
Tom Stavraky

Human Physiology Wednesday, December 2, 2009 “Renal V” **CORRECTION pp. 30  it is a Na/Ca antiporter, not a symporter (basolateral membrane) Regulation of Water Balance • Increased fluid uptake = lots of dilute urine • Dehydration = low volume of concentrated urine • Humans & terrestrial animals can regulate water & sodium balance independently • Intake: from food, fluids, and some from metabolic production; output: through skin and lungs, sweat, feces, and urine • Anti-diuretic hormone (ADH)  Aka vasopressin  Released during dehydration  Results in increased water reabsorption in collecting duct  Peptide hormone, so it must signal via a cell surface receptor (highly charged, can’t cross membrane) on the basolateral membrane (no blood flow on luminal side, too big to be filtered)  Increases cAMP levels, which inserts more aquaporin II channels into luminal membrane  Regulation of ADH release 1. Osmoreceptors: located in hypothalamus, detect changes in plasma osmolality • Water loss produces higher plasma osmolality • Osmoreceptors decrease in volume in response to dehydration; water leaves osmoreceptor as it moves into the plasma • Decrease in volume causes ADH release • If plasma osmolality decreased, ADH not released, so less water would be reabsorbed, which increases volume of urine 2. Baroreceptors: located in aortic arch, and carotid sinus, detect ECF volume and BP • Directly innervate ADH cells in hypothalamus • Water loss causes lower ECF and lower BP • Decrease in activity of baroreceptors (fewer APs) • Decrease in APs causes ADH release from posterior pituitary gland, where it is stored • Diuresis = increased production of urine (1) Alcohol  Easily passes the BBB  Inhibit ADH release, which inhibits additional water reabsorption, causing increased urine production  Overall result is dehydration (2) Diabetes insipidus  Failure of pituitary to release ADH, or by failure of kidney to respond to ADH  Increased urine production and dehydration (3) Caffeine  NOT a diuretic  Some studies show no difference in urine production between water, decaf, and coffee;
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