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Lecture 5

Lecture 5 Water and ion balance.doc

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Michelle French

Lecture 5: Water and ion balance October-25-12 1:07 PM Water distribution in body • ~55% water • 2/3 intracellular fluid • ECF 1/3 o Interstitial fluid: plasma = 3:1 of ECF • Water balance o Intake (food, drink @2.2 L/day) + metabolic production (.3 L/day) = output [ urine @ 1.5 L/day, sweat, breath @ .9L/day, feces @ .1 L/day o 2.5 L out = 2.5 L in Urine formation • In kidneys. Osmolarity difference between surface and medulla • Nephron is the functional unit of the kidney • Urine is formed in the nephron by filtration, reabsorption, secretion Isosmotic fluid leaving the proximal tubule becomes progressively more 1. concentrated in the descending limb, only water absorbed 1. Inner part of medulla (where loop of henle is located, 1200 mOsM, from 300 mOsM near cortex 1. Removal od solute in the thick ascending limb created hyposmotic fluid (100 mOsM) - only ions absorbed 1. Permeability to water and solutes is regulated by hormones 1. Urine osmolarity depends on variable reabsorption in the collecting duct, (50 - 1200 mOsM urine excreted) Hormone regulation of water, Na+ and K+ balance • Vasopressin increases water reabsorption (less urine) • Aldosterone increases Na+ reabsorption and K+ secretion (more Na in blood less K) • Atrial naturietic peptide decreases Na+ and water reabsorption, increases K+ reabsorption (counter-effects both of the above, increases the amount of urine produced) Vasopressin (antidiuretic hormone , ADH) • Synthesized in the hypothalamus (contained in vesicles) • Secreted from the posterior pituitary • Sometimes called arginine vasopressin Regulation of vasopressin release • Detected by (positive regulation) o reduced stretch of walls of the atria of the heart, aortic and carotid arteries (measure blood volume) o High plasma osmolarity detected by osmoreceptors in hypothalamus o Hypothalamic neurosecretory cells give out impulses -> neurosecretory terminals in pituitary o ADH release -> increased water permeability -> water absorption in collecting duct • Osmolarity is the most potent stimulus for vasopressin release (thirst sign of high OsM) • Inserts water pores into distal convoluted tubule/collecting duct cell membranes o Binds to receptors on basal membrane -> cAMP o Signal cascade o Aquaporin-2 water channels created o Join in apical membrane -> water absorbtion • Osmolarity driving force for entry of water into the cell • When vasopressin present, water leaces the colecting duct a
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