Countercurrent Mechanism Establishes Sodium Gradient From Cortex to Medulla

Countercurrent Mechanism Establishes Sodium Gradient From Cortex to Medulla

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Urinary System Countercurrent Mechanism Establishes Sodium Gradient From Cortex to Medulla * The Kidney Uses Osmosis in the Collecting Duct to Control the Concentration and Volume of Urine * •The concentration of the urine is adjusted in the collecting ducts of the kidney. * •The collecting ducts pass through tissue with a very high osmotic pressure in the medulla. * •Water will be sucked out of the tubules by osmosis if the tubules are permeable. Collecting Duct & Urine Production * •As the urine passes into the collecting duct it first passes through a region of isotonic osmotic pressure (300 milliosmoles/liter) and then through a region of hypertonic osmotic pressure (up to 1200 milliosmoles/liter) * If the collecting duct has low water permeability the dilute urine in the kidney tubule passes through with little uptake of water * This produces large amounts of dilute urine (diuresis) * If the collecting duct has high water permeability much of the water will be reabsorbed from the collecting duct into the interstitial fluid * This produces small amounts of concentrated urine (antidiuresis) ADH Controls Kidney Osmosis by Inserting Water Pores into the Collecting Duct * The permeability of the collecting duct is determined by water pores ( aquaporin-2 ) which are under the control of antidiuretic hormone (ADH- also called vasopressin) from the posterior pituitary * The water pores are made by the cells lining the collecting duct * Pores are stored in vesicles called endosomes and are inserted when needed * If ADH is present they are inserted into the cell membranes facing the tubule * If ADH is low the channels are removed from the membranes (down-regulated) Effects of Antidiuretic Hormone * If ADH is High the Kidney Makes Concentrated Urine and Conserves Water * When a person is dehydrated the blood osmotic pressure rises * Under these conditions osmoreceptors in the hypothalamus fire, causing the posterior pituitary to secrete large amounts of ADH * The ADH will cause the kidney collecting ducts to insert water pores * More water will be conserved, preventing further dehydration * The dehydration will also make the person thirsty and drinking will restore the water volume Effects of Antidiuretic Hormone * If ADH is Low the Kidney Makes Large Volumes of Dilute Urine * If a person has recently consumed a lot of water the ADH secretion will be low * Water channels will be down-regulated from the collecting duct * Less water will be reabsorbed * The increased urine production will remove the excess water
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Urinary System Defects in the ADH Mechanism Cause Diabetes Insipidus * Diabetes insipidus is the continuous production of large amounts of watery urine (5-10 L/day) * The urine does not contain sugar, as it does in diabetes mellitus * ADH mechanism can fail in 2 ways:
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This note was uploaded on 01/30/2012 for the course BSC BSC1085 taught by Professor Sharonsimpson during the Fall '10 term at Broward College.

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Countercurrent Mechanism Establishes Sodium Gradient From Cortex to Medulla

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