Water Reabsorption

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  • Water Reabsorption
    • Reabsorption of Water
      • After selective reabsorption in the proximal convoluted tubule about 45 Cm3 is left
      • Each minute about 125 cm3of fluid is filtered from the blood and enters the nephron tubules.  
      • The role of the loop of Henle is to create a low (very negative) water potential in the tissue of the medulla
      • This ensures that even more water can be reabsorbed from the fluid in the collecting duct
    • The Loop of Henle
      • The loop of Henle consists of a descending limb that descends into the medulla and an ascending limb that ascends back out to the cortex
      • The arrangement of the loop of Henle allows salts (Na ions and Cl ions) to be transferred from the ascending limb to the descending limb.
      • The overall effect is to increase the concentration of salts in the tubule fluid.
      • Consequently they diffuse out from the thin walled ascending limb into the surrounding medulla tissue
        • This gives the tissue fluid in the medulla a very low (very negative) water potential
    • How is this (low water potential) achieved?
      • As the fluid in the tubule descends deeper into the medulla its water potential becomes lower (more negative. This is due to two things
        • Loss of water by osmosis to the surrounding  tissue fluid
        • Diffusion of sodium and chloride ions into the tubule from the surrounding tissue fluid
      • As the fluid then goes up the ascending limb, Na+ and Cl? ions are actively pumped out so it gets more and more dilute.
      • Filtrate passing down the descending limb of the loop of Henle is flowing in the opposite direction to fluid in the ascending limb.
      • The fluid is increasingly concentrated as it moves down and increasingly dilute as it moves up. This countercurrent flow (or countercurrent multiplier) allows concentrated urine to be produced.
    • The Collecting Duct
      • This is important because yet more water is drawn out of the tube (at this point called the collecting duct) when it passes through the medulla again.
      • This allows you to make concentrated urine. Any filtrate not reabsorbed - most of the urea, some water and some salt - is drained into the bladder.
      • Obviously the amount of water reabsorbed is controlled by the quantity of water in the blood. The less water in the blood, the more that it must be reabsorbed.
  • Reabsorption of Water
    • After selective reabsorption in the proximal convoluted tubule about 45 Cm3 is left
    • Each minute about 125 cm3of fluid is filtered from the blood and enters the nephron tubules.  
    • The role of the loop of Henle is to create a low (very negative) water potential in the tissue of the medulla
    • This ensures that even more water can be reabsorbed from the fluid in the collecting duct

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