Water regulation in the kidney

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Water regulation in the kidney
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The loop of Henle

• The descending limb of the loop of Henle is
  permeable to water
• The ascending limb is impermeable.
• Na and Cl- ions are actively transported out of
  the ascending limb and passively move into the
  descending limb.
• This creates a concentration gradient across the
  medulla.
• There is a lower ion concentration gradient at
  the top of the ascending limb and a much higher
  ion concentration gradient at the bottom.

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The Role of ADH

• The water potential of the tissue fluid in the
  medulla is always more negative than that of the
  filtrate in the collecting duct.
• Whether the water actually leaves the collecting
  duct (by osmosis) is determined by the hormone
  ADH (anti-diuretic hormone)
• Osmoreceptors in the hypothalamus detect the low
  levels of water,so the hypothalamus sends an
  impulse to the pituitary gland which releases ADH
  into the bloodstream.
• ADH makes the wall of the collecting duct more
  permeable to water.
• Therefore, when ADH is present more water is
  reabsorbed and less is excreted.