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•  The loop of Henle maintains hypertonic conditions in the medulla

Osmoregulation is the third of three processes by which blood is filtered and urine is formed

  • Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism

Osmoregulation occurs in the medulla of the kidney and involves two key events:

  • The loop of Henle establishes a salt gradient (hypertonicity) in the medulla
  • Anti-diuretic hormone (AD) regulates the level of water reabsorption in the collecting duct

Establishing a Salt Gradient

  • The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
  • The descending limb of the loop of Henle is permeable to water but not salts
  • The ascending limb of the loop of Henle is permeable to salts but not water
  • This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic
  • Additionally, the vasa recta blood network that surrounds the loop of Henle flows in the opposite direction (counter-current)
  • This means that salts released from the ascending limb are drawn down into the medulla, further establishing a salt gradient

Creating Hypertonicity in the Medulla

loop of Henle

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•  ADH controls reabsorption of water in the collecting duct

Water Reabsorption

  • As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis
  • The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH)
  • ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
  • ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
  • This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis)
  • When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine)
    • Remember:  ADH is produced when you Are DeHydrated

Role of ADH in Water Reabsorption