Osmoregulation

• 4 step process by which water is reabsorbed by the kidneys

  1. Ultrafiltration in Bowman's capsule

  2. High hydrostatic pressure in glomerulus forces water, glucose, ions, urea out against osmotic gradient

  3. Through fenestrations in capillary endothelium

  4. Through capillary basement membrane, blood cells and large molecules remain in capillary
• Selective reabsorption

  1. Glucose and water are reabsorbed into blood

  2. Glucose by facilitated diffusion and active transport

  3. Water down a water potential gradient
• Cells in the proximal convoluted tubule adapted for selective reabsorption

  • Microvilli: large surface area for co-transporter proteins
  • Many mitochondria: ATP for active transport of glucose
  • Folded basal membrane: large surface area
• Loop of Henle


  1. Active transport of Na+ and Cl- out of ascending limb

  2. Osmosis of water out of the descending limb (ascending limb is impermeable to water)

  3. Water potential of filtrate decreases going down descending limb, lowest in medullary region, highest at top of ascending limb
• Distal convoluted tubule and the collecting duct

  1. Reabsorption of water via osmosis

  2. Reabsorption of ions via active transport

  3. Permeability of walls determined by action of hormones
• Role of the collecting duct

  Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins
• Maintaining an Na+ gradient

  • Countercurrent multiplier: filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential
  • Maintains water potential gradient for maximum reabsorption of water
• Role of the hypothalamus in osmoregulation


  1. Osmosis of water out of osmoreceptors in the hypothalamus causes them to shrink

  2. This triggers the hypothalamus to produce more antidiuretic hormone (ADH)
• Role of the posterior pituitary gland in osmoregulation


  Stores and secretes the ADH produced by the hypothalamus
• The permeability of the collecting duct can be altered by hormones

  1. Osmoreceptors in hypothalamus detect changes in blood water potential

  2. When low, osmoreceptors shrink, causing ADH to be released

  3. ADH binds to receptors on surface of collecting duct and activates phosphorylase

  4. Causes vesicles containing aquaporins to be embedded in cell surface membrane

  5. Increases water permeability and urea permeability

  6. Urea leaves collecting duct, causing water to leave and be reabsorbed in the blood
• Role of ADH in osmoregulation

  • Makes cells lining collecting duct more permeable to water
  • Makes cells lining collecting duct more permeable to urea
  • More water reabsorbed = more concentrated urine