MAINTENANCE OF SODIUM ION GRADIENT

• WHAT DOES THE LOOP OF HENLE ACT AS?

  counter-current multiplier responsible for reabsorption of water from the collecting ducts

  -> concentrating urine and decreasing the water potential of blood
• TWO LIMBS OF LOOP OF HENLE?

  DESCENDING

  • narrow
  • thin walls
  • highly permeable to water

  ASCENDING

  • wider
  • thick walls
  • impermeable to water
• PROCESS?

  1. Na+ AT out AL
  2. Lower water potential in IS in medulla
  3. Water passes out of filtrate by osmosis into IS through DL walls
  4. Enters capillaries
  5. Carried away
  6. Filtrate loses water
  7. Lowers water potential
  8. At base of AL, Na+ diffuse out filtrate
  9. Na+ actively pumped out as filtrate moves up AL
  10. Increasing water potential of filtrate
  11. In IS, highest water potential in cortex, lowest in medulla
  12. CD permeable to water
  13. Filtrate moves down and passes out by osmosis
  14. Passes into vessels
  15. Carried away
  16. Lowers water potential of filtrate and IS
  17. Continues to move out across whole length of CD
• WHAT DOES THE COUNTER-CURRENT MECHANISM ENSURE?

  • water potential gradient
  • water passes out through aquaporins (channel proteins specific to water)
  • number of them is altered by ADH
  • filtrate then called urine
  • leaves collecting duct on way to bladder
  • lost lots of water so now has a lower water potential than blood
• DISTAL CONVOLUTED TUBULE?

  • many microvilli and mitochondria
  • can reabsorb rapidly by active transport form filtrate
  • makes final adjustments to water and salts that are reabsorbed
  • control blood pH by selecting which ions to reabsorb
  • wall permeability altered by hormones