function of the kidney 2

• reabsorption of water


  • each minute 125cm3 of fluid filtered from blood and enters nephrons.
  • After selective reabsorption in proximal convoluted tubule, about 45cm3 of fluid is left.
• Loop of Henle 


  • loop of Henle consists of a descending limb that descends into the medulla and an ascending limb that ascends back out to the cortex.
  • arrangement of loop of Henle allows mineral ions to be transferred from ascending limb to descending limb.
  • overall effect is to increase the concentration of mineral ions in the tubule fluid which has a similar effect upon concentration of mineral ions in tissue fluid. gives tissue fluid in medulla a very low water potential.
• loop of Henle1


  • as mineral ions enter descending limb, concentration of fluid in descending limb rises. this means that water potential decreases, becomes increasingly more negative the deeper the tubule descends into the medulla
• loop of Henle 2


  • as fluid rises up ascending limb, mineral ions leave fluid. at base movement is by diffusion. Higher up the ascending limb active transport is used to move mineral ions out. Upper portion of ascending limb is also impermeable to water.
• loop of Henle 3


  • effect of these ionic movements is to create higher water potential in fluid of ascending limb. It also decreases water potential in tissue fluid of medulla. Water potential of tissue fluid becomes lower towards bottom of loop of Henle.
• loop of Henle 4


  • as fluid passes down collecting duct, it passes through tissues with a decreasing water potential - there is always a water potential gradient between the fluid in the collecting duct and that in tissues. This allows water to be moved out of collecting duct and into tissue fluid by osmosis
• loop of Henle 5


  • arrangement of the loop of Henle is known as a hairpin countercurrent multiplier system. The overall effect of this arrangement is to increase efficiency of transfer of mineral ions from ascending limb to descending limb, in order to create water potential gradient seen in medulla
• collecting duct 1

  • from top of ascending limb, tubule fluid passes along short distal convoluted tubule where active transport is used to adjust concentrations of mineral ions.
  • fluid flows into collecting duct - tubule fluid has high water potential
  • collecting duct carries fluid back down through medulla to pelvis
  • as tubule fluid passes down collecting duct, water moves by osmosis from tubule fluid into surrounding tissue
  • enters capillaries by osmosis and is carried away
• collecting duct 2

  • amount of water reabsorbed depends on permeability of collecting duct walls
  • by the time urine reaches pelvis, it has a low water potential and concentration of minerals and urea is higher than in blood
• concentration changes in tubule fluid

  1. glucose decreases in conc as its selectively reabsorbed from proximal tubule
  2. sodium ions diffuse into descending limb of loop of Henle causing concentration to rise. They are then pumped out of ascending limb so concfalls
  3. urea conc rises as water withdrawn from tubule. Urea also actively moved into the tubule
  4. sodium ions removed from tubule but their conc rises as water removed from tubule and potassium ions increase in conc as water is removed. Potassium ions also actively transported into tubule to be removed in urine