Formation in Human · Part 3

Na and water in the filtrate is reabsorbed in the PCT. Sodium is reabsorbed by active transport through sodium- potassium (Na K ) pump in the PCT. Small amounts of urea and uric acid are also reabsorbed. Descending limb of Henle’s loop is permeable to water due the presence of aquaporins, but not permeable to salts. Water is lost in the descending limb, hence Na and Cl gets concentrated in the filtrate. Ascending limb of Henle’s loop is impermeable to water but permeable to solutes such as Na , Cl and K . The distal convoluted tubule recovers water and secretes potassium into the tubule. Na , Cl and water remains in the filtrate of the DCT. Most of the XI Std Biology-Zoology Chapter- Osmolarity - The solute concentration of a solution of water is known as the solutions osmolarity, expressed as milliosmoles /liter (mOsm/L) Formation of concentrated urine Formation of concentrated urine is accomplished by kidneys using counter current mechanisms. The major function of Henle’s loop is to concentrate Na and Cl . There is low osmolarity near the cortex and high osmolarity towards the medulla. This osmolarity in the medulla is due to the presence of the solute transporters and is maintained by the arrangement of the loop of Henle, collecting duct and vasa recta. This arrangement allows movement of solutes from the filtrate to the interstitial fluid. At the transition between the proximal convoluted tubule and the descending loop of Henle the osmolarity of the interstitial fluid is similar to that of the blood – about 300mOsm. Ascending and descending limbs of Henle, create a counter current multiplier (interaction between flow of filtrate through the limbs of Henle’s and JMN) by active transport. Figure . (a) shows the counter current multiplier created by the long loops of Henle of the JM nephrons which creates medullary osmotic gradient. As the fluid enters the descending limb, water moves from the lumen into the interstitial fluid and the osmolarity of interstitial fluid decreases.