Selective reabsorption

• What is selective reabsorption?

  -When all glucose, most water and sodium and chloride ions are reabsorbed into the blood in the proximal convoluted tubule

  -small amounts of water and salt are reabsorbed in the distal convoluted tubule

  -most of the remaining water is reabsorbed in the collecting duct
• In selective reabsorption, useful substances are reabsorbed, and urea remains in the filtrate to be excreted
• All glucose and most water and salt are reabsorbed
• How is the proximal convoluted tubule adapted for selective reabsorption? 1

  • cells lining the walls are cuboidal epithelium with a microvilli brush border, increases SA for reabsorption
  • cells contain lots of mitochondria, provides ATP for active transport
  • the base of the cells are folded to form basal channels which increase the concentration of molecules to increase the concentration gradient
• How is the proximal convoluted tubule adapted for selective reabsorption? 2

  • capillaries are in close contact with the basement membrane of the cells of the PCT to reduce the diffusion distance
  • blood flow in capillaries removes substances in the filtrate so maintains the concentration gradient
• As glomerular filtrate moves along the PCT, ions and polar substances are reabsorbed into the blood. This increases the water potential of the filtrate, so water moves into the cytoplasm by osmosis.
• Na+ are reabsorbed at the same rate as water and Cl- are reabsorbed a little less than water. So even though nearly 100% of amino acids and glucose (polar molecules) are reabsorbed, the overall water potential of the filtrate remains isotonic to that of the blood plasma
• How is water reabsorbed from the PCT into the blood?

  • water moves from a high water potential in the filtrate in the PCT lumen to a low water potential in the cytoplasm of PCT cells by osmosis
  • the cytoplasm is diluted until the water potential is higher than that of the blood plasma
  • so water then moves by osmosis into the tissue fluid and then the blood
• How are ions reabsorbed from the PCT into the blood?

  • glucose/amino acids are reabsorbed from the filtrate into the cytoplasm of PCT cells with Na+ by co-transport
  • Na+ is then reabsorbed into the blood by facilitated diffusion and/or active transport
• How are glucose/amino acids reabsorbed from the PCT into the blood?

  • Na+ move from PCT cells into the blood by active transport
  • so the PCT cells now have a low Na+ concentration
  • so Na+ move by facilitated diffusion from the filtrate into the PCT cells through carrier proteins
  • at the same time, glucose/amino acids are co-transported with Na+ through the carrier protein from the filtrate into the PCT cells
  • so glucose/amino acid concentration increases in the PCT cells
  • so glucose/amino acids diffuse from the PCT cells into the blood