Selective reabsorption

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  • Selective reabsorption
    • 85% of the filtrate is reabsorbed in the proximal convoluted tubule.
    • ALL glucose, all amino acids and some salts are reabsorbed with some water.
    • Cells lining the proximal convoluted tubule are specialised to achieve reabsorption. Adaptations:
      • Cell surface membrane in contact with tubule fluid is highly folded to form microvilli. These increase SA for reabsorption.
      • Membrane contains special co-transporter proteins that transport glucose/amino acids in association with Na+ ions fro the tubule into the cell (facilitated diffusion).
      • Opposite membrane of cell (close to tissue fluid+blood capillaries) is also folded to increase SA. Membrane contains sodium--potassium pumps that pump sodium ions out of the cell+potassium ions into the cell.
      • Cell cytoplasm has many mitochondria which indicates an active process is involved as many mitochondria will produce a lot of ATP.
    • Sodium-potassium pumps remove Na+ ions from the cells lining the PCT which reduces conc of Na+ ions in the cell cytoplasm.
    • Na+ ions are transported into the cell with glucose or amino acid molecules by facilitated diffusion.
    • As glucose/amino acids conc rise in cell, these substances are able to diffuse out of the opposite side of the cell into this tissue fluid. May be enhanced by active removal of glucose+amino acids from cells.
    • From tissue fluid, these substances diffuse into the blood and are carried away. Reabsorption of salts, glucose+amino acids reduces water potential in cells and increases water potential in tissue fluid. This means water will enter the cells and be reabsorbed into the blood by osmosis.
    • Larger molecules e.g. small proteins that entered the tubule will be reabsorbed by endocytosis.


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