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Water Regulation; unaffected lungs

Water Regulation; unaffected lungs

  • Cells that line the airways produce mucus
  • Water in the mucus is continuously regulated to maintain a constant viscosity
  • Must be runny enough to be moved by beating cilia
  • However it cannot be so runny that the fluid floods the airway
  • Regulation of water content is achieved by transport of sodium ions and chloride ions across epithelial cells
  • Water then follows the ions due to osmosis

Excess water:

  • Too much water; detected by epithelial cells
  • Carrier proteins in the basal membranes on the epithelial cells actively pump sodium ions out of the cells
  • The concentration of sodium ions in the cell falls
  • Creating a concentration gradient across the apical membrane
  • Sodium ions diffuse down this concentration gradient
  • They then enter to cell by facilitated diffusion through sodium channels in the apical membrane
  • The raised concentration of sodium ions creates a potential difference between the tissue fluid and the mucus on the apical membrane side
  • Electrical gradient created – causes negative charged chloride ions to diffuse out the mucus into the tissue fluid
  • Elevated Na+ and Clconcentrations draw water out of the cell by osmosis across the basal membrane in tissue fluid
  • Water loss increases overall solute concentration within cell; higher in cell
  • Causing water to be drawn out the mucus by osmosis across the apical membrane into the cell

Too little water:

  • Chloride ions are transported across the basal membrane into epithelial cells
  • Creates a concentration gradient across the apical membrane
  • At same time, the CFTR protein channels open
  • Chloride ions diffuse out of the cell through the CFTR channels down this concentration gradient into the mucus
  • When open, the CFTR channels block the sodium ion channels
  • Build-up of negative charge chloride ions in mucus creates an electrical gradient between mucus and tissue fluid
  • Sodium ions diffuse out of the tissue fluid and move down the gradient, passing between cells into mucus
  • Movement of ions into mucus draws water out of the cells by osmosis until solutions outside cell are isotonic (same concentration of water molecules)

People with CF:

  • CFTR protein may be missing or not functioning properly
  • When there is too little water; Clcannot be secreted across the membrane
  • No blockage of sodium ions, sodium is continuously absorbed by the cells
  • This draws chloride ions and water out the mucus into the cells making it more viscous