Too little water in mucus

• 1. Chloride in

  • Chloride ions pumped across the basal membrane
  • From tissue fluid into epithelial cell
  • Via chloride ion pump
  • Against conc gradient
  • By active transport
• 2. Chloride out

  • Chloride ions diffuse across the apical membrane
  • From epithelial cell into mucus
  • Via CFTR channel protein
  • Down conc gradient
  • By facilitated diffusion
• 3. Sodium up to Chloride:

  • Accumulation of negatively charged chloride ions in mucus
  • Creates an electrochemical gradient
  • Positively charged sodium ions diffuse down this gradient
  • In the gaps between neighbouring epithelial cells
  • By electrochemical diffusion
• 4. Water out up:

  • High solute conc in mucus
  • Water moves across the apical membrane
  • From epithelial tissue into mucus
  • Down the conc gradient
  • By osmosi
• 5. Water in from down:

  • Water moves across the basal membrane
  • From tissue fluid into epithelial cell
  • Down the conc gradient
  • By osmosis
  • Thus, increasing water content of mucus