Too much water in mucus

• 1. Sodium out:

  • Sodium ions pumped across the basal membrane
  • From epithelial cell into tissue fluid
  • Via sodium ion pump
  • Against the conc gradient
  • By active transport
• 2. Sodium in:

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

  • Accumulation of positively charged sodium ions in tissue fluid
  • Creates electrochemical gradient
  • Negatively charged chloride ions diffuse down this gradient
  • In the gaps between neighbouring epithelial cells
  • From mucus to tissue fluid
  • By electrochemical diffusion
• 4. Water out (same way as Na+ in 1)

  • High solute conc in tissue fluid
  • Water moves across the basal membrane
  • From epithelial cell into tissue fluid
  • Down the conc gradient
  • By osmosis
• 5. Water in (same way as Na+ in 2)

  • Water loss in epithelial cell caused a increase in overall solute conc in epithelial cell
  • Water moves across the apical membrane
  • From mucus into epithelial cell
  • Down the conc gradient
  • By osmosis
  • Thus lowering water content in mucus