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 Cl– concentrations 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; Cl– cannot 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
