What happens in the membranes of cells lining the airways?

With excess water:
1. Na+ is actively pumped across the basal membrane.
2. Na+ diffuses through sodium channels in the apical membrane
3. Cl- diffuses down electrical gradient
4. Water is drawn out of the cells by osmosis due to the high salt concentrations in the the tissue fluid.
5. Water is drawn out of the mucus by osmosis.

With too little water.
1. Cl- is pumped into the cell across the basal membrane.
2. Cl- diffuses through the open CFTR channels
3. Na+ diffuses down the electrical gradient into the mucus.
4. Elevated salt concentrations in the mucus draws water out of the cell by osmosis.
5. Water is drawn into the cell by osmosis.

With CF
1. CFTR chanel is absent or not functional
2. Na+ channel is permanently open
3. Water is continually removed from mucus buy osmosis. 

In a person who has CF, the CFTR protein may be missing, or if present it does not function properly. When there is too little water in the mucus, Cl- cannot be secreted across the apical membrane, and there is no blockage of the epithelial sodium ion channels. Since the Na+ channels are always open, there is continual Na+ absorption by the epithelial cells. The raised levels of Na+ draw chloride ions and water out of the mucus and into the cells. This makes the mucus more viscous, which makes it harder for cilia to move it, so the mucus is not effectively cleared up and out of the lungs. Sticky mucus builds up in the airways. This mucus frequently becomes infected with bacteria, causing a downward spiral of airway inflammation and damage.

The effect of CF on the digestive system

In a person with CF, the pancreatic duct becomes blocked by sticky mucus, impairing the release of digestive enzymes. The lower the concentration of enzymes within the small intestine reduces the rate of digestion. Food is not fully digested, so not all of the nutrients can be absorbed. A higher proportion of partially digested and undigested food means energy is lost in the faeces. This is called malabsorption syndrome. An additional complication occurs when the pancreatic enzymes become trapped behind the mucus blocking the pancreatic duct. These enzymes damage the pancreas itself.