The Mucas cleans our respiratory system and its part of our defence system. Sometimes out mucas can get too sticky (hypotonic) and water will move into the mucas by osmosis to 'loosen' it up a bit. In CF sufferers this does not happen.
The Normal Situation
Picture of Lungs
The normal situation continued
In the normal situation there is excess water in the mucas. The Na+ pump produces a high concentration of Na+ ions outside the cell the the basal end. Water is lost from the cell by osmosis, drawing water out of the mucas at the apical end.
How the cell responds when the mucas becomes dehyd
When the mucas in the airways becomes too viscous, for example during exercise, the CFTR channel opens. The CFTR channels also inhibit the NA+ ion channels that allow Na+ ions to enter the cell from the mucas. Cl- ions are secreted into the mucas, and Na+ ions also build up in the mucas. This reverses the direction of osmosis. More water enters the mucas, reducing its viscosity.
In people with cystic fibrosis , the CFTR channel is absent or it does not function correctly. Without the CFTR channel there is no Cl- secretion, and no regulation of the Na+ channel. The direction of osmosis cannot be reversed in response to the dehydrated mucas.