The defective protein in cystic fibrosis results in a thick, sticky mucus outside the cells, instead of a loose, fluid covering because it prevents movement of chloride to the outside of the cell. In a healthy individual, when chloride moves to the outside of the cell it creates a higher extracellular concentration gradient of chloride on the outside which leads to movement of water to the outside, as well. Since the chloride remains within the cell so does the water, as a result, the water cannot thin the mucus outside the cell and the mucus accumulates and clogs things up.
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Cystic fibrosis is an inherited disease resulting in a defect in the chloride channel protein. Due to this defect, water cannot move to the outside of the cell and in places, like the lungs and pancreas, where water is needed to thin mucus, the absence of water leads to the build up of mucus.