Describe the pathophysiology.
The CFTR gene encodes the CFTR protein, which is an important component of the ATP-gated chloride channel in cell membranes.
Mutated CFTR gene → misfolded protein → retention for degradation of the defective protein in the rough endoplasmic reticulum (rER) → absence of ATP-gated chloride channel on the cell surface of epithelial cells throughout the body (e.g., intestinal and respiratory epithelia, sweat glands, exocrine pancreas, exocrine glands of reproductive organs) 
In sweat glands
The chloride channel is responsible for transporting Cl- from the lumen into the cell (reabsorption).
Defective ATP-gated chloride channel → inability to reabsorb Cl- from the lumen of the sweat glands → reduced reabsorption of Na+ and H2O → excessive loss of salt and elevated levels of NaCl in sweat
In all other exocrine glands (e.g., in the GI tract or lungs)
The chloride channel is responsible for transporting Cl- from the cell into the lumen (secretion).
Defective ATP-gated chloride channel → inability to transport intracellular Cl- across the cell membrane → reduced secretion of Cl- and H2O → accumulation of intracellular Cl- → ↑ Na+ reabsorption (via ENaC) → ↑ H2O reabsorption → formation of hyperviscous mucus → accumulation of secretions and blockage of small passages of affected organs → chronic inflammation and remodeling → organ damage (see “Clinical features” below)
↑ Na+ reabsorption → transepithelial potential difference between interstitial fluid and the epithelial surface increases (i.e., negative charge increases; e.g., from normal -13 mv to abnormal -25 mv)