Cystic fibrosis is inherited in an autosomal recessive manner.
This disease occurs due to a defect in the CFTR gene on chromosome 7. Though there are many ways to effect the CFTR gene, commonly, a deletion of Phe508 occurs.
CFTR encodes an ATP-gated Cl- channel. In the lungs and gut, this channel secretes Cl-, which leads to an H2O gradient. In sweat glands, this channel reabsorbs Cl-. A defect in CFTR leads to defects in Cl- secretion through these channels.
Defective Cl- channels lead to increased chloride on the skin (not reabsorbed in sweat glands), as well as decreased chloride secretion (and subsequently water) in the gut and lungs.
In this disorder, Cl- is not secreted into the lungs and GI tract. Thus, there is increased intracellular Cl-, which then causes a compensatory increase in Na+ reabsorption. Due to the high concentration of accumulated salt (NaCl) intracellularly, water is then reabsorbed.
In this disorder, Cl- is not reabsorbed through sweat glands. Increased epithelial Cl- causes a compensatory increase in Na+ excretion via epithelial channels.
As Cl- is not secreted into the lungs and GI tract and is "trapped" intracellularly, Na+ follows and H2O is reabsorbed. This leads to abnormally thick mucus secreted into the lungs and GI tract.
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