Outline how cystic fibrosis leads to recurrent lung infecions

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   Cystic Fibrosis is a genetic condition that results from defective chloride ion channels. Normally a protein known as the “Cystic Fibrosis Transmembrane conductance Regulator” (CFTR) transports chloride ions across the cell membrane and out of the cell. This movement of Cl- ions out of the cell causes Sodium ions to also move out of the cell due to the electrostatic attractions between the Cl- and Na+ ions. The increased levels of Na+ and Cl- outside of the cell mean that the fluid outside the cell has a higher concentration of solute (IE there is a higher concentration of water molecules on the inside of the cell.) This causes water to move by osmosis out of the cell. In the cells lining the airways, These Chloride ion channels help to pump chloride and therefore sodium and water out of the cell into the surrounding mucus (which is also produced by the cells lining the airways.) This osmotic movement of water makes the mucous more watery and less viscous. In Cystic Fibrosis a mutation in the genes coding for the CFTR protein means that the ion channel is defective and cannot pump chloride ions out of the cell. This means that sodium and therefore water cannot be added to the extracellular mucus lining the airways making it very thick and sticky. This excessively viscous mucous damages the cilia (small hairs) lining the airways. The cilia cannot properly clear mucous (which contains trapped pathogens) from the airways, meaning that those pathogens linger in the airways and are more likely to enter the respiratory tract and lungs. This results in recurrent chest infections.

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