Question

What specific part of the cell/structure allows the mucus secretions to be thick and viscous rather than fluid (i.e., is it a membrane channel, organelle or something else)? Please be specific and describe the step-by-step mechanism whereby this altered structure/part of the cell causes the secretions to contain less water.

Answer 1

In a normal respiratory system, the movement of Cl- ions out of the cell and the maintenance of an electronegative environment outside the cell attract positive ions like Na+. The accumulation of both Cl- and Na+ ions creates solute-rich mucus, which has a low concentration of water molecules. Through osmosis, water moves from cells and the extracellular matrix into the mucus, "thinning" it out and allowing it to be propelled out of the respiratory system.

Step-by-step explanation:

In normal lung tissue, the movement of Cl- out of the cell maintains a Cl-rich, negatively charged environment immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside.

In order to be effectively moved upward, the mucus cannot be too viscous; rather it must have a thin, watery consistency. The transport of Cl and the maintenance of an electronegative environment outside of the cell attract positive ions such as Na+ to the extracellular space. The accumulation of both Cl- and Na+ ions in the extracellular space creates solute-rich mucus, which has a low concentration of water molecules. As a result, through osmosis, water moves from cells and extracellular matrix into the mucus, "thinning" it out. This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down to be propelled out of the respiratory system.

Answer 2

The CFTR behaves like a channel for chlorine. Its dysfunction affects both the transport of this ion and other ions and the transport of water, which causes a thickening of secretions, an alteration of mucociliary transport and local defenses, facilitating bacterial colonization and promoting the release of pro-inflammatory mediators in the airway

Step-by-step explanation:

CFTR is a protein expressed in the epithelial cells of the respiratory system, pancreas, bile ducts, sweat glands and genitourinary system. It is made up of a single chain made up of 1,480 amino acids. It contains 12 hydrophobic regions embedded in the lipid membrane and acts as a channel for chlorine.The highest levels of expression of the CFTR protein are found in serous cells of the submucosal glands of the proximal airway. In them, Cl- is released to the outside. In addition, there are channels for Na +, through which this ion is also secreted in the same direction. These movements cause the displacement of water and also of mucins, originating in the submucosal glands, allowing their presence on the surface of the airway. For all this to occur normally, a basolateral Na + - K + - ATPase cotransporter must function, another basolateral cotransporter formed by Na +, K + and 2 Cl-, which allows the latter to enter the cell, and an apical CFTR channel through which it exits the Cl- of the cell towards the acinar lumen. Na + leaves the cell following Cl- by a paracellular pathway accompanied by water. When CFTR malfunctions, Cl- does not exit through this channel and this implies a decrease in Na + and water in the canalicular lumen, with the consequent thickening of secretions.