Please enable JavaScript.
Coggle requires JavaScript to display documents.
Water in the Mucus - Coggle Diagram
Water in the Mucus
Excess
If the mucus layer is too runny, the presence of excess water is detected by the epithelial cell membranes that line the airways.
Carrier proteins in the basal membranes of the epithelial cells actively pump Na ions out of the cells. The conc. of Na ions in the cell falls, setting up a conc. gradient across the apical membrane.
Na ions diffuse from the mucus down this conc. gradient into the epithelial cell by facilitated diffusion through epithelial Na ion channels in the apical membrane. The raised conc. of Na in the tissue fluid on the basal membrane side of the epithelial cells creates a p.d. between the tissue fluid and the mucus on apical membrane side.
The tissue fluid now contains more + charged ions than the mucus -> This creates an electrical gradient between the tissue fluid and the mucus.
This electrical gradient causes - charged Cl ions to diffuse out of the mucus into the tissue fluid via the gaps between neighbouring epithelial cells. The overall effect of these processes is to increase the Na & Cl conc.'s in the tissue fluid
Water is then drawn out of the epithelial cells by osmosis across the basal membrane into the tissue fluid.
This water loss increases the overall solute conc within the cell ->Since the solute conc. is now higher within the cell than in the mucus, water is drawn out of the mucus by osmosis across the apical membrane and into the epithelial cell.
Having too much water in the mucus is normal -> This is because the cilia are continuously moving mucus along the airways.
Movement of mucus from many smaller bronchiole branches into fewer larger bronchioles means that water must be removed to reduce the volume of mucus to avoid the larger airways flooding with fluid.
-
Too little
When there is too little water in the mucus Cl ions are transported across the basal membrane into the epithelial cell.
This creates a conc. gradient across the apical membrane, with the conc. of Cl ions being higher inside the cell than outside.
In a person who does not have CF, the Cl ion imbalance causes the cystic fibrosis transmembrane conductance regulator (CFTR) protein channels to open -> The CFTR protein is a type of gated channel protein.
Cl ions now diffuse out of the cell through the CFTR channels down this conc. gradient into the mucus.
When open, the CFTR channels block (close) the epithelial Na ion channels (ENaC) in the apical membrane.
The build up of - charged Cl ions in the mucus creates an electrical gradient between the mucus and the tissue fluid.
Na ions diffuse out of the tissue fluid and move down this electrical gradient, passing between the cells into the mucus.
The movement of the Na and Cl ions into the mucus draws water out of the epithelial cells by osmosis until the solutions are isotonic -> This movement of water prevents the mucus that lines the airways from becoming too viscous (sticky).