<P> During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined . As a result, the nasal cavity can fill up with mucus . At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils . This causes an excess amount of water to build up inside nasal cavities . In these cases, the excess fluid usually spills out externally through the nostrils . </P> <P> Excess mucus production in the bronchi and bronchioles, as may occur in asthma, bronchitis or influenza, results from chronic airway inflammation, and hence may be treated with anti-inflammatory medications . Impaired mucociliary clearance due to conditions such as primary ciliary dyskinesia may also result in its accumulation in the bronchi . The dysregulation of mucus homesotasis is the fundamental characteristic of cystic fibrosis, an inherited disease caused by mutations in the CFTR gene, which encodes a chloride channel . This defect leads to the altered electrolyte composition of mucus, which triggers its hyperabsorption and dehydration . Such low - volume, viscous, acidic mucus has a reduced antimicrobial function, which facilitates bacterial colonisation . The thinning of the mucus layer ultimately affects the PCL, which becomes dehydrated, compromising ciliary function and impairing mucociliary clearance . </P> <P> In the human digestive system, mucus is used as a lubricant for materials that must pass over membranes, e.g., food passing down the esophagus . Mucus is extremely important in the intestinal tract . It forms an essential layer in the colon and in the small intestine that helps reduce intestinal inflammation by decreasing bacterial interaction with intestinal epithelial cells . A layer of mucus along the inner walls of the stomach is vital to protect the cell linings of that organ from the highly acidic environment within it . Mucus is not digested in the intestinal tract . Mucus is also secreted from glands within the rectum due to stimulation of the mucous membrane within . </P> <P> In the human female reproductive system, cervical mucus prevents infection and provides lubrication during sexual intercourse . The consistency of cervical mucus varies depending on the stage of a woman's menstrual cycle . At ovulation cervical mucus is clear, runny, and conducive to sperm; post-ovulation, mucus becomes thicker and is more likely to block sperm . Several Fertility Awareness methods, such as the Creighton Model and Billings method rely on this fact to prevent or to improve the odds of pregnancy . However, its effectiveness as a contraceptive has yet to be established . </P>

Why is it essential that the cells lining the stomach produce mucus
find me the text answering this question