Explanation: This statement is incorrect. While mucus contains lipids and proteins, water is its predominant component, typically comprising approximately 95% of its volume. The high water content is crucial for its lubricating and barrier functions.

Explanation: This assertion is accurate. MUC5AC and MUC5B are the major gel-forming mucins in airway mucus, and their polymeric structure is directly responsible for the mucus's viscoelasticity.

Explanation: This statement is incorrect. The tunable swelling capacity of mucus is significantly influenced by environmental pH, which affects the charge state of mucin molecules and consequently their interaction with water.

Explanation: This statement is incorrect. The bottlebrush structure of mucins features hydrophilic segments that facilitate significant water absorption and swelling, rather than a hydrophobic core.

Explanation: This statement is correct. A polyelectrolyte is defined as a polymer chain that possesses charged functional groups along its backbone.

Explanation: This statement is incorrect. The polyelectrolyte effect in mucus involves the attraction of counter-ions to the charged mucin molecules, which creates an osmotic gradient that draws water into the mucus, leading to swelling, not dehydration.

Explanation: Water is the primary component of mucus, constituting approximately 95% of its volume. This high water content is essential for its function as a lubricant and for enabling the swelling capacity of mucins.

Explanation: Cholesterol is not typically listed as a primary component of mucus. Mucus is mainly composed of water, mucin glycoproteins, inorganic salts, and antimicrobial agents like lysozymes and immunoglobulins.

Explanation: MUC5AC is the primary gel-forming mucin secreted by goblet cells in the airways, contributing significantly to the mucus layer's structure and properties.

Explanation: The bottlebrush structure of mucins, characterized by numerous attached carbohydrate chains, presents a high density of hydrophilic segments. This structure is fundamental to mucus's capacity to absorb substantial amounts of water, contributing to its swelling and rheological properties.

Explanation: The polyelectrolyte effect in mucus arises from the charged nature of mucins, which attract counter-ions. This increases the solute concentration within the mucus, creating osmotic pressure that drives water movement and contributes to mucus hydration and swelling.

Explanation: Mucus possesses a tunable swelling capacity, enabling it to absorb or release water in response to environmental cues, thereby altering its volume and hydration state.

Explanation: The polyelectrolyte effect in mucus involves the attraction of counter-ions to charged mucins, creating an osmotic gradient that draws water into the mucus, thereby contributing to its hydration and swelling capacity.

Explanation: MUC5B mucin, secreted by submucosal glands and some goblet cells, forms strands that are crucial for the structural integrity and viscoelastic properties of airway mucus.

Explanation: This statement is inaccurate. The largest volume of mucus in the human body is produced within the gastrointestinal tract, underscoring its significant role in digestion and intestinal protection.

Explanation: This statement is correct. Mucus forms an essential protective layer over epithelial surfaces in numerous systems, including the digestive, respiratory, and urogenital tracts, thereby defending against pathogens and physical damage.

Explanation: This statement is correct. Mucus lines structures within the auditory system and serves a protective role by trapping pathogens and foreign particles, preventing them from causing damage or infection.

Explanation: This statement is correct. Umbilical cord lining epithelial stem cells express a specific mucin (MUC1, termed CLEC-muc) that has demonstrated potential utility in the regeneration of the cornea.

Explanation: Umbilical cord lining epithelial stem cells, which express MUC1, have shown potential for use in the regeneration of the cornea, indicating a possible therapeutic application in ophthalmology.

Explanation: While moisturizing inhaled air is a function of the respiratory mucus blanket, its primary role is to trap foreign particles and infectious agents. Humidification is a secondary but important function.

Explanation: This statement is incorrect. The periciliary liquid layer is a watery, low-viscosity fluid that surrounds the cilia, enabling their effective beating motion. The overlying mucus layer is the thick, gel-like substance responsible for trapping inhaled particles.

Explanation: This statement is incorrect. Phlegm and sputum refer to mucus that has accumulated cellular debris, inflammatory cells, or pathogens, typically during illness or inflammation. Normal respiratory mucus is cleared subconsciously and is not termed phlegm or sputum.

Explanation: This statement is incorrect. Cystic fibrosis is fundamentally characterized by the production of abnormally viscous and dehydrated mucus due to defects in ion transport, which leads to impaired clearance and airway obstruction.

Explanation: While increased mucus production is associated with the common cold, the term 'mucus hypersecretion' typically refers to excessive and persistent mucus production characteristic of chronic conditions like chronic bronchitis. The common cold involves a temporary increase in mucus.

Explanation: This statement is incorrect. While color changes can occur during infection, yellow or green mucus does not exclusively indicate a bacterial infection or necessitate antibiotics. Viral infections can also cause these changes, often due to the presence of immune cells like neutrophils.

Explanation: This statement is correct. Mucoobstructive lung diseases are defined by impaired mucociliary clearance, often resulting from mucus hypersecretion or other factors that lead to airway obstruction.

Explanation: This statement is incorrect. Myeloperoxidase, an enzyme released by white blood cells, contributes a green hue to mucus during infections, not yellow.

Explanation: This statement is incorrect. The airway surface liquid (ASL) is composed of two distinct layers: the periciliary liquid layer (sol layer) and the overlying mucus layer (gel layer).

Explanation: This statement is incorrect. Cold, dry weather tends to cause nasal mucus to dry out, potentially increasing its viscosity and making it more difficult to clear, often leading to increased mucus production.

Explanation: The primary function of the mucus layer in the respiratory system is to act as a physical barrier, trapping inhaled particles, pathogens, and other foreign materials before they can reach the delicate lung tissues.

Explanation: The airway surface liquid (ASL) is comprised of the periciliary liquid layer (a watery sol layer) and the overlying mucus layer (a gel layer).

Explanation: Cystic fibrosis is a genetic disorder characterized by defective ion transport, leading to dysregulated mucus homeostasis and the production of abnormally viscous mucus that obstructs airways.

Explanation: Mucus transforms into phlegm or sputum when it thickens due to the accumulation of cellular debris, pathogens, and inflammatory cells, typically during respiratory illness or infection.

Explanation: The periciliary liquid layer is a watery fluid surrounding the cilia on respiratory epithelial cells. Its primary role is to provide a low-viscosity medium that allows the cilia to beat effectively for mucociliary clearance.

Explanation: Excessive mucus production, or mucus hypersecretion, is a hallmark feature of chronic bronchitis, a condition characterized by a persistent cough and inflammation of the bronchi.

Explanation: The green color often observed in mucus during infections can be attributed to the presence of myeloperoxidase, an enzyme released by neutrophils (a type of white blood cell) as part of the inflammatory response.

Explanation: Bacterial sinusitis is often associated with discolored mucus and responds to antibiotics. Viral sinusitis, more common, typically begins with clear mucus that may thicken, but it does not respond to antibiotics.

Explanation: Mucoobstructive lung diseases result from impaired mucociliary clearance, predominantly caused by excessive mucus production (hypersecretion) or other conditions that obstruct the airways.

Explanation: In cold, dry weather, the nasal mucus lining tends to dry out, stimulating increased mucus production. Simultaneously, condensation of exhaled warm, moist air can lead to a runny nose.

Explanation: This statement is correct. The gastric mucosal barrier, formed by alkaline mucus secreted by gastric glands, is essential for protecting the stomach lining from the harsh environment created by gastric acid and digestive enzymes.

Explanation: This statement is incorrect. Cervical mucus becomes more watery and clear *around* ovulation to facilitate sperm transport. After ovulation, it typically becomes thicker and less permeable, serving to impede sperm.

Explanation: This statement is incorrect. Cervical mucus undergoes significant changes in consistency throughout the menstrual cycle, becoming thinner and more watery around ovulation to facilitate sperm transport and thicker at other times.

Explanation: This statement is correct. The gastric mucosal barrier is indeed formed by alkaline mucus secreted by gastric glands, providing crucial protection against the corrosive environment of the stomach.

Explanation: This statement is partially correct but incomplete. While mucus does lubricate passage in the digestive tract (e.g., esophagus), its primary role in the intestines is to act as a protective barrier, limiting bacterial interaction with epithelial cells and reducing inflammation, rather than solely lubricating food passage.

Explanation: The gastrointestinal tract generates the largest volume of mucus in the human body, highlighting its critical role in digestion and the protection of the gut lining.

Explanation: The protective layer formed by mucus in the stomach is termed the gastric mucosal barrier, which shields the underlying tissue from the acidic and enzymatic environment.

Explanation: Around ovulation, cervical mucus becomes significantly more watery and clear, exhibiting increased elasticity to facilitate the passage of sperm towards the reproductive tract.

Explanation: In the esophagus, mucus primarily functions as a lubricant, facilitating the smooth and frictionless passage of food boluses from the pharynx to the stomach.

Explanation: Cervical mucus is integral to the female reproductive system, providing lubrication during intercourse and acting as a barrier to prevent ascending infections.

Explanation: The mucus layer in the intestines acts as a physical barrier, significantly reducing the direct contact between gut microbiota and the intestinal epithelial cells, thereby mitigating inflammatory responses.

Explanation: In the digestive system, mucus serves dual roles: lubricating the passage of food and forming a protective barrier against mechanical damage, acidic environments, and microbial invasion.

Explanation: This statement is incorrect. Helicobacter pylori facilitates its movement by increasing the local pH (making it less acidic), which causes mucins to deprotonate, swell, and decrease viscosity, thereby easing penetration.

Explanation: This statement is correct. The charged nature of mucins within mucus creates a barrier that selectively impedes or permits the passage of molecules and particles based on their own electrical charge.

Explanation: This statement is incorrect. Cationic particles typically penetrate mucus the least due to electrostatic repulsion. Anionic particles generally achieve the greatest depth of penetration.

Explanation: This statement is incorrect. Native mucus, in its physiological context, exhibits a stronger barrier function against agent penetration compared to purified mucus, suggesting the presence of other protective factors in its natural state.

Explanation: Helicobacter pylori can elevate the local pH, leading to the deprotonation of acidic amino acids in mucins. This increases mucin charge, enhancing the polyelectrolyte effect, causing mucus swelling, increased pore size, and reduced viscosity, thereby facilitating bacterial penetration.

Explanation: The charge of a particle significantly influences its mucus penetration. Cationic particles experience electrostatic repulsion and penetrate least, while anionic particles are attracted to the negatively charged mucins and generally penetrate the deepest.

Explanation: Native mucus, in its physiological context, demonstrates a significantly stronger barrier function against the penetration of agents compared to purified mucus, indicating the importance of its natural composition and environment.

Explanation: Cationic particles are least likely to penetrate mucus due to electrostatic repulsion between their positive charge and the predominantly negative charge of the mucin network.

Explanation: This statement is incorrect. While protection against pathogens is a key function, external mucus in fish also serves to facilitate locomotion and line the gills.

Explanation: This statement is incorrect. While mucus may contribute to adhesion, its primary functions for snails and slugs are to facilitate movement across surfaces and prevent desiccation.

Explanation: This statement is correct. Hagfish produce a unique slime that rapidly expands upon contact with water, serving as an effective defense mechanism by overwhelming predators.

Explanation: This statement is incorrect. In tunicates and larval lampreys, mucus is produced by the endostyle, and its primary function is related to filter feeding, not digestion in the stomach.

Explanation: This statement is incorrect. While both mucus and mucilage are viscous secretions, they differ in their origin and specific chemical composition. Mucus is an animal secretion, whereas mucilage is produced by plants and microorganisms.

Explanation: This statement is incorrect. The endostyle in these organisms produces mucus primarily for filter feeding, trapping food particles from the water, not for respiration.

Explanation: The mucus secreted by snails and slugs primarily serves to lubricate their path, enabling locomotion, and to prevent their bodies from drying out, which is crucial for their survival.

Explanation: Mucus is a secretion from animal mucous membranes, whereas mucilage is a comparable viscous substance produced by plants and microorganisms, serving functions such as water retention.

Explanation: While epidermal mucus in fish protects against pathogens, aids locomotion, and lines the gills, regulating body temperature is not cited as a primary function.

Explanation: The mucus produced by the endostyle in tunicates and larval lampreys serves the crucial function of trapping food particles from the water, aiding in filter feeding.