Mucus is a substance normally present in the respiratory system, serving a continuous and protective function. This aqueous fluid is composed primarily of about 95% water, along with salts, immune cells, and large protein molecules called mucins. These mucins are glycoproteins that give the secretion its characteristic sticky, gel-like consistency. The sensation of “mucus in the lungs” usually refers to an abnormal increase in quantity or a change in thickness, making it noticeable and difficult to clear.
Where Respiratory Mucus Is Produced
The production of this airway surface liquid occurs along the entire respiratory tract, from the nose down through the bronchioles. Two main structures generate respiratory mucus: goblet cells and submucosal glands. Goblet cells are specialized, single-celled glands scattered throughout the epithelial lining of the airways, constantly secreting mucins that swell upon contact with water to form the mucus gel.
Submucosal glands are more complex structures found deeper within the walls of the larger airways. They contribute a significant volume of both water-rich fluid and gel-forming mucins, playing a larger role in total mucus production.
The Protective Function of Mucus
The presence of mucus creates a defense layer that shields the underlying cells of the respiratory tract. This layer acts as a physical trap for foreign materials inhaled during breathing, such as dust, pollen, pollutants, and microbes, preventing them from reaching the deeper, gas-exchange areas of the lungs.
Mucus also contains protective components, including antimicrobial enzymes like lysozyme and antibodies like Immunoglobulin A (IgA), which provide a chemical defense against pathogens. The mucus layer helps to humidify the inhaled air, preventing the drying out of airway surfaces. This moisturizing function is necessary for the proper movement of the cilia, which clear the mucus layer.
Cilia and Mucus Clearance
The respiratory system has a self-cleaning mechanism known as the mucociliary escalator, which continuously prevents mucus from accumulating in the lungs. This system relies on the coordinated action of the mucus and tiny, hair-like projections called cilia that cover the surface of the airway cells. Mucus is structured into two distinct layers to facilitate this movement.
The lower layer, called the periciliary liquid layer or sol layer, is a thin, watery fluid that surrounds the base of the cilia. The cilia, which are approximately seven micrometers long, beat freely within this low-viscosity fluid. The upper layer is the viscous gel layer, which traps the inhaled particles.
Cilia beat in a rhythmic, wave-like pattern, performing a two-part motion. During the power stroke, the cilia tips extend to make contact with the gel layer, propelling it forward. They then perform a recovery stroke, bending backward and remaining within the watery sol layer to avoid dragging the thick mucus back down the airway.
This synchronized beating moves the mucus blanket and its trapped debris upwards, away from the lungs and toward the throat. This transport can reach up to 20 millimeters per minute in the trachea, effectively clearing the entire healthy lung in less than 24 hours. Once the mucus reaches the pharynx, it is normally swallowed, removing the material from the respiratory system entirely.
Conditions Leading to Mucus Buildup
When people feel that mucus has “gotten into their lungs,” it is a sign that the mucociliary escalator has failed or been overwhelmed. One common cause is infection, such as bronchitis or pneumonia, where the body increases mucus production as an immune response to trap and eliminate pathogens. This excess volume of mucus can overwhelm the clearance capacity of the cilia.
Inflammation from irritants, like cigarette smoke or allergens, can cause goblet cells and submucosal glands to increase their output, leading to hypersecretion. Simultaneously, inflammation can damage or slow the rhythmic beating of the cilia, impairing the transport process. The resulting thicker, more abundant mucus then stalls in the airways, causing congestion and requiring a cough to clear the material.
Chronic conditions permanently disrupt this balance, leading to persistent mucus accumulation. For example, in Chronic Obstructive Pulmonary Disease (COPD), the number of mucus-producing goblet cells increases significantly, and the cilia can be damaged. Genetic conditions like cystic fibrosis cause the mucus to become abnormally thick and sticky due to issues with water secretion, making it extremely difficult for the cilia to move.