Respiratory system

In the respiratory system mucus aids in the protection of the lungs by trapping foreign particles that enter, particularly through the nose, during normal breathing. Phlegm is a specialized term for mucus that is restricted to the respiratory tract, while the term mucus more globally describes secretions of the nasal passages as well. Nasal mucus is produced by the nasal mucosa, and mucus lining the airways (trachea, bronchus, bronchioles) is produced by specialized airway epithelial cells (goblet cells) and submucosal glands. Small particles such as dust, particulate pollutants, and allergens as well as infectious agents such as bacteria become caught in the viscous nasal or airway mucus. This event along with the continual movement of the respiratory mucus layer toward the oropharynx, helps prevent foreign objects from entering the lungs during breathing. Additionally, mucus aids in moisturizing the inhaled air and prevents tissues such as the nasal and airway epithelia from drying out. Nasal and airway mucus is produced constitutively, with most of it swallowed unconsciously.

Increased mucus production in the respiratory tract is a symptom of many common illnesses, such as the common cold and influenza. Similarly, hypersecretion of mucus can occur in inflammatory respiratory diseases such as respiratory allergies, asthma, and chronic bronchitis. The presence of mucus in the nose and throat is normal, but increased quantities can impede comfortable breathing and must be cleared by blowing the nose or expectorating phlegm from the throat. Tears are also a component of nasal mucus.

Mucin

Mucus is produced by submucosal cells as well as goblet cells found in the airway epithelium in the respiratory tract. It consists of mucin, a highly glycosylated peptide. Upon stimulation, MARCKS (myristylated alanine-rich C kinase substrate) protein coordinates the secretion of mucin from mucin filled vesicles within the specialized epithelial cells. Fusion of the vesicles to the plasma membrane causes release of the mucin, which as it exchanges Ca²⁺ for Na⁺ expands up to 600 fold. The result is a viscoelastic product of interwoven molecules which, combined with other secretions from the airway epithelium and the submucosal glands, is called mucus.

Diseases involving mucus

Generally mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change colour to yellow or green either as a result of trapped bacteria, or due to the body's reaction to viral infection. Such coloured mucus or phlegm usually has an offensive putrid odour.