The mucous membranes are a thin tissue layer or epithelium that lines the body"s hollow structures (cavities) opening to the exterior, such as the upper respiratory system. They secrete a slightly viscous fluid called mucus, which is actually a complex viscoelastic gel that lubricates and moistens the cavity surfaces. As a part of normal, healthy physiology, mucus is the first line of defense in the nose and lungs to trap and filter inhaled dust, pollutants, and other potentially damaging agents. Proteolytic enzymes such as serrapeptase can break down inappropriate protein complexes in the mucus, thereby favorably influencing its properties and helping to support the healthy balance between adequate mucous viscosity/elasticity and airway comfort levels associated with the mucous membranes of the respiratory system. Research with serrapeptase for maintenance of healthy sinus function began decades ago in Japan. Studies thus far indicate that serrapeptase is a safe and useful supplement to promote favorable viscosity of nasal and bronchial mucus. Two of the earliest human trials found that serrapeptase had favorable effects on upper airway viscosity. In these assessments, mucus was collected from participants after a 4-week supplementation period and evaluated using an rheometer to determine its viscoelastic properties. The enzyme"s action most likely involves the breakdown of mucus protein structures, allowing the cilia (microscopic hair like projections in the mucosal epithelium of the upper respiratory tract) to helpfully transport mucus in the airway. Serrapeptase can also significantly affect sputum viscoelasticity. In an investigation of forty individuals who were randomly assigned to one of five groups, six participants were given serrapeptase for one week. Sputum viscoelasticity was analyzed using a rheometer and researchers concluded that serrapeptase altered the "relaxation behavior" of sputum-meaning that serrapeptase helped support airway clearance.