Which membranes line cavities and tubes that open to the outside?

Mucous membranes are specialized layers of tissue that line cavities and tubes that open directly to the exterior environment, such as the respiratory, digestive, and urinary tracts. These membranes are crucial for protecting the underlying tissues and facilitating functions such as absorption and secretion. They are comprised of epithelial cells and a layer of connective tissue, which help to produce mucus, a viscous fluid that keeps the surfaces moist and aids in trapping pathogens and debris.

In contrast, serous membranes line body cavities that are not open to the exterior, such as the thoracic and abdominal cavities. Synovial membranes are found in joint cavities and provide lubricating fluid for joint function, while cutaneous membranes refer to the skin itself, which serves as a protective barrier but does not line internal tubes or cavities. Thus, the primary distinction for mucous membranes is their direct contact with external environments, fulfilling vital roles in various bodily systems.