What substance is secreted by the mucous membranes?

Mucus is a viscous secretion produced by the mucous membranes that line various body cavities and organs, playing a crucial role in protecting and lubricating these surfaces. It serves several important functions, such as trapping pathogens and debris, facilitating the passage of materials within the body, and maintaining moisture in the respiratory and digestive tracts. The secretion of mucus helps to create a barrier against infections and contributes to the overall health of the tissues it surrounds.

Sputum is specifically a thick fluid produced in the lungs that may contain mucus, but it is not a substance that is solely secreted by mucous membranes. Saliva is produced by salivary glands and, while it contains mucus, it is not a direct secretion of the mucous membranes. Pus is a thick fluid that accumulates at sites of infection and is a product of the immune response, not a normal secretion of mucous membranes. Therefore, mucus is the appropriate answer as it is the specific substance secreted by the mucous membranes themselves.