While it's true that mucous membranes are found throughout the body and are involved in many bodily functions, they are considered part of the integumentary system, which is the largest organ system in the human body.

Here's why:

* The integumentary system's primary function is protection. Mucous membranes, which line the openings and cavities of the body, provide a vital layer of protection against foreign invaders, pathogens, and physical damage.

* Mucous membranes are a type of epithelial tissue. The integumentary system is primarily composed of epithelial tissues, and mucous membranes are a specialized form of epithelial tissue.

* The integumentary system includes the skin, hair, and nails. While mucous membranes are not on the surface of the skin, they are a component of the system that acts as a barrier and contributes to overall protection.

So, even though mucous membranes are found in many different organs and systems, they are considered part of the integumentary system due to their primary function and their composition.