The mucous membranes are located in areas that are contiguous with the skin. Like the latter, their role is to protect the body against external elements. Most of them secrete a thick, viscous substance called mucous, hence their name.
Location of the mucous membranes
The mucous membranes are thin and soft tissue that lines the cavities of the body which are contiguous with the skin and exposed to the external environment. Thus, mucous membranes can be found in five parts of the body:
• The digestive system, from the mouth to the anus.
• The respiratory system, from the nostrils to the lungs.
• The urogenital system: urethra, bladder, ureter, as well as the uterus and vagina in women, and the glans in men.
• The inside of the eyelids.
• The inside of the ears.
Structure of the mucous membranes
The mucous membranes are structured in three layers:
• First, on the surface there is a layer of epithelial tissue, composed of cells that are set closely against one another. The shape and arrangement of the epithelial cells vary depending on their location.
• The epithelial tissue rests on the corium, a supporting tissue composed of connective fibres and elastic fibres. Along this layer runs a network of blood vessels which nourish the epithelial tissue and absorb certain substances.
• Lastly, the deepest layer of the mucous membranes is composed of muscle cells.
Function of the mucous membranes
First and foremost, the role of the mucous membranes is to protect the body from harmful external agents. This protection occurs in two ways:
• Because of its dense structure, the epithelial tissue in the mucous membranes forms a barrier which prevents pathogens from entering.
• At the same time, most mucous membranes secrete mucous, a viscous substance that keeps them constantly slightly moist. This thick gel, produced by the cells in the epithelial tissue called goblet cells, contains natural antibiotics called defensins. Thus, any pathogens (viruses, bacteria, etc.) that attempt to penetrate the mucous membrane are both caught in the mucous and attacked by the defensins. Only the mucous membranes of the urinary tract are free of mucous, because they are constantly soaked in urine, which is sterile.
The role of some mucous membranes is also to absorb. Thus, the mucous membranes in the digestive tract are capable of absorbing part of the nutrients to transfer them elsewhere in the body. The nasal mucous membranes are covered in a multitude of short hairs and their role is also to block out most inhaled particles.