Membranes cover or line body structures to provide protection from injury and infection. There are four major classes of membranes.
- Mucous membranes line those structures that open to the outside world (for example, the mouth, the airway, digestive tract, urinary tract, and vagina)
- Serous membranes line the closed body cavities and cover the outsides of organs
- The cutaneous membrane is the skin
- Synovial membranes line joints to reduce friction during movement
A serous membrane that covers an organ is called a visceral layer. The term parietal layer is used for the part of the serous membrane that lines a cavity. The serous membrane in the thoracic cavity is called pleura. In the abdominal cavity, it is called peritoneum. A double layer of peritoneum is called mesentery. The membrane that lines the sac surrounding the heart is pericardium.
Of interest:
The skin is the largest organ of the body. In the adult the skin covers about 3000 square inches and weighs about 6 pounds. It is involved with protection, insulation, thermal regulation, excretion, and the production of vitamin D.