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.
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.