Membrane is a term used to describe biological structures that cover surfaces and separate spaces in organisms. For example, cellular membranes provide the external boundary for cells and divide cells into functional compartments. They serve as barriers to the passage of molecules in and out of cells and between compartments (see Cell (Inside a living cell)).
The term membrane is also used to describe thin sheets of tissues that cover surfaces or separate spaces in the body. There are three types of these membranes: (1) fibrous, (2) serous, and (3) mucous. They vary in thickness and in the types of cells composing them.
Fibrous membranes
are tough and add strength to the parts they cover. They are made up entirely of fibrous connective tissue (see Tissue (Animal tissues)). The fibrous membrane that lines the inside of the skull is called dura mater. The periosteum is a fibrous membrane that covers the bones. The periosteum also serves as an attachment for muscles, and contains the blood vessels and nerves of the bones.
Serous membranes
line body cavities that do not open to the outside, such as the thorax and abdomen. They also cover the outside of the digestive organs and support them. Serous membranes secrete a watery fluid that keeps them moist and prevents their sticking to each other or to the organs they touch. A serous membrane lines the pericardium, the sac around the heart (see Heart (The structure of the heart)). Other serous membranes include the pleura, which lines the lung cavities, and the peritoneum, which lines the cavity of the abdomen (see Pleura). Inflammation of the peritoneum is known as peritonitis. A serous membrane called the synovial membrane lines the cavities of the joints. It secretes a watery fluid that lubricates the joints and helps them move easily and smoothly. The largest of the synovial cavities is in the knee.
Mucous membranes
line organs and passages of the body that open to the outside. A clear, sticky fluid called mucus covers mucous membranes (see Mucus). Glands just under the membranes produce the mucus. Mucous membranes form the lining of the mouth, throat, alimentary canal, reproductive system, nose, trachea and lungs, the inner surfaces of the eyelids, and the Eustachian tube.
