The Human Body

Membrane

Membrane (Latin membrana,”parchment”), in biology, any thin layer of connective tissue coating individual cells and organs of the body, or lining the joints and the ducts and tracts that open to the exterior of the body. The membrane surrounding single-celled animals and plants and individual cells in multicellular organisms is important in the nutritive, respiratory, and excretory processes of these cells. Such cell membranes are semipermeable; that is, they allow the passage of small molecules, such as those of sugars and salts, but not large molecules, such as those of proteins. Structures inside cells, such as the nucleus, may also have membranes.

Each organ in the animal body is surrounded by a membrane, extensions of which often anchor the organ to the body wall. Three membranes, known as meninges, surround the brain and spinal cord; the outermost is known as the dura mater, the middle layer as the arachnoid, and the innermost as the pia mater. Each lung is coated with a membrane known as a visceral pleura. The visceral pleurae anchor the lungs to the wall of the pleural cavity by extensions, known as the parietal pleurae, which line the cavity. The abdominal cavity is lined by a large membrane called the peritoneum, which is attached to the mesenteries—the membranes coating the abdominal organs. A double membrane from the stomach, known as the omentum, hangs like an apron in the abdominal cavity and is interlaced with fat; the omentum is one of the major fat-storage areas of the body. The articular surfaces of bones making up a joint are lined with lubricating membranes. Small membrane sacs, or bursae, occur in the space between the bones of most joints. The hollow tracts, such as the respiratory and alimentary tracts, and the blood vessels and glandular ducts are lined with membranes. The membranes lining body cavities and coating organs are generally known as serous membranes because the cavities usually contain a serumlike fluid; the membranes lining joints are known as synovial membranes because they secrete synovial lubricating fluid; and the membranes lining the hollow tracts are known as mucous membranes because they secrete mucus. Inflammations of the membranes are assigned names by adding the suffix -itis to their anatomic name.