In anatomy, serous membrane (or serosa) is a smooth membrane consisting of a thin layer of cells which secrete serous fluid, and a thin epithelial layer. The Latin anatomical name is tunica serosa. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid which reduces friction from muscle movement. Serosa is entirely different from the adventitia, a connective tissue layer which binds together structures rather than reducing friction between them. The serous membrane covering the heart and lining the mediastinum is referred to as the pericardium, the serous membrane lining the thoracic cavity and surrounding the lungs is referred to as the pleura, and that lining the abdominopelvic cavity and the viscera is referred to as the peritoneum.
The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells, and also serves as the binding layer which allows the whole serous membrane to adhere to organs and other structures.
For the heart, the surrounding serous membranes include:
Other parts of the body may also have specific names for these structures. For example, the serosa of the uterus is called the perimetrium.
Schematic diagram of an organ invaginating into a serous cavity
The pericardial cavity (surrounding the heart), pleural cavity (surrounding the lungs) and peritoneal cavity (surrounding most organs of the abdomen) are the three serous cavities within the human body. While serous membranes have a lubricative role to play in all three cavities, in the pleural cavity it has a greater role to play in the function of breathing.
The serous cavities are formed from the intraembryonic coelom and are basically an empty space within the body surrounded by serous membrane. Early in embryonic life visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom. Therefore each organ becomes surrounded by serous membrane - they do not lie within the serous cavity. The layer in contact with the organ is known as the visceral layer, while the parietal layer is in contact with the body wall.
The splanchnopleure is associated with the underlying endoderm which it is in contact with, and later becomes the serous membrane in contact with visceral organs within the body.
The somatopleure is associated with the overlying ectoderm and later becomes the serous membrane in contact with the body wall.
The intraembronic coelom can now be seen as a cavity within the body which is covered with serous membrane derived from the splanchnopleure. This cavity is divided and demarcated by the folding and development of the embryo, ultimately forming the serous cavities which house many different organs within the thorax and abdomen.