Serosa (or serous membrane) is a smooth membrane consisting of a thin layer of cells, found on the outer wall of the organs of the abdominal cavity known as the serous cavity. It secretes serous fluid, and a thin connective tissue layer. They secrete a lubricating fluid which reduces friction from muscle movement.
Serosa is not to be confused with 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 called the pericardium, the serous membrane lining the thoracic cavity and surrounding the lungs is called the pleura, and that lining the abdominopelvic cavity and the viscera is called the peritoneum.
The portions of the gastrointestinal tract that do not have a serosa include the esophagus, as well as the middle and distal rectum.