A serous membrane is a thin layer of tissue that lines the body cavities and organs. It is composed of two layers of cells, an epithelial layer and a connective tissue layer. The epithelial layer secretes a fluid called serous fluid which helps to lubricate the organs and reduce friction between them. The connective tissue layer provides support and protection for the organs. Serous membranes are found in the pleural cavity, pericardial cavity, and peritoneal cavity. They also line the heart, lungs, and abdominal organs.