Line body cavities that do not open directly to the outside, and they cover the organs located in those cavities covered by a thin layer of serous fluid that is secreted by the epithelium Serous fluid lubricates the membrane and reduces friction and abrasion when organs move against each other or against the cavity wall havespecialnamesgivenaccordingtotheir location e.g. pleura – serous membrane that lines the thoracic cavity and covers the lungs