serosae

Serosae, or serous membranes, are thin, slippery tissues that line closed body cavities and surround many internal organs. Each serosa consists of a simple squamous epithelium, called the mesothelium, supported by a thin underlying connective tissue. The mesothelium secretes serous fluid, which collects in a serous cavity to lubricate surfaces and minimize friction as organs move.

Serosae form two continuous layers: the parietal serosa lines the walls of a cavity, while the visceral serosa covers the organs within that cavity. The space between these layers, when present, is a serous cavity containing serous fluid. In health, this fluid allows smooth movement of organs such as the lungs, heart, and abdominal viscera.

Three major serous membranes are commonly described in humans: the pleura surrounding the lungs, the pericardium around the heart, and the peritoneum lining the abdominal cavity and covering abdominal organs. The peritoneum also forms folds such as mesenteries, omenta, and ligaments. Serosae arise from mesoderm during embryonic development as the coelomic cavities expand and become lined by mesothelium.

Functions of serosae include reducing friction between moving organs and body walls, enabling smooth organ movement, and helping to compartmentalize organs within the body cavities. They also create barriers and provide a slippery surface for the rapid movement of viscera relative to one another.

Clinical notes: inflammation of serous membranes is termed serositis (e.g., pleuritis, pericarditis, peritonitis). Serous effusions can accumulate in these spaces, leading to pleural effusion, pericardial effusion, or ascites in the peritoneal cavity.