In the body, membrane has more than one meaning. A membrane can be the lipid bilayer that surrounds a cell - a cell membrane - or it can be a thin layer of tissue made up of many different cells. The cell membrane controls what enters and leaves the cell, but at the tissue layer, a membrane forms a barrier. A serous membrane is this second type of membrane.

Serous membranes line the cavities within the body and secrete serous fluid. They can be found around organs that move a lot, such as the lungs, heart, stomach and intestines. A serous membrane acts as a covering membrane within body cavities that do not open to the outside of the body. Serous membranes are also referred to as serosa or serous tunic.

A serous membrane is a type of tissue that is made up a thin layer of mesothelium covering connective tissue. Mesothelium is formed from a single layer of flattened cells, while connective tissue consists of many layers of cells. Connective tissue has a very specific function within the body, which is to support, connect, or surround other tissues and organs.

Within vertebrates, the three main examples of serous membranes are the pericardium, the pleura and the peritoneum. The pericardium is found within the thorax and provides a lining around the heart. The pleura are made up of two cavities that line the lungs. The peritoneum lines the abdomen region and provides a cover for the stomach, liver and intestines.

Each serous membrane is made up of two distinct layers - the parietal layer and the visceral layer. The parietal layer is the outer layer of the membrane and is attached to surrounding tissues in the cavity. The visceral layer is the inner layer and it covers the organ. The two layers are separated by a thin layer of fluid, called serous fluid, which has an extremely important role in the upkeep and function of the body’s organs.

Not only does serous fluid keep the two layers of the serous membrane separate, it acts as a lubricant. Serous fluid reduces friction between organs and other organs or tissues. This allows free movement by the organs that are covered by a serous membrane. Depending on where the serous fluid is found, it is often called pericardial fluid if collected from around the heart, peritoneal fluid if collected from the abdominal area, or pleural fluid if collected from around the lungs.