What is a Serous Membrane?

A serous membrane is a thin membrane that helps body parts move more easily and reduces friction between them. Also known as serosa, these membranes are found throughout the body, usually surrounding organs. There are several medical conditions associated with the serous membane, some of which can be life threatening.

Serosa are made up of two layers of tissue with a thin layer of serous fluid in the middle. One layer is made up of mesothelium, which produces the fluid, while the other is made up of connective tissue, which attaches the membrane to other parts of the body and provides blood and nerves for the epithelial layer. Serous fluid itself is a thin liquid that's normally pale yellow, but can become milky, dark yellow, or red in people with infections or other medical conditions.

Many places in the body have serosa, including the body cavities surrounding the heart, lungs, and the abdomen and pelvic area. It's also found in the stomach, uterus, testicles, and vagina. Depending on where the membrane is located in the body, it may be called different things: for instance, the serous membrane around the heart is called the pericardium, while that around the lungs is called the pleura, and that surrounding the testes is called the tunica vaginalis.

Though all serosa are primarily used for reducing friction, their exact function differs slightly by location. The pericardium, for example, makes it easy for the heart to expand and contract as it beats as well as protecting is and holding it in place. Similarly, the pleura makes it easier for the lungs to expand by reducing friction between them and the surrounding tissues, but also makes the outer layer of the lungs cling to the surrounding tissue, which helps keep them from collapsing inwards.

Problems with a serous membrane can be related to the serous fluid or to the two tissue layers. An overproduction of fluid can cause serious problems, especially if enough fluid is produced to put pressure on a person's organs. This is called effusion when it happens around the heart or lungs, and ascites if it happens in the peritoneum. All three can be life-threatening if not treated promptly, and usually have to be treated by draining the fluid with a needle. Serous fluid can also become infected with bacteria or mixed with blood because of a problem somewhere else in the body.

Another condition that affects the tissue layers of the serous membrane, rather than the fluid, is malignant mesothelioma. This is a cancer that most often affects the pleura, though it can affect any of the serosa. It's primarily associated with exposure to asbestos, and is generally very difficult to treat.