3.4 Serous Cavities of the Human Body

The term serous refers to a watery- type fluid. Serous cavities are sacs lined with serous membranes. These cavities serve as lubricating devices. They reduce the friction during the motion between organs.

BURSA

A bursa is the simplest of serous cavities. Each bursa is a small sac located between two moving structures, usually a muscle moving over a bony surface. The bursa reduces the friction between the two structures. For example, a bursa prevents excessive friction between the skin and patella (knee cap). This bursa, called the prepatellar bursa, allows the skin to move freely over the patella. (When injured, it produces excessive amounts of the serous fluid and is known as "housemaid's knee.")

As a fibrous sac, each bursa has a central cavity which is lined with a serous membrane. This membrane is a simple squamous epithelium. The serous membrane secretes a serous fluid into the serous cavity. The serous fluid is the lubricant, minimizing friction.

OTHER SEROUS CAVITIES OF THE BODY

Other important serous cavities are associated with the major hollow organs, referred to as visceral organs. Each lung is encased in a serous cavity called the pleural cavity. The heart lies in a serous cavity called the pericardial cavity. The intestines are allowed to move freely during the digestive processes within the peritoneal cavity.

Each serous cavity has an inner and an outer membrane. The inner membrane is intimately associated with the surface of the visceral organ. The outer membrane forms the outer wall of the cavity. The serous lining of the cavity secretes the serous fluid into the cavity to act as a lubricant between the membranes, allowing freer motion for the organs.