The mesothelium is a membrane surrounding the chest cavity, the abdominal cavity, and the cavity around the heart, and is made up of mesothelial cells. Mesothelial cells also cover the outer surface of most internal organs. The mesothelium helps protect the organs by producing a lubricating fluid that lets organs move around. An example of this is the lungs: this fluid makes it easier for the lungs to expand and move around inside the chest during breathing. The mesothelium in the chest is called the pleura and the mesothelium around the abdomen is known as the peritoneum. The mesothelium around the heart (or the pericardial cavity, a sac like space around the heart) is called the pericardium. It also exists on structures such as the ovaries and testes.
