Epithelium is one of the four main types of tissue in the body of animals. It is made up of one or more layers of cells. Epithelium is found on the surface of organs and many other parts of the body. The main function of epithelium is moving fluids into or out of an organ or body part. It can also help protect organs and sense things. There are many different types of epithelium that do different things in the body, including the excretory system.