The submucosa is the mucous membrane that lines the inner surface of the gastrointestinal (GI) tract. It covers the entire length of the small and large intestines, the rectum, the colon and the anus. The submucosa is made up of many layers of cells, each of which is tightly pressed against the one beneath it. This barrier protection against moistening and hostile microorganisms is one of the main reasons why the GI tract is considered to be one of the most protected organs in the body.

The submucosa is also a main surface for contact with the environment.