The adventitia (/ædvɛnˈtɪʃə/ ad-ven-TI-shuh) is the outer layer of fibrous connective tissue surrounding an organ.
To some degree, its role is complementary to that of the serosa, which also provides a layer of tissue surrounding an organ.
In the abdomen, whether an organ is covered in adventitia or serosa depends upon whether it is peritoneal or retroperitoneal: In the gastrointestinal tract, the muscular layer is bounded in most cases by serosa.
However, at the oral cavity, thoracic esophagus, ascending colon, descending colon and the rectum, the muscular layer is instead bounded by adventitia.
The muscular layer of the duodenum is bounded by both tissue types.