1) The adventitia is the outer external layer of fibrous connective tissue surrounding an organ.

2) The outer layer of connective tissue that surrounds an artery, or vein—the tunica externa, is also called the tunica adventitia.

In the abdomen, whether an organ is covered in adventitia or serosa depends upon whether it is peritoneal or retroperitoneal:

• Intraperitoneal organs are covered in serosa (a layer of mesothelium, the visceral peritoneum)

• Retroperitoneal organs are covered in adventitia (loose connective tissue)

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