adventitia

Adventitia is the outermost layer of connective tissue that surrounds many tubular structures in the body, including most arteries and veins and organs such as the esophagus and trachea. In blood vessels, the adventitia lies external to the tunica media and intima and blends with surrounding connective tissue. It is composed primarily of collagen fibers, fibroblasts, and loose connective tissue, and may contain adipose tissue in larger vessels. The adventitia commonly contains the vasa vasorum, nerves (nervi vasorum), and lymphatics, supplying the vessel wall and anchoring the vessel to surrounding tissues.

Functionally, the adventitia provides mechanical support and anchorage and serves as a conduit for nerves and microvasculature reaching the vessel wall. In many organs, it is the outermost connective tissue layer; where organs are covered by a serosa, the adventitia may be absent or merge with the serosa.

In the respiratory and digestive tracts, the adventitia blends with surrounding fascia and mediastinal or peritoneal tissue. In the lungs, it surrounds the bronchi and large vessels and connects with the visceral pleura. The composition and thickness of the adventitia vary by organ and species.

Pathology involving the adventitia can include inflammatory conditions such as aortitis or retroperitoneal fibrosis, which can affect surrounding tissues and vessel integrity. Because the adventitia houses nerves and microvasculature, disease processes can impact nourishment and stability of the covered structure.

Overall, adventitia is a structural and functional layer dedicated to anchoring, supporting, and nourishing the outer region of tubular structures, distinct from the media, intima, or serous coverings.