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The adventitia () is the outer layer of fibrous
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
surrounding an
organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic keyboard instrument ** Hammond ...
. The outer layer of connective tissue that surrounds an
artery An artery (plural arteries) () is a blood vessel in humans and most animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pu ...
, or
vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...
– the
tunica externa The tunica externa (New Latin "outer coat"), also known as the tunica adventitia (New Latin "additional coat"), is the outermost tunica (layer) of a blood vessel, surrounding the tunica media. It is mainly composed of collagen and, in arteries, i ...
, is also called the ''tunica adventitia''. To some degree, its role is complementary to that of the
serosa The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...
, 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: * intraperitoneal organs are covered in serosa (a layer of
mesothelium The mesothelium is a membrane composed of simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura ( pleural cavity around the lungs), peritoneum ( abdominopelvic cavity including the mes ...
, the
visceral peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesothel ...
) * retroperitoneal organs are covered in ''adventitia'' (
loose connective tissue Loose connective tissue, sometimes called areolar tissue, is a cellular connective tissue with thin and relatively sparse collagen fibers. Its ground substance occupies more volume than the fibers do. It has a viscous to gel-like consisten ...
) In the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
, the muscular layer is bounded in most cases by serosa. However, at the
oral cavity In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on t ...
,
thoracic The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
,
ascending colon ''Ascending'' is a science fiction novel by the Canadian writer James Alan Gardner, published in 2001 by HarperCollins Publishers under its various imprints.HarperCollins, Avon, HarperCollins Canada, SFBC/Avon; paperback edition 2001, Eos Books. ...
,
descending colon In the anatomy of humans and homologous primates, the descending colon is the part of the colon extending from the left colic flexure to the level of the iliac crest (whereupon it transitions into the sigmoid colon). The function of the descendi ...
and the
rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the l ...
, the muscular layer is instead bounded by adventitia. The muscular layer of the
duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
is bounded by both tissue types. Generally, if it is a part of the
digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
that is free to move, it is covered by serosa, and if it is relatively rigidly fixed, it is covered by adventitia. The connective tissue of the
gallbladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although ...
is covered by adventitia where the gallbladder bounds the
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
, but by serosa for the rest of its surface.


See also

*
Lumen (anatomy) In biology, a lumen (plural lumina) is the inside space of a tubular structure, such as an artery or intestine. It comes . It can refer to: *The interior of a vessel, such as the central space in an artery, vein or capillary through which blood ...


References


External links

* - "Mammal, whole system (LM, Low)" * (
vermiform appendix The appendix (or vermiform appendix; also cecal r caecalappendix; vermix; or vermiform process) is a finger-like, blind-ended tube connected to the cecum, from which it develops in the embryo. The cecum is a pouch-like structure of the large ...
) {{Urinary system anatomy Connective tissue