Ascending Colon (Posterior)

Structure/Morphology

The ascending colon is the portion of the large intestine between the cecum and the hepatic flexure.

The cross-sectional microarchitecture of the large intestine demonstrates mucosa, submucosa, and muscular (inner circular and outer longitudinal) layers with a thin outer serosal covering (Standring, 2016).

As with the rest of the large intestine, the longitudinal muscle arrangement of the ascending colon contains three muscular bands called teniae coli. Contraction of the teniae coli shortens the ascending colon to create sacculations or bulges called haustra. Between the haustra are the semilunar folds. These promote churning of chyme and increase the surface area of mucosa for absorption (Standring, 2016). Additionally, small pockets of fat called the omental appendices project from the external (non-mesenteric) surface of most of the large intestine.

The ascending colon is part of the midgut, which helps explain the patterns of vascularization, innervation, and lymphatic drainage of this tissue.

Anatomical Relations

Superior to the ileocecal junction on the right side of the abdominal cavity is the ascending colon. It is secondarily retroperitoneal. Where the lateral border of the ascending colon lies in contact with the posterior abdominal wall, a small recess is formed called the right lateral paracolic gutter. A less distinct right medial paracolic gutter is present at the medial margin, beyond which lie the small intestines. The ascending colon extends superiorly to the level of the right kidney where it variably lies ventral to its anterior surface. Further ascension is blocked by the inferior surface of the liver (approximately level of ribs nine and ten). As a result, the distal end of the ascending colon makes a sharp bend to the left as the hepatic or right colic flexure (Standring, 2016).

Function

The ascending colon absorbs water, salts, vitamins, and minerals from the waste material. As a result, as it passes through the colon it becomes more and more solid, with the contents of the ascending colon being more liquid than the rest. The colon contains a significant proportion of gastrointestinal bacteria, which aids in the formation of feces and in the synthesis of essential vitamins (Koeppen and Stanton, 2009).

Arterial Supply

The ascending colon is supplied primarily by the right colic artery which is a major branch of the superior mesenteric artery (Standring, 2016). Another branch of the superior mesenteric artery, the ileocolic artery, supplies the proximal ascending colon, cecum, and ileocecal junction.

A significant anastomotic arterial “circle” known as the marginal artery of the colon (of Drummond) allows blood from an artery serving one area of the colon to pass to territories normally served by a different artery. In this way, blood from the ileocolic artery can serve the more distal ascending colon or blood from the right colic artery could serve more proximal ascending colon.

Venous Drainage

The venous drainage is into the portal circulation. The pattern largely parallels the arterial supply, with blood from the right colic vein and ileocolic vein draining into the superior mesenteric vein, then into the portal vein, and finally into the liver.

Although oxygen poor, the venous blood from the ascending colon is nutrient rich.

Innervation

Innervation of the colon includes the enteric nervous system (sensory and motor), the autonomic nervous system (sympathetic and parasympathetic), and extrinsic sensory innervation (visceral afferents) (Standring, 2016).

The vagus nerve (CN X) provides parasympathetic innervation to the ascending colon.

Sympathetic innervation is derived from the aortic plexus. The ascending colon is supplied by postganglionic neurons from the superior mesenteric plexus.

Visceral afferent nerves from the ascending colon travel back to the CNS along the greater splanchnic nerve or vagus nerve.

The enteric system consists of two plexuses of densely packed small neurons. Meissner’s plexus lies in the submucosal layer and Auerbach’s myenteric plexus lie between the outer longitudinal and inner circular smooth muscle layers. These systems of nerves control mucosal and peristaltic function.

Lymphatic Drainage

Lymphatic drainage from the midgut component of the large intestine parallels the arterial supply and drain into superior mesenteric lymph nodes (Földi et al., 2012). Paracolic nodes drain to the intermediate ileocolic nodes, and into the central superior mesenteric nodes. Ultimately, all lymph is drained via the cisterna chyli and the thoracic lymphatic duct.

List of Clinical Correlates

- Colitis

- Colonoscopy

- Crohn’s disease

- Diverticulitis

References

Földi, M., Földi, E., Strößenreuther, R. and Kubik, S. (2012) Földi's Textbook of Lymphology: for Physicians and Lymphedema Therapists. Elsevier Health Sciences.

Koeppen, B. M. and Stanton, B. A. (2009) Berne & Levy Physiology, Updated Edition E-Book. Elsevier Health Sciences.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.