Ciclación borilativa de aleninos y enalenos

Sections of the colon

Defined as the lower segment of intestine from the ileocecal valve to the anus, the

large bowel is approximately 150 cm in length. It is divided into three segments

defined by different morphology and location (extraperitoneal and retroperitoneal

location): the caecum, the colon (the ascending colon, the transverse colon, the

descending colon and the sigmoid colon), and the rectum. The large intestine, which

can be distinguished from the small intestine by its increased diameter, has a muscular

wall. The muscular wall includes taenia coli, haustra coli and appendices epiploicae.

The taenia coli consists of three longitudinal muscles along the longitudinal axis of

the colon starting near the base of the appendix and ending in the ends of sigmoid

colon. The length of taenia coli is 30cm shorter than the bowel it is attached to, and

the enlarged pouch formed by the shrinkage of colonic wall is called haustracoli.

Appendices epiploicae are accumulations of subserosal fat tissue along the external

surface of the colon.

The right colon is made up of the caecum (with appendix) and ascending colon. It is

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Blood supply to the colon

The vascular supply to the caecum and ascending colon arises from branches of the

superior mesenteric artery (SMA), which divides into the inferior

pancreaticoduodenal artery, the middle colic artery, the right colic artery and the

ileocolic artery. The middle colic artery immediately forms two large arcades in the

transverse mesocolon, which connects with the SMA and ileocolic arterial branches.

The ileocolic artery splits towards a right colic artery to the upper ascending colon,

which starts with the right border of the superior mesenteric artery and forms two

branches at the caecum (the colic branch and the ileal branch). The colic branch forms

an anastomosis with descending branches from the right colic artery. The ileal branch

forms branches to the distal small bowel and caecum and anastomoses with the ends

of superior mesenteric artery. The right colon is a retroperitoneal structure.

The transverse colon is supplied by branches of the middle colic artery. It is the first

portion of the colon considered to be intraperitoneal, and its length is around 20cm to

50cm. Its boundaries are defined from the hepatic flexure on the right to the splenic

flexure on the left. Both of these points are fixed by the transverse mesocolon. The

length of transverse mesocolon fixed both hepatic flexure and splenic flexure is

shorter than the rest of the transverse colon section, hence the shape of transverse

colon is described as arched prolapsed. The hepatic flexure abuts the gallbladder fossa,

while the splenic flexure lies anterior to the tail of the pancreas.

The descending colon is defined as the segment of colon from the splenic flexure to

the sigmoid colon, where the colon becomes a retroperitoneal structure and its length

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mesenteric artery (IMA). The IMA arises from the aorta and ends at the upper part of

rectum. It gives off the left colic artery and three to four sigmoidal arteries, which

supply the intraperitoneal sigmoid colon. The arterial arcade between the ileocecum

and the trial of sigmoid flexure, which belongs to the part of vessels anastomose

between mesenteric artery and its adjacent branches is known as the marginal artery

of Drummond. The arcade, which effectively connects the left and right circulations,

is defined as the arc of Riolan.

Venous and lymphatic drainage of the colon

The venous and lymphatic drainage of the colon parallels the arterial supply, and all

three vessels (superior mesenteric vein, inferior mesenteric vein and splenic vein)

course and divide within the colonic mesocolon, which belong to the hepatic portal

system. The mesocolon therefore contains the regional lymph nodes for the segment

of colon it supplies and drains. The different lymphatic channels pass from the

submucosa to the intramuscular and subserosal plexus of the bowel to the first tier of

lymph nodes lying adjacent to the large intestine and known as epicolic nodes

[15].Paracolic nodes lie on the marginal vessels along the mesenteric side of the colon

and are frequently involved in metastases. Intermediate nodes are found along the

major arterial branches of the SMA and IMA in the mesocolon. The principal nodes

are found around the origin of these vessels from the aorta, and they drain into

retroperitoneal nodes. The drainage of the superior and inferior mesenteric veins,

which drain the ascending, transverse, descending, and sigmoid colon, is via the

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The extent of resection of the colon is defined by the vascular supply and by the

lymph nodes belonging to the mesentery [16-17]. Therefore, the whole bowel

supplied by related vascular should be resected thereby ensuring that all related lymph

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Figure 1.3.The anatomy of the colon with the vascular supply

Source: DeVita VT, Lawrence TS, Rosenberg SA: DeVita, Hellman, and Rosenberg’s

Cancer: Principles & Practice of Oncology, 9th Edition, 2011. By Lippincott

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Anatomy of the rectum

In the upper portion of the rectum there are changes both in the musculature of the

large bowel and in the relationship to the peritoneal covering. In the lower portion of

the rectum, the mucosal changes occur at roughly the same location as the anal

sphincter. The lower rectum is the area approximately from 3 to 6 cm from the anal

verge. The midrectum goes from 5 to 6, to 8 to 10 cm, and the upper rectum extends

approximately from 8 to 10, to 12 to 15 cm from the anal verge, although the

retroperitoneal portion of the large bowel often reaches its upper limit approximately

12 cm from the anal verge. In some patients, especially elderly women, the

peritonealised portion of the large bowel can be located much lower than these

definitions. The determination of the location of the boundary between rectum and

sigmoid colon is important in defining adjuvant therapy, with the rectum usually

being operationally defined as that area of the large bowel that is at least partially

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Figure 1.4.Rectal anatomy.Source: Shrieve DC, Loeffler JS: Human Radiation Injury.

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1.1.5 Staging and Prognosis of Colorectal cancer