Colon, Rectum & Anus

Colon​​

The colon is approximately five feet (1.5 meters) in length, begins at the ileocecal valve, and ends at the rectosigmoid junction. Arterial blood supply to the colon from cecum to splenic flexure is through the superior mesenteric artery which gives rise to the ileocolic, right colic, and middle colic arteries. The left and sigmoid colon is supplied by the inferior mesenteric artery which gives rise to the left colic and sigmoidal arteries. There can be several anatomic variations in the colic arteries including absent middle colic artery, absent right colic artery, common trunk for right and ileocolic artery, and the presence of an Arc of Riolan between the middle and left colic artery. The colonic wall histologically from lumen outward consists of: (1) a simple columnar epithelium which forms crypts, (2) lamina propria, (3) muscularis mucosa, (4) submucosa, (5) muscularis propria formed by an inner circular and outer longitudinal layer of smooth muscle, and (6) serosa.



The typical colonic malignancy is an adenocarcinoma. Once the neoplastic epithelial cells penetrate the muscularis mucosa and into the submucosa, a malignant (the ability to metastasize) adenocarcinoma is formed. The mainstay for treatment is operative resection of the involved colonic segment along with the draining lymph nodes located in the mesentery. Neoplastic cells confined by the muscularis mucosa are termed carcinoma-in-situ or severe dysplasia and are not as yet malignant thereby typically eliminating the need for segmental colonic resection.



The outer longitudinal smooth muscle of the colon thickens in three locations called tenia coli. The rectosigmoid junction is the point at which the three tenia fan out and form a complete outer longitudinal layer. This anatomic point has clinical significance. Carcinomas proximal to this point are colonic; whereas distal tumors are rectal and as such may benefit from adjuvant radiation therapy. Likewise, operative resection for classic sigmoid diverticular disease should include the rectosigmoid junction with the anastomosis located at the upper rectum.



The function of the colon is (1) absorption of water and electrolytes, and (2) propulsion and storage of unabsorbed fecal waste for evacuation. Approximately one liter of fluid chyme enters the cecum each day with an average of only 100cc excreted in the feces. Parasympathetc innervation by preganglionic vagal fibers and pelvic fibers result in colonic motility. Sympathetic innervation by the superior mesenteric plexus, inferior mesenteric plexus, and the hypogastric plexus inhibits colonic motility. It appears that the major control of motility depends on the colonic wall intrinsic plexus (myenteric or Auerbach’s/submucous or Meissner’s). An absence of intrinsic plexuses occurs in Hirschsprung’s Disease resulting in tonic wall contraction and functional obstruction.



Disorders of colonic motility including irritable bowel syndrome, slow-transit constipation, colonic pseudo-obstruction, and post-operative ileus are poorly understood but may represent an imbalance in this autonomic imput to the smooth muscle wall of the colon. Normal colon transit arbitrarily results in one to three bowel movements per day to one bowel movement every 3 days. Colonic transit is measured by obtaining abdominal radiographs after ingesting radiopaque markers. Markedly constipated patients with slow-transit constipation (markers remaining disbursed throughout the colon after 5 days) but with normal defecation mechanics may benefit from abdominal colectomy.



Rectum​​

The rectum is the terminal portion of the large intestine beginning at the confluence of the three tenia coli of the sigmoid colon and ending at the anal canal. Generally the rectum is 15 cm in length, is intraperitoneal at its proximal and anterior end, and is extraperitoneal at its distal and posterior end. The epithelial lining or mucosa of the rectum is of a simple columnar mucous secreting variety. Therefore, the characteristic malignancy of the rectum is an adenocarcinoma.



Anal Canal​​​

The anal canal begins a few centimeters proximal to the classic and well visualized dentate line and it ends at the anal verge. The anal canal is about 5 cm in length. Histologically the proximal end of the anal canal is the point at which the columnar epithelium of the rectum becomes a transitional epithelium. This epithelium transitions to a stratified squamous variety at the dentate line. The distal most end of the anal canal is the anal verge which is the point where the stratified squamous epithelium becomes true skin marked by the presence of hair follicles and sweat glands. The anal verge is readily identified by noting the point at which hair shafts are seen. The anoderm is a term used to describe the zone between the dentate line and the anal verge. Perianal skin then describes the anatomic area beyond the anal verge. Malignancies of the perianal skin are typical skin cancers usually squamous cell carcinomas. Anal canal carcinomas are described as epidermoid carcinoma, squamous cell carcinoma, cloacogenic carcinoma, or baseloid carcinoma depending on their particular histologic features. The importance of locating and anatomically defining the particular malignancy of the anorectal region is in their treatment.



The dentate line is a clearly observed undulating line near the midpoint of the anal canal. It is at this location where the anal crypts are found. Anal glands secrete mucus that empty into the anal crypts by way of anal ducts. The pathologic significance of anal glands, ducts, and crypts is infection. Cryptoglandular infection can occur leading to anorectal abcess and its sequelae anorectal fistula.

Autonomic nerves supply the rectum and upper anal canal whereas somatic nerves supply the lower anal canal and perianal skin. Rectal polyps, tumors, and mucosa can be biopsied without anesthesia. Internal hemorrhoids located beneath the autonomically innervated upper anal canal classically present as painless bleeding and can be treated without anesthesia using simple fixation techniques. Conversely, lesions of the distal anal canal and perianal skin such as anal fissure and external hemorrhoids are painful.



The blood supply to the anorectal region is rich. The terminal branch of the inferior mesenteric artery is the superior hemmorrhoidal (rectal) artery. The superior hemorrhoidal artery branches into right and left branches; the right branch further divides into anterior and posterior branches. The classic hemorrhoidal plexes are then located at the left later, right anterolateral, and right posterolateral locations. The middle hemorrhoidal (rectal) arteries are direct branches from the internal iliac arteries. The inferior hemorrhoidal (rectal) arteries are branches off the pudendal arteries which also arise from the internal iliac arteries. The superior, middle, and inferior hemorrhoidal arteries then complete the rich arterial supply to the anorectal region.



The venous drainage of the anorectal region consists of superior hemorrhoidal veins draining into the portal venous system (by way of the inferior mesenteric vein) and the middle and inferior hemorrhoidal veins draining into the caval system (by way of the internal iliac veins). Thus the anorectal region can provide a means of portal decompression when portal hypertension exists.

The main pathologic significance of the anorectal vasculature is in hemorrhoidal disease. Two hemorrhoidal plexes are formed in each of the classic locations (left lateral, right anterolateral, and right posterolateral). The internal (superior) hemorrhoidal plexus is proximal to the dentate line and the external (inferior) hemorrhoidal plexus is located distal to the dentate line.

The musculature of the anorectal region forms the anal sphincter mechanism. The internal anal sphincter is smooth, involuntary muscle and is simply the terminal thickening of the inner visceral smooth muscle layer of the rectal wall. The role of the internal anal sphincter in fecal continence may be for flatus control. Division of the internal anal sphincter is the operative treatment for anal fissure and most often has no effect on fecal continence.



The external anal sphincter is skeletal muscle and thus under voluntary control. There is a distinct anatomic plane between the internal and external anal sphincter occupied by longitudinal connective tissue fibers continuous with the outer longitudinal muscle wall of the rectum. The external anal sphincter is arbitrarily separated into subcutaneous, superficial and deep components. The puborectalis muscle is felt to represent the deep component of the external anal sphincter and appears to be the most significant muscle for maintainting fecal continence. The puborectalis muscle originates and inserts on the pubis after encircling the rectum at the anorectal junction. When contracted the puborectalis muscle creates a 90 degree angle between the anal canal and the rectum. Puborectalis relaxation allows the anorectal angle to approach 180 degrees which in combination with relaxation of the other components of the external allows defecation.

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Lymphatic drainage of the rectum travels along the internal iliac vessels as well as the aorta. Lymphatic drainage of the anal canal can follow the internal iliac vessels but also may travel through channels in the inguinal region.