Large Intestine

The small intestine continues on to become the large intestine on the lower right side of the abdominal cavity. The large intestine is approximately 8 cm in diameter and is where water resorption occurs; approximately 70% of the material entering the colon is resorbed. Undigested residue that cannot be resorbed is consolidated into the feces for excretion.

The parts of the large intestine can be categorized as:

The following are some features of the ascending, transverse and descending colons:

1. Absence of villi

2. Absence of plicae circularis, therefore its inner luminal surface is smooth

3. Site of water and electrolyte absorption

4. Presence of intestinal glands (crypts)

5. Presence of teniae coli (three longitudinal bands of smooth muscle)

6. Presence of appendices epiploicae (fatty tags)

7. Presence of haustra (sacculations of the wall)

Sigmoid Colon

The sigmoid colon lies between the descending colon and rectum and serves as a temporary storage point for feces. It begins at the S1 vertebral level and ends at S3. The sigmoid colon is suspended by the sigmoid mesocolon (intraperitoneal).

The teniae coli from the more proximal portions of the colon are replaced by a complete circular layer of smooth muscle of the rectum.

Blood Supply:

1. The ileocolic artery and right colic artery supply the cecum and the ascending colon.

2. The middle colic artery supplies the proximal two thirds of the transverse colon.

3. The left colic artery supplies the distal third of the transverse colon and the descending colon.

4. The sigmoid artery supplies the sigmoid colon.

Venous Drainage:

1. The ileocolic vein and right colic veins drain the cecum and the ascending colon.

2. The middle colic vein drains the proximal two thirds of the transverse colon.

3. The left colic vein drains the distal third of the transverse colon and the upper portion of the descending colon.

4. The sigmoid veins drain the distal portion of the descending and sigmoid colon.

Rectum

The rectum is a segment of the large intestine between the sigmoid colon and the anal canal. It beings at vertebral level S3 and ends at the tip of the coccyx (anorectal junction), where the puborectalis muscle forms a U-shaped sling causing a 90-degree perineal flexure.

The ampulla of the rectum lies just above the pelvic diaphragm and generates the urge to defecate when feces moves into it. The rectum contains three transverse rectal folds (Houston valves) formed by the mucosa, submucosa, and inner circular layer of smooth muscle that permanently extend into the lumen of the rectum to support the fecal mass.

Blood Supply

1. The superior rectal artery, middle rectal artery and inferior rectal arteries supply the rectum.

Venous drainage:

1. The superior rectal vein, middle rectal vein and inferior rectal veins draing the rectum.

Anal Canal

The anal canal extends from the rectum at the anorectal junction to the surface of the body at the anus.

The parts of the anal canal can be divided by the pectinate line into:

Blood Supply:

1. The superior rectal artery supplies the upper anal canal.

2. The inferior rectal artery supplies the lower anal canal.

3. The middle rectal artery forms an anastomosis with the superior and inferior rectal arteries

Venous drainage:

1. The superior rectal vein drains the upper anal canal.

2. The inferior rectal vein drains the lower anal canal.

Innervation:

The innervation of the large intestine is by the enteric nervous system, which in the large intestine consists of the submucosal plexus of Meissner and the myenteric plexus of Auerbach.

The motor component of the submucosal plexus controls primarily mucosal and submucosal gland secretion and blood flow, whereas the sensory component consists of mucosal mechanosensitive neurons. The motor component of the myenteric plexus controls primarily GI motilitiy (contraction/relaxation of GI smooth muscle), whereas the sensory component consists of tension-sensitive neurons and chemosensitive neurons. The enteric nervous system is modulated by the parasympathetic and sympathetic nervous systems.

Parasympathetic

1. Proximal to the splenic flexure the innervation is provided by the superior mesenteric nerve plexus.

2. Distal to splenic flexure, rectum and upper anal canal the innervation is provided by the inferior hypogastric plexus.

Sympathetic

1. Proximal to splenic flexure the innervation is provided by the superior mesenteric ganglion.

2. Distal to splenic flexure the innervation is provided by the lumbar (L1-2) splanchnic nerves.

Anal canal:

• The upper anal canal is innervated by the autonomic nervous system (parasympathetic and sympathetic nervous systems) such that the internal anal sphincter is under autonomic, nonvoluntary control and sensation is limited to stretch sensation.

  The lower anal canal is innervated by the somatic nervous system by the pudendal nerve such that the external anal sphincter is under voluntary control and sensation is expanded to pain, temperature, and touch.

Clinical Notes:

1. Diverticulosis occurs when there are outpouchings of the mucosa of the colon that may occur along weak points of muscles fibres. These outpouchings can get infected and rupture causing diverticulitis or they may erode blood vessels leading to hemorrhage.

2. Volvulus or twisting of the colon occurs at the movable region of the sigmoid colon and could result in obstruction, constipation, and ischemia.