Lecture Notes on the Large Intestine

Overview of the Large Intestine

• Part of the digestive tract.
• Specializes in absorbing water and forming/storing feces.
• Begins at the ileocecal junction (where it connects to the ileum).
• Larger caliber than the small intestine.

Distinguishing Features

1. Omental Appendices: Fatty outgrowths covered by visceral peritoneum.
2. Tinea Coli: Three strips of smooth muscle running lengthwise, merging at the rectosigmoid junction.
3. Hostra: Pouch-like bulges of the intestinal wall formed between the tinea.

Major Components of the Large Intestine

• Cecum: First part; receives terminal ileum; intraperitoneal, mobile, no mesentery.

  • Lies in the right iliac fossa; looks like an intestinal pouch.
  • Contains the appendix on its posteromedial wall.

• Appendix: Blind-ended organ; varies in length (up to 10 cm); usually retrocecal.

  • Contains lymphoid tissue and attached to cecum by the mesoappendix.

• Ascending Colon: Extends from the cecum to the right colic flexure (hepatic flexure).

  • Retroperitoneal; positioned behind the peritoneum.

• Transverse Colon: Extends from right colic flexure to left colic flexure (splenic flexure).

  • Intraperitoneal; has transverse mesocolon.

• Descending Colon: From left colic flexure to the sigmoid colon; also retroperitoneal.

• Sigmoid Colon: Makes an S shape; connects to the rectum; intraperitoneal.

  • Has a mesentery called the sigmoid mesocolon.

• Rectum and Anal Canal: Terminal parts of the large intestine; rectum is fixed retroperitoneally.

Blood Supply

• Cecum: Supplied by superior mesenteric artery (SMA) via the ileocolic artery.
• Appendix: Supplied by the appendicular artery (branch of ileocolic artery).
• Ascending Colon: Supplied by right colic and ileocolic arteries (anastomose).
• Transverse Colon: Supplied by middle colic artery (anastomoses with right colic).
• Descending and Sigmoid Colon: Supplied by inferior mesenteric artery (IMA) via left colic and sigmoid arteries.
• Rectum: Supplied by superior rectal artery (terminal branch of IMA).
• Marginal Artery: Formed by anastomoses of SMA and IMA branches.

Venous Drainage

• Cecum and appendix drain into ileocolic vein; ascending colon drains into ileocolic and right colic veins; transverse colon drains into middle colic vein.
• Descending and sigmoid colon drain into left colic and sigmoid veins (to IMA).
• Ultimately drain into hepatic portal vein.

Lymphatic Drainage

• Lymphatic vessels drain into local lymph nodes and then into mesenteric lymph nodes.
• Cecum, appendix, and ascending/ transverse colon drain into superior mesenteric lymph nodes.
• Descending and sigmoid colon drain into inferior mesenteric lymph nodes.

Innervation

• Sympathetic Fibers: Come from abdominopelvic splanchnic nerves (lesser, least, lumbar).
• Parasympathetic Fibers: From vagus nerve (cecum to left colic flexure) and pelvic splanchnic nerves (below left colic flexure).
  • Reflex sensations follow parasympathetic innervation.

Pain Sensation in the Large Intestine

• Visceral afferent fibers conduct reflex and pain sensations.
• Pain fibers above middle of the sigmoid colon travel with sympathetic fibers; below this point, they travel with parasympathetic fibers.
• Exception: Pain from the large intestine follows a different rule based on the sigmoid colon.

Recap

• Components: Cecum, appendix, ascending, transverse, descending, sigmoid colon, rectum, anal canal.
• Retroperitoneal: Ascending, descending colon, rectum; Intraperitoneal: Cecum, transverse, sigmoid colon.
• Blood supply through SMA and IMA; venous drainage follows arteries to hepatic portal vein.
• Sympathetic and parasympathetic innervations differ above and below the middle of the sigmoid colon.