Lecture Notes: The Large Intestine

Introduction

• The large intestine is part of the digestive tract.
• Specializes in absorbing water from residual digested food from the small intestine.
• Forms and stores feces until defecation.

Anatomy of the Large Intestine

• Begins at the ileocolic junction (connected to the ileum, the last part of the small intestine).
• Main Features Distinguishing Large Intestine from Small Intestine:
  1. Omental Appendices: Fatty outgrowths covered by visceral peritoneum.
  2. Tena Coli: Three strips of smooth muscle running lengthwise.
  3. Haustra: Pouch-like bulges of the intestinal wall formed between the tenia when they contract.

Major Components of the Large Intestine

• Cecum:
  • First part of the large intestine.
  • Receives terminal ileum, located in the right iliac fossa.
  • Appendix is a blind-ended organ attached to cecum.
• Colon:
  • Ascending Colon: Extends from cecum upwards towards the liver (right colic flexure).
  • Transverse Colon: Runs horizontally from the right to left colic flexure.
  • Descending Colon: Extends downwards to the left iliac fossa, connects to sigmoid colon.
  • Sigmoid Colon: S-shaped, continues to rectum.
• Rectum: Terminal part of the large intestine, continues as anal canal.

Arterial Supply

• Cecum: Supplied by the Superior Mesenteric Artery (SMA) via the ileocolic artery.
• Appendix: Supplied by Appendicular Artery (branch of ileocolic artery).
• Ascending Colon: Receives blood from right colic artery (branch of SMA).
• Transverse Colon: Supplied by Middle Colic Artery (branch of SMA).
• Descending & Sigmoid Colon: Supplied by Inferior Mesenteric Artery (IMA) through left colic and sigmoid arteries.
• Marginal Artery: Formed by anastomoses of the major arteries supplying the colon.

Venous Drainage

• Cecum and appendix drain into ileocolic vein.
• Ascending colon drains into right colic vein.
• Transverse colon drains into middle colic vein.
• Descending and sigmoid colons drain into left colic and sigmoid veins, respectively, which flow into the inferior mesenteric vein.

Lymphatic Drainage

• Lymphatics drain into local lymph nodes around intestines, then to superior and inferior mesenteric lymph nodes.

Innervation of the Large Intestine

• Sympathetic fibers: Derived from abdominopelvic splanchnic nerves.
  • Cecum, appendix, ascending, and transverse colon: Lesser thoracic splanchnic nerves.
  • Descending and sigmoid colon: Least thoracic and lumbar splanchnic nerves.
• Parasympathetic fibers:
  • Vagus nerve for cecum, appendix, ascending, and transverse colon.
  • Pelvic splanchnic nerves for descending and sigmoid colon.
• Visceral Afferent Fibers: Pain sensation follows sympathetic fibers above midpoint of sigmoid colon and parasympathetic fibers below.

Recap

• Large intestine includes: cecum, appendix, ascending, transverse, descending, sigmoid colon, rectum, and anal canal.
• Various structures have specific arterial supplies and innervations, with distinct features that differentiate it from the small intestine.

---

End of Notes