Overview of the Large Intestine

• The large intestine extends from the ileocecal valve to the anus
• It measures 1.5 meters in length
• Encases the small intestine like a frame

Large Intestine Parts

• Cecum
• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon
• Rectum
• Anal canal

Cecum

• Located in the right iliac fossa on the iliac and Psoas major muscle
• Its length measures 6 cm and its width measures 8 cm
• Mobile part of the large intestine
• Covered by the peritoneum
• The vermiform appendix opens about 2 cm inferior to the ileocecal opening

Structure of the Large Intestine

• Serous layer has appendices epiploicae (peritoneum)
• The muscular layer includes longitudinal and circular fibers
• Free taenia
• Mesocolic taenia
• Omental taenia
• Taeniae coli are longitudinal fibers which thicken to form free taenia, mesocolic taenia, and omental taenia.
• Mucosa is pink and has solitary lymphatic follicles mostly in the cecum and appendix
• Knuckles of the large intestine are Haustra coli

The Colon

• Ascending Colon
• Transverse Colon
• Descending Colon
• Sigmoid Colon

Ascending Colon

• It has a length of 15 cm and ends at the right colic flexure
  • Anterior relations are the small intestine and greater omentum
  • Posterior relations are iliac fascia, quadratus lumborum muscle, and right kidney
• Right colic flexure's relations: right kidney and lobe of liver, and descending part of duodenum and gallbladder
  • Vessels include the Ileocolic artery and right colic artery, which are branches of the superior mesenteric artery
  • Lymph nodes are the superior mesenteric lymph nodes
  • Sympathetic nerve supply is from the celiac and superior mesenteric ganglia
  • Parasympathetic nerve supply is from the vagus nerve

Transverse Colon

• It goes from the right to left colic flexures
• Measures 50 cm and is a mobile, intraperitoneal part
  • Superior relations: liver, gallbladder, greater curvature of the stomach, and side
  • Posterior relations: descending part of the duodenum and head of the pancreas
  • The tail of the pancreas is near the inferior part of the spleen
  • The phrenicocolic ligament attaches the spleen
  • The right two-thirds are supplied by the right and middle colic arteries and veins from the superior mesenteric artery and vein
  • The left one-third is supplied by the left colic artery and vein (inferior mesenteric artery and vein)
  • Lymphatic drainage is to the superior mesenteric lymph nodes
  • The right two-thirds has sympathetic innervation from the celiac ganglion and superior mesenteric ganglion
  • Parasympathetic innervation is via the vagus nerve
  • The left one-third's sympathetic supply is from the sympathetic trunk
  • Parasympathetic innervation is via the vagus nerve
  • The superior hypogastric plexus and inferior mesenteric ganglion also contribute to some innervation in the area

Descending Colon

• It has a length of 25 cm and is covered by the peritoneum except for its posterior surface

Sigmoid Colon

• L = 40 cm
• Covered by peritoneum (sigmoid mesocolon)
• Location may vary
  • Vessels include the: left colic artery and sigmoid artery sourced from the inferior mesenteric artery
  • Includes left colic and sigmoid lymph nodes
  • Supply to the inferior mesenteric lymph nodes
    • Sympathetic nerve supply from the superior hypogastric plexus
    • Parasympathetic nerve supply from the pelvic splanchnic nerve (nn. Erigentes)

Rectum

• Begins at the level of the third vertebral line of the sacrum
• Measures 12 cm in length
• It does not contain haustra coli, epiploic appendices, or a mesentery
• There are two flexures: sacral flexure (anteroposterior curve) and perineal flexure (downward and backward)
• The anorectal junction is also known as the perineal flexure - it's located 2-3 cm in front of the tip of the coccyx
• Has lateral curvatures in the full state
• Has transverse or Houston's plicae which are constant
• The ampulla region is located just superior to the anal canal

Anal Canal

• Extends 3-4 cm
• Encircled by external and internal anal sphincter muscles
• Has internal surface with Anal Columns, which are longitudinal mucosal folds numbering 6-10
• Anal columns contain the superior rectal artery and vein
• Anal valves are inferior parts of the anal columns that form the anal valve
• Anal sinuses are pits of anal valves
• The anorectal line is the junction between the rectum and the anal canal, located at the superior border of the anal columns
  • The transitional zone is 15 mm in area inferior to the anal valve, and contains the inferior rectal venous plexus, identified as a blue line
  • The anal canal extends to the Anocutaneous line (Hilton line) and the skin area is below the Hilton line

Sphincter Muscles of the Anal Canal

• Internal anal sphincter consists of the muscular layer of the rectum and is involuntary
• External anal sphincter is voluntary with three parts: subcutaneous, superficial and deep

Muscles of Anal Canal

• Anorectal ring consists of the puborectalis muscle, deep part of the external sphincter ani muscle, and internal sphincter ani muscle

Arteries of the Large Intestine

• Supplies the cecum, appendix, ascending colon and transverse colon via the ileocolic, right colic, and middle colic arteries, all branches of the SUPERIOR MESENTERIC ARTERY
• Supplies the descending colon, sigmoid colon & upper rectum via the left colic, sigmoidal branches, and superior rectal artery, all branches of the INFERIOR MESENTERIC ARTERY
• Middle rectal artery which is a branch of the INTERNAL ILIAC ARTERY
• The inferior rectal artery branches off of the INTERNAL PUDENDAL ARTERY
• Veins share the same names as the arteries