LE 3.04 SMALL AND LARGE INTESTINES .docx

GROSS ANATOMY OF SMALL AND LARGE INTESTINES

SMALL INTESTINE

Duodenum

  • Characteristics:

    • Shortest and widest part (~25 cm long, C-shaped).

    • Primarily retroperitoneal except for first 2.5 cm.

  • Parts of Duodenum:

    • Superior Part: Begins at pyloric sphincter.

    • Descending Part: Curves around pancreas. Common bile and pancreatic ducts enter here.

    • Horizontal Part: Runs horizontally at L3 vertebra.

    • Ascending Part: Meets jejunum at duodenojejunal flexure.

  • Ligament of Treitz: A muscular fibrous band that aids in surgical landmarks.

Functions of the Duodenum

  • Mixes Chyme: With pancreatic juice and bile for breakdown; neutralizes gastric acid via pancreatic/biliary secretions.

  • Absorption: Significant absorption of electrolytes and nutrients.

  • Hormonal Regulation: Releasing hormones (secretin, cholecystokinin) that stimulate liver/gallbladder bile release and pancreas enzyme secretion.

Internal Portion

  • Variable position but important for receiving bile and pancreatic duct openings.

  • Plicae Circulares: Circumferential folds to enhance absorption surface area.

Duodenum Structure

  • Anatomical Relations: Covered anteriorly by peritoneum, with specific artery and vein supply from pancreaticoduodenal arteries (branches of mesenteric arteries).

Jejunum and Ileum

  • General Characteristics:

    • Slung from the posterior abdominal wall by mesentery, highly mobile.

    • Located centrally in the abdomen.

  • Jejunum: 2.5 m long, starts at duodenojejunal flexure, located in left upper quadrant.

  • Ileum: 4 m long, ends at ileocecal junction, in right upper quadrant.

  • Differences Between Jejunum and Ileum:

    • Jejunum: Thicker walls, greater vascularity, deeper red color, tall plicae circulares.

    • Ileum: Thinner walls, less vascular, paler color, fewer and shorter plicae circulares, contains Peyer’s patches.

Function

  • Jejunum: Specialized for nutrient absorption (e.g., magnesium).

  • Ileum: Absorbs Vitamin B12 and bile salts; walls with villi and microvilli for increased absorption surface area.

Vascular Supply

  • Blood supply from Superior Mesenteric Artery; paired with corresponding venous drainage via superior mesenteric vein.

  • Lymphatic Drainage: Follows arteries; drains to mesenteric and celiac lymph nodes.

LARGE INTESTINE

  • Function: Absorbs water from indigestible residues, forming semi-solid stool (~1.5 m long).

  • Components: Cecum, appendix, ascending, transverse, descending and sigmoid colon, rectum, anal canal.

Distinct Features

  • Taeniae Coli: Longitudinal muscle bands; important in colonic structure.

  • Haustra: Pouches formed by tonic contraction, allowing for content storage.

  • Omental Appendices: Fatty projections along the colon.

Cecum

  • 6 cm long, blind pouch in right iliac fossa.

  • Appendix: Long variable tube with lymphoid tissue, average length 10 cm, with variable positioning.

  • Nerve and Blood Supply: Supplied by ileocolic artery and drained by the superior mesenteric vein.

Ascending Colon

  • 12 to 20 cm long, ascends from cecum to liver.

  • Surrounded by peritoneum, attached to posterior abdominal wall.

Transverse Colon

  • About 45 cm long, crosses abdomen freely.

Descending and Sigmoid Colon

  • Descending Colon: 22 to 30 cm, smaller diameter, often lacks mesentery.

  • Sigmoid Colon: S-shaped loop, around 40 cm; considerable mobility, involving various adjacent structures.

ARTERIAL SUPPLY TO THE COLON

  • Superior Mesenteric Artery: Supplies ileocolic, right colic, and middle colic arteries.

  • Inferior Mesenteric Artery: Supplies left colic and sigmoid branches.

VENOUS DRAINAGE OF THE COLON

  • Portal Venous System: Drains from lower esophagus to upper rectum; corresponding veins for arteries.

LYMPHATIC DRAINAGE

  • Follows arterial supply paths, draining to para-aortic nodes and ultimately the cisterna chyli.

INNERVATION OF THE COLON

  • Sympathetic and Parasympathetic Supplies: Vagus and pelvic splanchnic nerves, providing visceral sensation and peristalsis regulation.

RECTUM AND ANAL CANAL

  • Continuous with sigmoid colon at S3 vertebra; terminal section of the large intestine.