P2.02 SG1 RIHAN ALI

Gastrointestinal System Study Guide

1. Anatomy of the Small Intestine

  • Structure

    • Approximately 6.5m long.

    • Extends from the pylorus of the stomach to the ileocaecal junction (where it meets the large intestine).

    • Functions:

      • Segmentation: mixes chyme with digestive juices for nutrient absorption.

      • Peristalsis: propels chyme through the small intestine.

      • Completes digestion of carbohydrates, proteins, lipids; begins and completes digestion of nucleic acids.

      • Absorbs about 90% of nutrients and water.

  • Divisions of the Small Intestine: duodenum, jejunum, ileum

    • Duodenum:

      • Runs from pylorus to duodenojejunal junction.

      • C-shaped, 25cm long, wraps around the pancreas.

      • Divided into 4 parts:

        1. D1 - Superior: from pyloric orifice to neck of gallbladder; contains duodenal cap; connected to liver via hepatoduodenal ligament.

        2. D2 - Descending: curves around the pancreas; contains major and minor duodenal papillae for bile and pancreatic duct entrances.

        3. D3 - Inferior: longest section, located inferior to pancreas, crossing over major blood vessels.

        4. D4 - Ascending: connects to jejunum at duodenojejunal flexure; suspensory muscle aids movement into jejunum.

    • Jejunum:

      • Distal part, attached to posterior abdominal wall by mesentery.

      • Represents proximal two-fifths, primarily in the left upper quadrant, with larger diameter and thicker walls.

      • Characterized by prominent folds (plicae circulares) and longer vasa recta.

    • Ileum:

      • Makes up distal three-fifths, primarily in right lower quadrant.

      • Thin walls, fewer plicae circulares, more fatty tissue, opens into the large intestine at ileocaecal junction.

      • Surrounded by ileocecal folds, regulating passage to the cecum.

  • Neurovascular Supply and Lymphatic Drainage:

    • Duodenum:

      • Arterial supply from gastroduodenal artery (foregut) and inferior pancreaticoduodenal artery (midgut).

      • Veins drain into the hepatic portal vein.

    • Jejunum and Ileum:

      • Supplied by superior mesenteric artery; venous drainage via superior mesenteric vein.

2. Anatomy of the Large Intestine

  • Structure:

    • Extends from cecum to anal canal, absorbs fluids and salts to form feces.

    • Comprises cecum, appendix, colon, rectum, and anal canal.

    • Functions:

      • Haustral churning, peristalsis for content movement.

      • Bacteria perform protein conversion, vitamin production.

      • Absorption of water and ions, feces formation, and defecation.

  • Divisions:

    • Ascending Colon: starts in right groin; ascends through the right flank and turns at the hepatic flexure.

    • Transverse Colon: stretches from right to left colic flexure; least fixed and may dip into pelvis.

    • Descending Colon: moves inferiorly past left kidney, leading to sigmoid colon.

    • Sigmoid Colon: enters pelvic cavity before leading into rectum, has mobility due to sigmoid mesocolon.

  • Neurovascular Supply and Lymphatic Drainage:

    • Arterial supply divided by embryological origin:

      • Midgut structures (ascending and proximal transverse colon): supplied by superior mesenteric artery.

      • Hindgut structures (distal transverse, descending, sigmoid): supplied by inferior mesenteric artery.

    • Venous Drainage: similar to arterial supply; drains into hepatic portal vein.

3. Histological Features of the Small and Large Intestine

  • Small Intestine Mucosa:

    • Contains circular folds, villi, and microvilli to increase absorptive capacity and surface area for digestion.

    • Villi: finger-like projections that enhance absorption.

    • Microvilli: projections of the absorptive cells, form a brush border, containing enzymes for digestion.

  • Large Intestine Mucosa:

    • Lacks circular folds and villi, has absorptive cells and goblet cells for mucus secretion; less surface area overall than the small intestine.

4. Vermiform Appendix

  • Attached to cecum, contains lymphoid tissue, variable positioning within the abdomen.

5. Rectum and Anal Canal

  • Rectum:

    • Most distal segment, serves as a temporary feces storage, begins at S3, has no teniae coli or haustra.

    • Characterized by sacral and anorectal flexures, reserves and prepares for defecation.

  • Anal Canal:

    • Final segment of GI tract, surrounded by sphincters to maintain continence, lined by columnar epithelium and anal valves.

6. Peritoneal Coverings

  • Superior Third: covered by peritoneum.

  • Middle Third: has anterior peritoneal covering.

  • Lower Third: no peritoneum; reflections differ in males and females.