L5 small and large intestine

Introduction

  • Course Code: NU2057, NU2097, NU2078, NU2095

  • Instructors: Dr. Kathy Quane, Prof. Aonghus Lavelle

Overview of the Digestive System

  • Main Components:

    • Luminal components: Duodenum, Jejunum, Ileum, Colon, Rectum, Anus

    • Accessory organs: Tongue, Salivary glands (Parotid, Sublingual, Submandibular), Esophagus, Stomach, Pancreas, Liver, Gallbladder

Small Intestine

  • Major Function: Digestion and absorption

  • Length: 5-7 meters; extends from pyloric sphincter to ileocecal valve

Subdivisions of the Small Intestine

  1. Duodenum

    • Retroperitoneal

    • Length: ~2.5 m

    • Main site for initial digestion and enzyme action (bile from liver, pancreatic enzymes)

  2. Jejunum

    • Attached to posterior abdominal wall (PAW) by mesentery

    • Length: ~3.5 m

    • Responsible for nutrient absorption

  3. Ileum

    • Attached to PAW by mesentery

    • Absorbs Vitamin B12 in its terminal portion

Functions of the Small Intestine

  • Digestion:

    • Mechanical (chewing) and Chemical (enzymes from saliva, stomach, pancreas, bile)

  • Absorption:

    • Primarily occurs within the small intestine (nutrients, water, electrolytes)

Intestinal Modifications for Absorption

  • Surface Area Increases:

    • Circular Folds (Plicae Circulares): ~1 cm deep folds in mucosa and submucosa

    • Villi: Finger-like projections of mucosa

    • Microvilli: Projections of absorptive cells, containing enzymes (brush border enzymes)

Components of the Small Intestinal Wall

  • Layers:

    • (i) Mucosa: Epithelium, Lamina Propria, Muscularis Mucosae

    • (ii) Submucosa: Connective tissue (CT)

    • (iii) Muscle Layer: Inner circular and outer longitudinal muscle

    • (iv) Serosa: Visceral peritoneum

Mesentery

  • Description: Double layer of peritoneum connecting organ to body wall.

  • Functions: Routes for vessels/nerves, fat storage

Large Intestine

  • Components:

    • Caecum, Ascending colon, Transverse colon, Descending colon, Sigmoid colon, Rectum

  • Length: ~1.5 m

  • Main Functions: Absorption of fluids and electrolytes, Temporary storage of feces

Subdivisions of the Large Intestine

  1. Caecum:

    • A dilated pouch where the appendix attaches

  2. Colon:

    • Ascending -> Hepatic Flexure -> Transverse -> Splenic Flexure -> Descending -> Sigmoid

    • Final segment enters the pelvis as the rectum

  3. Rectum:

    • Ends in the anal canal

Rectum Structure and Function

  • Continuous: From sigmoid colon to anal canal

  • Features:

    • Curved structure with transverse folds (rectal valves) stops gas and feces passing simultaneously

    • Ampulla: distensible lower part

Anal Canal

  • Sphincters for Continence:

    • Internal anal sphincter: circular smooth muscle (involuntary)

    • External anal sphincter: skeletal muscle (voluntary)

Innervation and Blood Supply

  • Superior Mesenteric Artery: Supplies midgut (jejunum, ileum, caecum, part of colon)

  • Inferior Mesenteric Artery: Supplies hindgut (transverse colon, sigmoid colon, rectum)

  • Blood Drainage: Superior and inferior mesenteric veins drain into portal system for liver filtration

Summary

  • The gastrointestinal tract includes multiple organs working in concert to perform essential functions of digestion, absorption, and waste elimination, with complex structural adaptations for maximum efficacy.