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Digestive system

Learning Objectives

  • Understand the component parts of the GI tract.

  • Anatomy of the stomach, small and large intestine.

  • Understand types of digestive system mobility.

  • Roles and functions of GI tract organs.

  • Digestion and absorption of different macronutrients.

  • Examples of GI tract diseases and their causes.

The Gastrointestinal (GI) Tract

  • A long tube (approximately 4.5 m) including the oesophagus, stomach, small intestine, large intestine, rectum, and anus.

  • Surface area is about 300 m², equivalent to the size of a tennis court, due to complex structures and associated digestive organs (e.g., pancreas).

  • Contains extensive immune cell areas and a diverse microbiome that contributes to controlling infections and immune responses.

The Digestive Processes

The digestive system engages in four primary processes:

  1. Motility: Movement of food through the GI tract due to muscle contractions.

  2. Secretion: Transfer of substances from cells into the lumen or extracellular fluid (ECF).

  3. Digestion: Breakdown of food into absorbable units via chemical and mechanical processes.

  4. Absorption: Moving material from the GI lumen into the ECF.

The Mouth and Oesophagus

  • Digestion begins in the mouth through mechanical breakdown (chewing) and enzymatic digestion (e.g., Amylase for carbohydrates).

  • The oesophagus has stratified squamous epithelium for protection against injury.

Stomach Functions

  1. Storage: Upper stomach region.

  2. Digestion: Lower stomach (lipids and proteins).

  3. Defense: Protects against swallowed pathogens by mixing food with enzymes and acid.

  • The stomach regulates the release of contents into the small intestine.

Hormonal Regulation in the Stomach

  • Gastrin: Triggers gut mobility, enzyme (pepsinogen) release, and HCl production, creating an acidic environment that kills bacteria.

  • Pepsin: Activated from pepsinogen by HCl to assist in protein digestion.

Small Intestine Anatomy and Function

  • The small intestine is the main site for digestion and absorption.

  • Villi and microvilli: Increase surface area for absorption.

  • Peyer's patches: Associated lymphoid tissues sampling antigens and contributing to immune response.

Digestion in the Small Intestine

  • Proteins: Further digested by pancreatic enzymes (e.g., trypsin, chymotrypsin).

  • Carbohydrates: Broken down by amylase into monosaccharides for absorption.

  • Fats: Chylomicrons transport dietary lipids into the bloodstream.

Motility in the GI Tract

  • Peristalsis: Moves the bolus along the GI tract.

  • Segmentation: Mixes food within the small intestine for effective digestion.

The Large Intestine Functions

  • Responsible for absorbing water, electrolytes, and vitamins from chyme.

  • Lacks villi, focusing on absorption rather than digestion.

  • Appendix may play a role in housing microbiota.

Diseases of the GI Tract

Peptic Ulcers

  • Caused by erosion of mucous membranes due to excess HCl, often associated with H. pylori infection.

Lactose Intolerance

  • Resulting from a decrease in lactase enzyme, leading to undigested lactose in colon, causing diarrhea and bloating.

Appendicitis

  • Inflammation of the appendix which can lead to peritonitis if ruptured.

Bacterial Infections

  • Various bacteria can cause severe diarrhea through toxin production (e.g., cholera toxin).

Summary of Anatomy and Function

Mouth and Oesophagus

  • M: Swallowing, chewing

  • S: Saliva, lipase

  • D: Minimal carbohydrate and fat

Stomach

  • M: Peristalsis

  • S: HCl, pepsinogen

  • D: Proteins, fats

Small Intestine

  • M: Segmentation

  • S: Enzymes, HCO3, bile

  • D: Carbohydrates, fats, polypeptides, nucleic acids

Large Intestine

  • M: Segmental mixing

  • S: Mucus

  • D: None (except by bacteria)

  • A: Water, ions, vitamins produced by bacteria