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