The Digestive System: Structure, Function, and Clinical Correlates

Chapter 5 – The Digestive System

Introduction to Digestion

The digestive system is responsible for breaking down ingested food into simpler products suitable for absorption and metabolism by the body. This process involves both mechanical and chemical actions, as well as the coordinated movement of food through the gastrointestinal tract.

• Digestion: The breakdown of food into absorbable units.

• Physical Digestion: Chewing, swallowing, and peristaltic movement to mix and propel food.

• Chemical Digestion: Secretion of digestive enzymes, breakdown of large molecules into smaller molecules easily absorbed through digestive tract and blood and lymph vessels.

Main Divisions of the Digestive System

Alimentary Canal (Gastrointestinal Tract)

The alimentary canal is a muscular tube approximately 20 feet long, extending from the mouth to the anus. It is the main passageway for food.

Accessory Organs

• Teeth

• Tongue

• Salivary Glands

• Liver

• Gallbladder

• Pancreas

Histology: Four Layers of the Digestive Tract

• Mucosa: Inner lining; secretes mucus and digestive enzymes.

• Submucosa: Connective tissue with blood vessels, lymphatics, and nerves.

• Muscularis: Two layers of muscle (circular and longitudinal) for peristalsis.

• Serosa: Outermost membrane; reduces friction between digestive organs and surrounding structures.

The Gastrointestinal Tract: Structure and Function

Mouth

• Vestibule: Area between the teeth and the lips/cheeks.

• Oral Cavity Proper: Space internal to the teeth.

• Palate: Roof of the mouth, divided into hard and soft palate.

Tongue

• Involved in digestion, vocalization, and taste.

• Parts: root, body, apex.

• Lingual frenulum: attaches tongue to floor of mouth.

• Taste buds: located on papillae of the tongue.

Salivary Glands

• Parotid: Largest, near the ear.

• Submandibular: Below the jaw, anterior to the ear.

• Sublingual: Anterior to the submandibular, under the tongue.

Teeth

• Specialized for chewing (mastication).

• Three parts: crown, neck, root.

Pharynx and Esophagus

• Pharynx: Funnel-shaped passageway divided into nasopharynx, oropharynx, and laryngopharynx.

• Esophagus: Muscular tube that delivers food from oral cavity to the stomach via peristalsis.

Stomach

• Acts as a storage and mixing organ for food with digestive juices before entering the small intestine.

• Regions: cardia, fundus, body, pylorus.

• Two valves: cardiac sphincter (entry), pyloric sphincter (exit).

Small Intestine

• Main site of digestion and absorption.

• Three sections: duodenum, jejunum, ileum.

• Receives secretions from the pancreas and liver.

• Contains villi and microvilli to increase surface area for absorption.

• Peyer’s patches: lymphoid tissue in the ileum that protects against bacteria.

Large Intestine

• Four sections: cecum (with appendix), colon (ascending, transverse, descending, sigmoid), rectum, anal canal.

• Absorbs water and electrolytes; forms and stores feces.

• Some vitamins are produced by bacteria in the colon.

Accessory Organs

Liver

• Largest gland in the body; produces bile for fat digestion.

• Other functions: detoxification, storage of glycogen, synthesis of blood proteins, metabolism of nutrients.

Gallbladder

• Stores and concentrates bile; releases it into the duodenum via the common bile duct.

Pancreas

• Produces digestive enzymes and bicarbonate (exocrine function).

• Produces insulin and glucagon (endocrine function).

Peritoneum

• Serous membrane lining the abdominal cavity.

• Greater omentum: large fold of peritoneum that covers the intestines.

Digestive System Diseases

• Peptic Ulcers: Erosions in the stomach or duodenal lining.

• Intestinal Blockage: Mechanical (e.g., tumors) or non-mechanical (e.g., paralysis of muscles).

• Inflammatory Bowel Disease: Includes Crohn’s disease and ulcerative colitis.

• Viral Hepatitis: Inflammation of the liver caused by viruses.

• Cystic Fibrosis: Pancreatic ducts blocked by thick mucus, impairing digestion.

Summary Table: Main Segments and Functions of the Digestive Tract

Segment	Main Function	Associated Structures
Mouth	Ingestion, mechanical digestion, chemical digestion (saliva)	Teeth, tongue, salivary glands
Pharynx & Esophagus	Transport of food to stomach	Pharyngeal muscles, esophageal sphincters
Stomach	Mixing, storage, protein digestion	Gastric glands, sphincters
Small Intestine	Digestion, absorption	Villi, microvilli, Peyer’s patches
Large Intestine	Water absorption, feces formation	Colon, rectum, anal canal
Liver	Bile production, metabolism	Hepatocytes, bile ducts
Gallbladder	Bile storage and concentration	Common bile duct
Pancreas	Digestive enzyme and hormone production	Pancreatic duct, islets of Langerhans

Key Terms and Concepts

• Peristalsis: Rhythmic contractions that propel food through the digestive tract.

• Sphincter: Circular muscle that regulates passage of material (e.g., cardiac and pyloric sphincters).

• Villi and Microvilli: Increase surface area for absorption in the small intestine.

• Bile: Emulsifies fats, produced by the liver, stored in the gallbladder.

• Enzymes: Biological catalysts that speed up chemical digestion (e.g., amylase, lipase, protease).

Sample Equation

General equation for carbohydrate digestion:

$\text{Polysaccharide} + \text{Water} \xrightarrow{\text{Amylase}} \text{Disaccharides} \xrightarrow{\text{Disaccharidase}} \text{Monosaccharides}$

Clinical Application Example

• Gallstones: Solid deposits in the gallbladder that can block bile flow and cause pain or jaundice.

• Appendicitis: Inflammation of the appendix, often requiring surgical removal.

Additional info: The notes have been expanded with academic context, definitions, and a summary table for clarity and completeness.