Chapter 25: The Digestive System – Structure and Function

Introduction to the Digestive System

Overview and Major Components

The digestive system is responsible for the breakdown, absorption, and elimination of food and nutrients. It consists of a series of organs that form the digestive tract, as well as accessory organs that aid in digestion.

• Primary Organs: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine

• Accessory Organs: Teeth, tongue, salivary glands, pancreas, liver, gallbladder

Functions of the Digestive System

• Ingestion: Intake of food and liquids into the mouth

• Mechanical Processing: Chewing and swallowing to physically break down food

• Digestion: Chemical breakdown of food into nutrient molecules

• Secretion: Release of digestive enzymes and other substances by the tract and accessory organs

• Absorption: Movement of nutrients from the digestive tract into the bloodstream

• Excretion: Removal of waste products from the digestive tract

• Compaction: Dehydration of indigestible materials and waste

• Defecation: Elimination of feces from the body

An Overview of the Digestive System

Histological Organization of the Digestive Tract

The wall of the digestive tract is organized into four major layers, each with distinct functions:

• Mucosa: Innermost lining; consists of mucous membrane and epithelium (stratified or simple)

• Submucosa: Areolar connective tissue containing blood vessels, lymphatics, and neural plexuses

• Muscular Layer: Smooth muscle fibers arranged in circular and longitudinal layers; responsible for peristalsis and segmentation; contains the myenteric neural plexus

• Serosa: Outermost layer; present only in organs within the peritoneal cavity

Example: The small intestine has prominent circular folds (plicae circulares) to increase surface area for absorption.

Movement of Digestive Materials

• Peristalsis: Coordinated contractions of circular and longitudinal muscle layers propel a bolus (small mass of food) through the tract in waves.

• Segmentation: Churning movements produced by the circular muscle layer; mixes and fragments digestive contents without directional movement.

Example: Peristalsis moves food from the esophagus to the stomach, while segmentation mixes chyme in the small intestine.

The Peritoneum and Mesenteries

• Peritoneum: A serous membrane with two layers:

  • Visceral peritoneum: Covers organs

  • Parietal peritoneum: Lines the abdominal wall

• Intra- vs. Retroperitoneal Organs:

  • Intraperitoneal: Stomach, liver, ileum (within peritoneum)

  • Retroperitoneal: Kidneys, ureters, abdominal aorta (behind peritoneum)

  • Secondary retroperitoneal: Pancreas, duodenum (initially within, then behind peritoneum)

• Mesenteries: Double sheets of peritoneal membrane that stabilize organ positions and provide routes for vessels and nerves.

  • Mesentery proper: Suspends most of the small intestine

  • Transverse mesocolon, sigmoid mesocolon: Suspend regions of the large intestine

  • Lesser omentum: Between stomach and liver

  • Greater omentum: Covers anterior abdominal organs; contains adipose tissue for energy and insulation

The Oral Cavity

Anatomy of the Oral Cavity

• Lined by oral mucosa, continuous with cheeks, lips, and gums

• Oral vestibule: Space between cheeks/lips and gums

• Roof: Hard palate (maxilla and palatine bone), soft palate (separates oral cavity from nasopharynx)

• Floor: Contains the tongue

• Palatine tonsils: Lymphatic tissue between palatoglossal and palatopharyngeal arches

• Fauces: Space between oral cavity and pharynx

The Tongue

• Functions: Mechanical digestion, manipulation of food, sensory analysis, secretion of enzymes (e.g., lingual lipase), movement for speech

• Regions: Body (anterior), root (posterior), dorsum (superior, contains lingual papillae and taste buds)

• Frenulum: Thin mucous membrane attaching tongue to mouth floor

• Muscles:

  • Intrinsic: Alter tongue shape

  • Extrinsic: Gross movements (hyoglossus, styloglossus, genioglossus, palatoglossus)

Salivary Glands

• Three pairs:

  • Parotid: Largest, lateral side of face near mandible

  • Sublingual: Floor of mouth, under mucous membrane

  • Submandibular: Floor of mouth, deep to mandible

• Saliva: Water, ions, buffers, salivary amylase (digests carbohydrates)

The Teeth

• Crown: Above gums, covered by enamel

• Neck: Boundary between root and crown

• Root: Center is the root canal

• Dentine: Mineralized matrix

• Periodontal ligament: Anchors tooth to alveolus

• Types of teeth:

  • Incisors: Clip and cut food

  • Canines (cuspids): Tear and slash food

  • Premolars (bicuspids): Crush, mash, and grind food

  • Molars: Crush and grind food

• Humans have 20 deciduous (baby) teeth and 32 permanent teeth

Type	Incisors	Canines	Premolars	Molars
Upper jaw	4	2	4	4-6
Lower jaw	4	2	4	4-6

Mastication (Chewing)

• Involves muscles of mastication, buccal, labial, and lingual muscles

• Food is moved between oral vestibule and occlusal surfaces of teeth

• Salivary secretions moisten and process food for swallowing

The Pharynx

Structure and Function

The pharynx serves as a common passageway for food, liquids, and air. It plays a key role in swallowing (deglutition).

• Pharyngeal constrictors: Push bolus toward esophagus

• Palatopharyngeus and stylopharyngeus: Elevate the larynx

• Palatal muscles: Raise the soft palate

The Esophagus

Structure and Function

• Hollow muscular tube, 25 cm long, 2 cm in diameter

• Located posterior to the trachea

• Enters peritoneal cavity via the esophageal hiatus of the diaphragm

• Contains upper and lower esophageal sphincters

The Stomach

Functions of the Stomach

• Temporary storage of ingested food

• Mechanical digestion via muscular contractions

• Chemical digestion via gastric secretions, producing chyme

Anatomy of the Stomach

• Cardia: Where esophagus enters

• Fundus: Superior to cardia

• Body: Main region

• Pyloric part: Funnel-shaped region leading to small intestine

• Lesser curvature: Superior curve

• Greater curvature: Inferior curve

• Omenta (greater and lesser): Mesenteries extending from curvatures

Internal Structure of the Stomach

• Gastric folds (rugae): Allow expansion of the stomach

• Three smooth muscle layers:

  • Circular muscle layer

  • Longitudinal muscle layer

  • Oblique muscle layer

Example: The stomach's muscular layers and rugae enable it to churn and mix food, facilitating both mechanical and chemical digestion.