Chapter 25: The Digestive System – Anatomy & Physiology Study Notes

Introduction to the Digestive System

Overview and Major Organs

The digestive system is responsible for the breakdown, absorption, and excretion of food and nutrients. It consists of primary and accessory organs, each with specialized functions.

• 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 food

• Digestion: Chemical breakdown of food into nutrient molecules

• Secretion: Release of digestive enzymes and other substances

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

• Excretion: Removal of waste products

• Compaction: Dehydration of organic wastes

• Defecation: Elimination of feces from the body

An Overview of the Digestive System

Histological Organization

The digestive tract is composed of four major layers, each with distinct structures and functions:

• Mucosa: Inner lining, includes mucous membrane and mucosal epithelium (stratified or simple)

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

• Muscular Layer: Smooth muscle fibers arranged in circular and longitudinal layers; responsible for movement and mixing of contents

• Serosa: Outermost layer, present only within the peritoneal cavity

Table: Layers of the Digestive Tract

Layer	Main Components	Function
Mucosa	Mucous membrane, epithelium	Protection, secretion, absorption
Submucosa	Connective tissue, vessels, nerves	Support, nutrient supply
Muscular Layer	Circular & longitudinal muscle	Motility (peristalsis, segmentation)
Serosa	Serous membrane	Protection, reduces friction

Movement of Digestive Materials

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

• Segmentation: Churning and mixing of contents by circular muscle, without directional movement

Example: Peristalsis Sequence

1. Contraction of circular muscles behind bolus

2. Contraction of longitudinal muscles ahead of bolus

3. Wave of contraction in circular muscle layer forces bolus forward

The Peritoneum and Mesenteries

Peritoneal Organization

• Peritoneum: Serous membrane with two layers: visceral (inner) and parietal (outer)

• Serous fluid lubricates surfaces

• Intraperitoneal organs: Stomach, liver, ileum

• Retroperitoneal organs: Kidneys, ureters, abdominal aorta

• Secondary retroperitoneal organs: Pancreas, duodenum

Mesenteries

• Mesenteries: Fused double sheets of peritoneal membrane that stabilize organ position and provide routes for vessels and nerves

• Mesentery proper: Suspends most of the small intestine

• Fusion fascia: Attaches ascending/descending colon and rectum to posterior wall

• Lesser omentum: Between stomach and liver

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

The Oral Cavity

Anatomy of the Oral Cavity

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

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

• Floor: Formed by 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

• Body: Anterior portion

• Root: Posterior portion

• Dorsum: Superior surface, contains lingual papillae (taste buds)

• Frenulum: Thin fold 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

  • Sublingual: Under mucous membrane of mouth floor

  • Submandibular: Floor of mouth, deep into 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 root to alveolus

• Types:

  • 20 deciduous (baby) teeth

  • 32 permanent teeth:

    • Incisors (clip/cut), canines (tear/slash), premolars (crush/mash/grind), molars (crush/grind)

Table: Types of Adult Teeth

Type	Function	Number per Jaw
Incisors	Clip and cut food	4
Canines (Cuspids)	Tear and slash food	2
Premolars (Bicuspids)	Crush, mash, grind	4
Molars	Crush and grind	4-6

Mastication (Chewing)

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

• Food moves between oral vestibule and occlusal surfaces

• Food is moistened and processed for swallowing

The Pharynx

Structure and Function

• Common passageway for food, liquid, and air

• Pharyngeal muscles involved in swallowing:

  • Pharyngeal constrictors: Push bolus toward esophagus

  • Palatopharyngeus & Stylopharyngeus: Elevate larynx

  • Palatal muscles: Raise soft palate

The Esophagus

Structure and Function

• Hollow muscular tube (25 cm long, 2 cm diameter)

• Located posterior to trachea

• Enters peritoneal cavity via esophageal hiatus of diaphragm

• Contains upper and lower esophageal sphincters

The Stomach

Functions

• Temporary storage of ingested food

• Mechanical digestion of food

• Chemical digestion (produces chyme)

Anatomy of the Stomach

• Cardia: Entry from esophagus

• Fundus: Superior portion

• Body: Main region

• Pyloric part: Funnel-shaped, leads to small intestine

• Lesser curvature: Superior curve

• Greater curvature: Inferior curve

• Omenta: Mesenteries (greater and lesser)

Stomach Wall Structure

• Gastric folds (rugae): Allow expansion

• Three smooth muscle layers:

  • Circular

  • Longitudinal

  • Oblique

Example: Expansion of the Stomach

Gastric rugae flatten as the stomach fills, permitting increased volume without increased pressure.

Additional info:

• Histological details, blood supply, and further accessory organ functions (liver, pancreas, gallbladder) are covered in subsequent sections of the chapter.