The Digestive System

Overview

The digestive system is responsible for the breakdown of food, absorption of nutrients, and elimination of waste. It consists of the alimentary canal (gastrointestinal tract) and accessory organs.

• Alimentary canal: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

• Accessory organs: teeth, tongue, salivary glands, pancreas, liver, gallbladder

Functions of the Digestive System

• Ingestion: Moving food into the oral cavity

• Propulsion (motility): Swallowing, peristalsis, segmentation

• Digestion: Conversion of large molecules into smaller, absorbable subunits

  • Mechanical – chewing, churning

  • Chemical – enzymes, acid, bile salts

• Absorption: Passage of digestion products from the gut lumen into the blood and lymph

• Defecation: Elimination of undigested material and wastes

Anatomy of the Oral Cavity

Structure and Features

The oral cavity is lined by mucous membrane (stratified squamous epithelium and lamina propria) and contains underlying skeletal muscle.

• Lips and cheeks

• Palate:

  • Hard palate (anterior): formed by maxillae and palatine bones

  • Soft palate (posterior): arch of skeletal muscle, rises to close nasopharynx during swallowing

  • Uvula: hanging posterior projection

• Tongue:

  • Skeletal muscle, attached to hyoid bone

  • Dorsal surface covered with:

    • Nodular lingual tonsil (posterior)

    • Papillae: projections of mucosa containing taste buds (nervous tissue)

Teeth

Teeth are located in the maxilla and mandible. Children have primary (deciduous) teeth, while adults have secondary (permanent) teeth.

Structure of a Tooth

• Crown: Above gum

• Dentin (internal): Calcified connective tissue

• Enamel (cover): Acellular Ca2+ phosphate salts

• Neck: Boundary of crown/root at gum

• Root: Dentin with overlay of cementum (calcified CT)

• Periodontal ligament: Attaches the root to jaw bones

• Pulp cavity: Contains CT, blood and lymph vessels, nerves

• Root canal: Internal extension of the pulp cavity

Paired Salivary Glands

Types and Functions

• Secrete mucus-containing fluid (saliva: 99.5% water) with digestive enzymes and ions

• Parotid: Inferior and anterior to the ears

• Submandibular: Floor of mouth, below parotid glands

• Sublingual: Below the tongue

• Mumps: Inflammation of one or both parotid glands

Pharynx and Esophagus

Pharynx

• Includes oropharynx and laryngopharynx

• Histology similar to oral cavity: mucous membrane connected to skeletal muscle

Esophagus

• Posterior to larynx/trachea, passes through diaphragm

• Muscularis externa:

  • Upper 1/3: skeletal muscle

  • Middle 1/3: mixed

  • Lower 1/3: smooth muscle

• No serosa above diaphragm; fibrous areolar CT (adventitia)

Histology: Esophagus to Rectum

Four Tissue Layers

A. Mucosa

• Deep (innermost) layer

• Epithelium with goblet cells

  • Stratified squamous: esophagus, anal canal

  • Simple columnar: stomach, small intestine, large intestine, rectum

• Lamina propria (areolar connective tissue): houses blood and lymph vessels, nerve fibers, lymphoid tissue

• Muscularis mucosae: smooth muscle, produces folds and local movements

B. Submucosa

• Areolar CT layer with blood and lymphatic vessels, network of nerve cells

C. Muscularis externa

• Two layers of smooth muscle:

  • Inner circular layer: constricts lumen

  • Outer longitudinal layer: shortens gut length

• Contains two nerve cell networks (enteric nervous system):

  • Regulate gut secretions and motility (mixing and propulsion of chyme in the lumen)

D. Serosa (or adventitia)

• Superficial layer: simple squamous epithelium and areolar CT

Peritoneum

Structure and Function

• Largest serous membrane of the body

• Thin, continuous double-walled membrane lining the abdominopelvic cavity and most organs

• Visceral peritoneum: Lines external surface of digestive organs

• Parietal peritoneum: Lines abdominal cavity wall

• Forms serous fluid-containing peritoneal cavity that lubricates mobile organs

Mesentery and Omenta

• Mesentery: Double-layered sheet of parietal peritoneum holding small and large intestine to posterior abdominal wall

• Route for blood/lymph vessels and nerves to digestive organs

• Greater omentum: "Fatty apron" descending from stomach and draping over intestines

• Lesser omentum: Connects liver and stomach

Retroperitoneal Organs

• Organs posterior to the peritoneum (e.g., pancreas, duodenum)

• Anterior surface covered by parietal peritoneum; posterior surface by adventitia (fibrous CT)

Stomach

Structure and Function

• Stores, partially digests, and regulates emptying of chyme (food + gastric juices) into the small intestine

• Regions:

  • Cardial (cardiac): Site where esophagus enters the stomach

  • Fundus: Superior to esophageal entrance

  • Body: Middle portion

  • Pyloric (pylorus): Inferior, funnel-shaped; pyloric sphincter regulates release of stomach contents into small intestine

Key Terms and Concepts

• Chyme: Semi-fluid mass of partly digested food expelled by the stomach into the duodenum

• Pyloric sphincter: Muscular valve controlling passage of chyme from stomach to small intestine

Example: Peristalsis

Peristalsis is the coordinated, rhythmic contraction of smooth muscle that propels food through the digestive tract.

Relevant Equation

Rate of digestion and absorption can be modeled as:

where is a rate constant and is the concentration of digestible material.

Additional info: The enteric nervous system is a complex network regulating digestive tract motility and secretion, functioning independently but also interacting with the central nervous system.