Digestive System: Structure, Function, and Associated Organs

The Digestive System

Overview

The digestive system is responsible for providing nutrients essential for growth and maintenance. It consists of the alimentary canal (from mouth to anus) and accessory organs that secrete substances aiding digestion.

• Alimentary Canal: Continuous tube from mouth to anus.

• Accessory Organs: Salivary glands, liver, gallbladder, pancreas.

Pathway of Food

Food follows a specific pathway through the digestive tract:

1. Mouth

2. Pharynx

3. Esophagus

4. Stomach

5. Small intestine

6. Large intestine

7. Rectum

8. Anus

Functions of the Digestive Tract

• Ingestion: Intake of food.

• Digestion: Mechanical and chemical breakdown of food.

• Secretion: Release of digestive enzymes and fluids.

• Absorption: Uptake of nutrients into the bloodstream.

• Compaction and Elimination: Formation and removal of feces.

Alimentary Canal Structure

Layers of the Alimentary Canal

The alimentary canal is a long, muscular tube with four distinct layers:

• Serosa: Outermost protective layer.

• Muscularis Externa: Responsible for movement; contains longitudinal and circular muscle.

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

• Mucosa: Innermost layer; involved in secretion and absorption.

Types of Movement

• Peristalsis: Coordinated contractions of longitudinal and circular muscles that propel food forward.

• Segmentation: Cyclic contractions that mix and separate the bolus of food, enhancing digestion and absorption.

Oral Cavity

Structure and Function

The mouth is a mucosa-lined cavity involved in ingestion and initial mechanical digestion.

• Lips and Cheeks: Keep food between teeth during chewing.

• Palate: Forms the roof of the mouth; includes the uvula which helps prevent choking.

• Tongue: Skeletal muscle that mixes food and saliva into a bolus.

• Teeth: Mechanically break down food.

Salivary Glands

Types and Functions

• Sublingual Glands

• Submandibular Glands

• Parotid Glands

Saliva functions:

• Flushes the mouth

• Buffers pH (protects enamel)

• Contains antibodies and lysozyme (antimicrobial)

• Contains amylase (digests carbohydrates)

Swallowing

Phases of Swallowing

• Buccal Phase: Voluntary movement of bolus from mouth to esophagus; involves epiglottis and uvula.

• Pharyngeal Phase: Involuntary; tactile receptors stimulate swallowing reflex via medulla oblongata.

• Esophageal Phase: Bolus moves through esophagus (top 1/3 skeletal muscle, bottom 1/3 smooth muscle).

Esophagus

Structure and Function

• Muscular tube (~10 inches long) for food passage.

• No digestion or absorption occurs here.

• Lower end contains the cardiac sphincter (LES) to prevent acid reflux.

Stomach

Structure and Function

• Expands via rugae to hold ~1.5 L of material.

• Food + stomach secretions = chyme.

• Extra oblique muscle layer for enhanced mechanical digestion.

• Mucosa: Simple columnar epithelium; contains gastric pits and glands.

Stomach Secretions

• ~1.5 L of gastric juice daily.

• Parietal cells: Secrete HCl and intrinsic factor (for vitamin B12 absorption).

• Chief cells: Secrete pepsinogen (protein digestion).

• Enteroendocrine cells: Secrete hormones regulating gastric activity.

Protection of the Stomach

• Stomach acid pH ≈ 3.

• Protection by alkaline mucosa, tight junctions, and rapid cell replacement (~3-4 days).

• Chemotherapy drugs can lead to ulcers.

Exit the Stomach

• Food mixed and churned with gastric secretions; proteins partially digested.

• Acidic chyme is released through the pyloric sphincter into the duodenum (~5cc/squirt, ~3X/min).

Small Intestine

Characteristics

• ~20 ft long, >1 inch diameter.

• Specialized for increased surface area: plicae circulares, villi, microvilli.

Regions of the Small Intestine

• Duodenum: Receives chyme, mixes with liver, gallbladder, and pancreas secretions; secretes mucus and hormones.

• Jejunum: Major site of chemical digestion and nutrient absorption.

• Ileum: Ileocecal valve controls movement to cecum.

Accessory Organs

Liver and Gallbladder

• Liver: Produces ~1L bile/day; bile salts emulsify lipids, bile pigments (bilirubin) excreted in feces.

• Gallbladder: Stores and concentrates bile; releases bile upon duodenal signal; cholesterol crystals may form gallstones.

5 "F" Risk Factors for Gallstones

• Female

• Fertile

• Forty

• Fair skin

• Fats (high-fat diet)

Pancreas

• Both endocrine and exocrine functions.

• Exocrine: ~1L pancreatic juice/day (water, ions, enzymes: amylase, lipase, nuclease, proteases; buffers to neutralize stomach acid).

Large Intestine

Structure and Function

• ~5 ft long, 3 inches diameter.

• Digestion and absorption mostly complete; remaining waste is watery/mucousy.

Functions

• Reabsorbs water.

• Compacts waste into feces.

• Intestinal flora produce vitamins B and K, which are absorbed.

• Stores feces until defecation.

Appendix

• Hangs off right side of colon (~9cm).

• Reservoir for gut bacteria.

• Appendicitis: Inflammation; untreated may rupture; treated by appendectomy.

Rectum and Defecation

• Internal anal sphincter: Involuntary control.

• External anal sphincter: Voluntary control.

• Rectal valves: Help separate feces from gas.

• Defecation reflex: Relaxation of internal sphincter, contraction of external sphincter.

Digestive System Disorders

• Heartburn: Stomach acid enters esophagus (acid reflux); chronic form is GERD.

• Hepatitis: Liver inflammation, usually viral.

• Cirrhosis: Fatty liver replaced by scar tissue; causes include alcohol, obesity, chronic hepatitis.

• IBS: Over-contraction of bowels; chronic diarrhea and pain.

Teeth

Types of Teeth

• Incisors: Cutting

• Canines: Tearing

• Premolars: Grinding

• Molars: Crushing

Parts of a Tooth

• Crown: Projects above gum line.

• Neck: Boundary between crown and root.

• Root: Below gum line; sits in alveolus (bony socket).

Components of a Tooth

• Dentin: Bulk of tooth; mineralized matrix, no cells.

• Pulp cavity: Interior chamber; contains nerves and blood vessels.

• Occlusal surface: Part of crown for crushing, slicing, chewing.

• Enamel: Covers crown dentin; hardest biological substance; composed of calcium phosphate; cannot be replaced once worn down.

• Gingival sulcus: Shallow groove at base of neck; blocks bacteria from deeper tissues.

• Cementum: Covers root dentin; less resistant to erosion than dentin.

• Periodontal ligament: Connects root dentin to alveolar bone; forms gomphosis joint.

• Root canal: Tunnel within root; passage for nerves and blood vessels; opening is the apical foramen.

Summary Table: Layers of the Alimentary Canal

Layer	Location	Main Function
Serosa	Outermost	Protection
Muscularis Externa	Second (outside-in)	Movement (peristalsis, segmentation)
Submucosa	Third	Support, blood supply, nerves
Mucosa	Innermost	Secretion, absorption

Summary Table: Types of Teeth

Type	Function
Incisor	Cutting
Canine	Tearing
Premolar	Grinding
Molar	Crushing

Key Formula

pH of stomach acid: $pH \approx 3$

Example: If the stomach lining is damaged and the mucosa cannot produce enough alkaline mucus, the acidic environment can lead to ulcer formation.

Additional info: The notes have been expanded with academic context, including definitions, examples, and tables for clarity and completeness.