The Digestive System: Structure, Function, and Disorders

The Digestive System

Overview of the Digestive System

The digestive system is responsible for the breakdown and absorption of nutrients necessary for cellular function. It consists of the gastrointestinal (GI) tract and accessory organs.

• Gastrointestinal tract (Alimentary canal): A muscular tube beginning at the mouth and ending at the anus, including the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory organs: Salivary glands, liver, gallbladder, and pancreas produce secretions that aid digestion.

• Integration: The digestive system works with other systems to supply nutrients to tissues not directly exposed to the external environment.

Histology of the GI Tract

All organs of the GI tract share a basic histological structure with four layers:

• Mucosa: Innermost layer with epithelial tissue, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle).

  • Epithelium:

    • Stratified squamous epithelium: Oral cavity, pharynx, upper esophagus, rectum.

    • Simple columnar epithelium: Lower esophagus, stomach, small and most of large intestine; contains goblet cells.

  • Lamina propria: Contains blood vessels, nerves, lymphatics, and mucous glands.

  • Muscularis mucosae: Two layers of smooth muscle (circular and longitudinal) create folds like rugae, plicae circularis, and villi.

• Submucosa: Dense irregular connective tissue with blood vessels, lymphatics, nerves, and exocrine glands.

• Muscularis externa: Two layers of smooth muscle (inner circular, outer longitudinal) for mechanical processing and propulsion.

• Serosa/Adventitia:

  • Serosa: Visceral peritoneum covering most abdominal organs.

  • Adventitia: Dense collagen sheath for oral cavity, pharynx, esophagus.

Peritoneum and Peritoneal Folds

• Greater omentum: Fat-laden apron over intestines.

• Falciform ligament: Anchors liver.

• Lesser omentum: Suspends stomach.

• Mesentery: Anchors small intestine.

• Mesocolon: Anchors large intestine.

Innervation and Blood Supply

• Enteric nervous system:

  • Myenteric plexus: Controls motility.

  • Submucosal plexus: Regulates secretions.

• Blood vessels:

  • Transport absorbed nutrients.

  • Supply organs with oxygen and nutrients.

• Lipids: Absorbed via lymphatic lacteals.

Digestive System Processes and Regulation

Digestive System Functions

• Ingestion: Entry of food and drink via the mouth.

• Mechanical processing: Chewing, mixing, churning, segmentation.

• Propulsion: Movement by swallowing and peristalsis.

• Chemical digestion: Enzymatic breakdown into absorbable molecules.

• Secretion: Release of water, enzymes, buffers, acids.

• Absorption: Uptake of nutrients, water, electrolytes into blood or lymph.

• Compaction: Dehydration and elimination of waste as feces.

Regulation of Digestive Processes

• Neural control: Mechanoreceptors, chemoreceptors, osmoreceptors regulate function.

• Hormonal control:

  • Gastrin: Stimulates gastric acid secretion.

  • Secretin: Stimulates pancreatic bicarbonate secretion.

  • Cholecystokinin (CCK): Stimulates pancreatic enzymes and bile release.

  • Gastric inhibitory peptide (GIP): Inhibits gastric secretion and motility.

The Mouth, Pharynx, and Esophagus (Upper GI Tract)

Mouth

• Epithelium: Non-keratinized stratified squamous.

• Borders:

  • Labia (lips): No sweat/sebaceous glands; attached by frenula.

  • Cheeks: Buccinator muscles.

  • Vestibule: Space between lips/cheeks and gums/teeth.

  • Hard palate: Bone; soft palate: Muscle.

  • Uvula: Prevents premature entry to oropharynx.

  • Palatine tonsils: Immunity.

  • Root/body of tongue: Lingual tonsils, circumvallate papillae.

Tongue

• Muscle: Skeletal muscle for manipulation and gustation.

• Lingual papillae:

  • Circumvallate: Posterior, V-shaped, ~100 taste buds each.

  • Fungiform: Anterior, mushroom-shaped, ~5 taste buds each.

  • Filiform: Hair-like, friction, no taste buds.

  • Foliate: Lateral, vertical folds, many taste buds.

• Taste buds: Sensory structures; decrease with age.

Salivary Glands

• Function: Moistens, lubricates, cleanses, dissolves food, initiates digestion.

• Types:

  • Parotid: Serous cells, amylase, lysozyme, IgA.

  • Sublingual: Mucous cells, mucin.

  • Submandibular: Mixed serous/mucous, amylase, mucin.

• Innervation: Parotid (CN VII), sublingual/submandibular (CN IX).

Teeth

• Function: Mastication increases surface area for digestion.

• Types:

  • Primary (deciduous): 20 teeth.

  • Secondary (permanent): 32 teeth; incisors, canines, premolars, molars.

• Anatomy:

  • Crown: Enamel, dentin, pulp cavity.

  • Neck: Boundary between crown/root.

  • Root: Cementum, periodontal ligament, root canal, apical foramen.

Pharynx

• Regions:

  • Nasopharynx: Air only, pseudostratified columnar epithelium.

  • Oropharynx: Food/air, stratified squamous epithelium.

  • Laryngopharynx: Food/air, stratified squamous epithelium.

Esophagus

• Structure: Hollow muscular tube; passes through esophageal hiatus.

• Muscle composition: Upper third: Skeletal; middle: Mixed; lower: Smooth.

• Epithelium: Upper: Stratified squamous; lower: Simple columnar.

• Glands: Mucus production for lubrication.

• Sphincters: Upper esophageal (prevents backflow); cardiac/lower esophageal (prevents reflux).

Deglutition (Swallowing)

• Phases:

  • Bucal: Voluntary.

  • Pharyngeal: Involuntary, autonomic control.

  • Esophageal: Involuntary.

The Stomach (Lower GI Tract)

Stomach Structure and Function

• Regions:

  • Cardiac: Entry from esophagus.

  • Fundus: Dome-shaped, superior.

  • Body: Largest, mixing bowl.

  • Pyloric: Sharp curve, ends at pyloric sphincter.

• Curvatures: Greater (lateral, omentum attached), lesser (medial, omentum attached).

• Rugae: Folds for expansion.

Microscopic Anatomy of the Stomach

• Mucosa: Simple columnar epithelium.

• Muscularis externa: Three layers (circular, longitudinal, oblique).

• Gastric pits: Lead to gastric glands.

• Gastric glands:

  • Chief cells: Pepsinogen (protein digestion), gastric lipase, rennin (infants).

  • Parietal cells: HCl (activates pepsinogen), intrinsic factor (B12 absorption).

  • G cells: Hormones (gastrin, somatostatin, ghrelin).

The Small and Large Intestines

Small Intestine

• Function: Major site for digestion and absorption.

• Motility: Peristalsis and segmentation.

• Regions:

  • Duodenum: Receives chyme, enzymes, bile; Brunner's glands.

  • Jejunum: Chemical digestion and absorption.

  • Ileum: Absorption, Peyer's patches, ileocecal valve.

• Structural adaptations:

  • Plicae circularis: Folds increase surface area.

  • Villi: Finger-like projections with blood and lymph capillaries.

  • Microvilli: Brush border for absorption.

• Mesenteries: Anchor intestines (mesentery proper).

Secretions and Enzymes

• Enteroendocrine cells:

  • Secretin: Stimulates bicarbonate and bile.

  • CCK: Stimulates enzyme-rich pancreatic juice and bile.

  • GIP: Slows gastric activity, stimulates insulin.

  • VIP: Stimulates intestinal glands, dilates capillaries, inhibits stomach acid.

• Brush border enzymes:

  • Aminopeptidase, dipeptidase, carboxypeptidase: Protein digestion.

  • Nucleosidases, phosphatases: Nucleic acid digestion.

  • Dextrinase, maltase, sucrase, lactase: Carbohydrate digestion.

Large Intestine

• Function: Reabsorbs water, absorbs vitamins, stores feces.

• Regions:

  • Cecum: Entry from ileum, compaction.

  • Colon: Ascending, transverse, descending, sigmoid.

  • Rectum: Storage, defecation control (internal/external sphincters).

  • Appendix: Lymphoid tissue, immunity.

• Structural features:

  • Teniae coli: Longitudinal muscle bands.

  • Haustra: Pocket-like sacs.

  • Mesocolon: Anchors colon.

• Microscopic anatomy:

  • Simple columnar epithelium (except rectum/anal canal: stratified squamous).

  • No villi or plicae circularis.

  • Many goblet cells for mucus.

  • Bacterial flora: Synthesize vitamins B5, biotin, K; produce odor compounds.

• Fecal composition: 75% water, 5% bacteria, 20% indigestible/inorganic/epithelial cells.

Accessory Organs of the Digestive Tract

Liver and Gallbladder

• Liver: Largest internal organ; produces bile, processes nutrients, metabolizes cholesterol, removes drugs/hormones.

• Gross anatomy: Right/left lobes (falciform ligament), caudate/quadrate lobes.

• Bile flow: Right/left hepatic ducts → common hepatic duct → merges with cystic duct (gallbladder) → common bile duct → duodenum (sphincter of Oddi).

• Microscopic anatomy:

  • Lobules: Functional units; hepatocytes produce bile/process nutrients.

  • Portal triad: Hepatic portal vein, hepatic artery, bile duct.

  • Kupffer cells: Remove debris.

  • Blood flow: Portal vein/artery → sinusoids → central vein → hepatic vein → inferior vena cava.

  • Bile flow: Hepatocytes → canaliculi → bile ducts → hepatic ducts.

• Gallbladder: Stores/concentrates bile; releases bile via cystic duct upon CCK stimulation.

Pancreas

• Exocrine function: Acinar cells produce pancreatic juice (buffers, enzymes).

• Pancreatic juice:

  • Sodium bicarbonate: Buffers stomach acid.

  • Proteases: Carboxypeptidase, chymotrypsin, trypsin (protein digestion).

  • Amylase: Carbohydrate digestion.

  • Lipases: Lipid digestion.

  • Nucleases: Nucleic acid digestion.

• Endocrine function: Islets of Langerhans release insulin and glucagon (carbohydrate metabolism).

Chemical Digestion and Absorption

Chemical Digestion

• Enzymatic breakdown of carbohydrates, proteins, lipids into absorbable molecules.

Absorption

• Movement of nutrients, water, electrolytes, vitamins, minerals from GI lumen into blood or lymph.

Disorders of the Digestive System

Disorder	Description
Dental caries	Erosion of enamel; cavities from bacterial action.
Gingivitis	Inflammation of gums; can lead to periodontal disease.
Halitosis	Bad breath; usually bacterial.
Mumps	Swollen parotid glands; viral infection.
Hiatal hernia	Stomach/intestine protrudes through esophageal hiatus.
Hemorrhoids	Varicose veins of rectum/anus.
Acid reflux/GERD	Backflow of stomach acid into esophagus.
Peptic/duodenal ulcers	Stomach/duodenum wall erosion; often Helicobacter pylori.
Ascites	Abdominal swelling from fluid accumulation.
Pancreatitis	Inflammation of pancreas; can be fatal.
Hepatitis	Liver inflammation; viral, alcohol, drugs.
Cirrhosis	Chronic liver inflammation; scarring.
Gallstones/cholecystitis	Cholesterol stones in bile; can block ducts.
Jaundice	Bilirubin accumulation; yellow skin.
Enteritis	Small intestine inflammation.
Colitis	Colon inflammation; diarrhea/constipation.
Diarrhea	Watery stool; rapid transit.
Constipation	Hard stool; slow transit or low fiber.
Diverticulitis	Inflamed herniation in large intestine wall.
Appendicitis	Appendix inflammation; blockage/infection.

Key Terms and Concepts

• Peristalsis: Rhythmic contraction for propulsion.

• Segmentation: Mixing movements in intestines.

• Enzyme: Protein catalyst for chemical reactions.

• Absorption: Uptake of nutrients into blood/lymph.

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

• Chyme: Partially digested food in stomach/intestine.

Important Formulas and Equations

• General formula for carbohydrate digestion: $\text{Polysaccharide} \xrightarrow{\text{amylase}} \text{Disaccharide} \xrightarrow{\text{sucrase, maltase, lactase}} \text{Monosaccharide}$

• Protein digestion: $\text{Protein} \xrightarrow{\text{pepsin, trypsin, chymotrypsin}} \text{Peptides} \xrightarrow{\text{peptidases}} \text{Amino acids}$

• Lipid digestion: $\text{Triglyceride} \xrightarrow{\text{lipase}} \text{Fatty acids} + \text{Glycerol}$

Summary Table: Digestive System Layers

Layer	Composition	Function
Mucosa	Epithelial tissue, lamina propria, muscularis mucosae	Secretion, absorption, protection
Submucosa	Dense connective tissue, blood/lymph vessels, nerves	Support, transport, secretion
Muscularis externa	Smooth muscle (circular, longitudinal)	Motility, mechanical processing
Serosa/Adventitia	Serous membrane or collagen sheath	Protection, anchoring

Example: Pathway of Food Through the Digestive System

1. Mouth: Mechanical and chemical digestion begins.

2. Pharynx: Swallowing moves food to esophagus.

3. Esophagus: Peristalsis propels food to stomach.

4. Stomach: Churning and enzymatic digestion.

5. Small intestine: Enzymatic digestion and absorption.

6. Large intestine: Water reabsorption, vitamin absorption, feces formation.

7. Rectum/anus: Elimination of waste.

Additional info: Brush border enzymes and enteroendocrine secretions are essential for final digestion and absorption. The enteric nervous system provides local control, while hormones coordinate digestive activity across organs.