The Digestive System: Structure, Function, and Disorders

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

The Digestive System

System Interconnections

The digestive system works closely with other organ systems to maintain homeostasis and support cellular function. It provides nutrients for cell maintenance and growth, while the respiratory and urinary systems manage gas exchange and waste removal, respectively.

Digestive system: Supplies nutrients for cellular processes.
Respiratory system: Supplies oxygen and removes carbon dioxide.
Urinary system: Eliminates organic wastes generated by metabolism.

Organization of the Digestive System

Digestive Tract and Accessory Organs

The digestive system consists of the digestive tract (gastrointestinal tract) and accessory organs. The tract is a muscular tube lined with mucous membrane, beginning at the mouth and ending at the anus. Accessory organs secrete products into the tract to aid digestion.

Digestive tract: Mouth → Esophagus → Stomach → Small intestine → Large intestine → Anus
Accessory organs: Salivary glands, liver, gallbladder, pancreas

Layers of the Digestive Tract

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

Mucosa: Inner lining; mucous membrane with epithelium and lamina propria (areolar tissue). Moistened by glandular secretions.
Submucosa: Dense irregular connective tissue containing blood vessels, lymphatics, and exocrine glands.
Muscularis externa: Two layers of smooth muscle (inner circular, outer longitudinal) for movement and mixing.
Serosa: Visceral peritoneum covering tract in peritoneal cavity; replaced by adventitia in oral cavity, pharynx, esophagus, and rectum.

Mucosal Components

Epithelium: Stratified squamous (oral cavity, pharynx, esophagus); simple columnar (stomach, small and most of large intestine).
Villi: Projections increasing surface area for absorption.
Lamina propria: Areolar tissue with blood vessels, nerves, and lymphatics.
Muscularis mucosae: Two smooth muscle layers for moving folds and villi.

Nerve Plexuses

Local control of digestive activities is mediated by nerve plexuses:

Submucosal plexus: Innervates mucosa and submucosa; regulates glandular secretion and local blood flow.
Myenteric plexus: Located between muscle layers of muscularis externa; controls motility.

Parasympathetic stimulation increases digestive activity; sympathetic stimulation inhibits it.

Motility of the Digestive Tract

Smooth Muscle Actions

Arranged in sheets, bundles, and sphincters to regulate movement.
Inner circular and outer longitudinal layers coordinate contractions.

Peristalsis

Sequential contractions propel a bolus (moist, compact mass of food) along the tract.

Segmentation

Mixes and fragments contents without directional movement; occurs mainly in small and part of large intestine.

Peristalsis is more efficient for propulsion; segmentation is for mixing.

Digestive Tract Subdivisions and Functions

Major Subdivisions

Oral cavity, teeth, tongue: Mechanical processing, moistening, mixing with saliva.
Pharynx: Muscular propulsion of food.
Esophagus: Transports food to stomach.
Stomach: Chemical and mechanical processing.
Small intestine: Enzymatic digestion and absorption.
Large intestine: Dehydration and compaction of indigestible materials.

Functions of the Digestive Tract

Protection: Against acids, enzymes, abrasion, and pathogens.
Ingestion: Entry of food and liquids.
Mechanical processing: Chewing, mixing, churning.
Digestion: Chemical breakdown into absorbable molecules.
Secretion: Release of water, acids, enzymes, buffers.
Absorption: Movement of nutrients into blood/lymph.
Compaction and defecation: Dehydration and elimination of wastes as feces.

The Oral Cavity and Teeth

Structure and Boundaries

Roof: Hard palate (maxillary and palatine bones), soft palate (posterior).
Floor: Muscular base.
Anterior: Lips (labia).
Lateral: Cheeks.
Posterior: Uvula, palatine tonsils, root of tongue.

Oral Cavity Features

Vestibule: Space between cheeks/lips and teeth.
Lingual frenulum: Connects tongue to floor.
Gingivae (gums): Surround tooth bases.
Mastication: Chewing increases surface area for enzymes; digestion of carbohydrates and lipids begins here.

Tooth Anatomy

Dentin: Mineralized matrix, bulk of tooth.
Pulp cavity: Central chamber with nerves and vessels.
Crown: Visible part above gum.
Neck: Junction between crown and root.
Root: Anchored in alveolus (bony socket).
Enamel: Hard covering of crown; requires calcium, phosphate, vitamin D.
Cementum: Covers root; less resistant than enamel.
Periodontal ligament: Collagen fibers anchoring tooth.
Root canal: Passage for nerves and vessels.

Types of Teeth

Type	Shape	Function
Incisors	Blade-shaped, single root	Clipping, cutting
Cuspids (Canines)	Conical, pointed tip, single root	Tearing, slashing
Bicuspids (Premolars)	Flattened crowns, 1-2 roots	Crushing, mashing, grinding
Molars	Large, flattened crowns, 3+ roots	Crushing, grinding

Dentition

Primary (deciduous) teeth: 20 teeth, appear by age 2.
Secondary (permanent) teeth: 32 teeth, including third molars (wisdom teeth).

Tooth Decay and Periodontal Disease

Dental plaque: Bacterial matrix causing decay.
Gingivitis: Inflammation of gums; can lead to periodontal disease and tooth loss.
Prevention: Brushing, flossing, gum massage.

Pharynx and Esophagus

Pharynx

Membrane-lined cavity posterior to nose and mouth.
Common passageway for food, liquids, and air.
Food passes through oropharynx and laryngopharynx.

Esophagus

Muscular tube (~25 cm long) transporting food to stomach.
Layers: Mucosa, submucosa (with folds for expansion), muscularis externa (skeletal → mixed → smooth muscle), adventitia (no serosa).
Upper esophageal sphincter prevents air entry; lower esophageal sphincter prevents reflux.

Swallowing (Deglutition)

Buccal phase: Voluntary; bolus pushed to oropharynx.
Pharyngeal phase: Involuntary; bolus moves through pharynx, airway blocked.
Esophageal phase: Involuntary; peristalsis moves bolus to stomach.

The Stomach

Structure and Regions

J-shaped, muscular, expandable organ (1–1.5 L capacity).
Regions (proximal to distal): Cardia, fundus, body, pylorus.
Chyme: Mixture of food, saliva, gastric secretions.
Rugae: Folds allowing expansion.

Muscularis Externa

Three layers: Circular, longitudinal, oblique (unique to stomach).
Enhances churning and mixing to form chyme.

Stomach Wall and Secretions

Mucosa: Simple columnar epithelium, replaced every 3–7 days; produces protective mucus.
Gastric glands: In fundus and body; secrete acid and enzymes.
Gastric juice: ~1500 mL/day; acidic, contains enzymes.
Gastric pits: Openings to glands; contain stem cells for renewal.

Cells of Gastric Glands

Cell Type	Secretion	Function
Parietal cells	HCl, intrinsic factor	Acidifies stomach, aids B12 absorption
Chief cells	Pepsinogen	Converted to pepsin (protein digestion)
Enteroendocrine cells	Hormones	Regulate digestion

Pepsinogen is activated by HCl to pepsin.

The Small Intestine

Surface Modifications

Circular folds: Permanent transverse folds (most in jejunum).
Villi: Fingerlike projections covered with epithelial cells.
Microvilli: Brush border on epithelial cells; contain digestive enzymes.

These features increase surface area for absorption by over 600 times.

Intestinal Glands and Villi

Glands at villi bases; stem cells renew epithelium.
Villi contain capillaries (for nutrient absorption) and lacteals (for fat absorption).
Muscularis mucosae moves villi, aiding absorption.

Regions of the Small Intestine

Region	Length	Function
Duodenum	25 cm	Receives chyme, pancreatic and liver secretions
Jejunum	2.5 m	Most chemical digestion and absorption
Ileum	3.5 m	Ends at ileocecal valve; controls flow into large intestine

Hormonal Regulation of Digestion

Gastrin: Stimulates gastric acid and enzyme production.
Secretin: Stimulates pancreas to release buffers; inhibits gastric activity.
Gastric inhibitory peptide (GIP): Inhibits gastric activity; stimulates insulin release.
Cholecystokinin (CCK): Stimulates gallbladder contraction and pancreatic enzyme secretion; increased by high-fat meals.
Vasoactive intestinal peptide (VIP): Dilates intestinal capillaries, enhancing absorption.

The Large Intestine

Functions

Absorbs vitamins produced by bacteria.
Reabsorbs water.
Compacts indigestible material into feces.
Stores feces prior to defecation.

Regions

Cecum: Receives material from ileum; begins compaction; appendix attached.
Colon: Four regions—ascending, transverse, descending, sigmoid.
Rectum: Last 15 cm; stores feces; triggers defecation reflex.

Colon Structure

Fatty appendices: Fat sacs in serosa.
Taeniae coli: Three longitudinal muscle bands.
Haustra: Pouches allowing expansion and elongation.

Mass Movements

Powerful peristaltic contractions moving feces toward rectum; triggered by stomach and duodenum distension.

Control of Digestive Activities

Neural reflexes: Parasympathetic stimulation (vagus nerve) increases gastric juice; enterogastric reflex inhibits gastric activity when chyme enters duodenum.
Hormonal control: Hormones adjust secretion and motility based on chyme composition.
Absorption: Occurs mainly in jejunum and ileum; water reabsorbed in large intestine.
Defecation: Triggered by rectal distension; voluntary relaxation of external anal sphincter required.

Feces Composition

75% water, 5% bacteria, indigestible material, inorganic matter, epithelial cell remnants.

Accessory Digestive Organs

Salivary Glands

Gland	Secretion
Sublingual	Mucus (buffer, lubricant)
Submandibular	Buffers, mucus, salivary amylase, antibodies
Parotid	Salivary amylase (starch digestion)

Functions of saliva: Lubrication, buffering, enzyme activity, antibacterial action.

Liver

Largest visceral organ; four lobes (right, left, caudate, quadrate).
Functions: Bile synthesis, nutrient storage, detoxification, plasma protein and clotting factor synthesis, blood storage, hormone/drug metabolism.

Gallbladder

Stores and concentrates bile.
Bile path: Hepatic ducts → Common hepatic duct → (to duodenum via common bile duct or to gallbladder via cystic duct).
Hepatopancreatic sphincter controls bile entry into duodenum.

Pancreas

Exocrine cells secrete digestive enzymes and buffers into duodenum via pancreatic duct.
Endocrine cells secrete insulin and glucagon.
Major enzymes: Pancreatic amylase (starch), lipase (lipids), nucleases (nucleic acids), proteolytic enzymes (proteins; e.g., trypsin, chymotrypsin).

Disorders of the Digestive System

Oral Cavity

Periodontal disease: Loss of teeth due to plaque and gum inflammation (gingivitis).

Salivary Glands

Mumps: Viral infection, often of parotid gland; can affect other organs; prevented by MMR vaccine.

Esophagus

Esophagitis: Inflammation from acid reflux (heartburn).
Gastroesophageal reflux: Backflow of stomach acid into esophagus.

Liver

Hepatitis: Inflammation from alcohol or viral infection (A, B, C).
Cirrhosis: Scarring and loss of function.
Jaundice: Bilirubin buildup causing yellow skin/eyes.
Portal hypertension: Increased pressure in portal vein due to liver disease; can cause bleeding and ascites.

Gallbladder

Gallstones: Crystals of minerals/salts; may block ducts.
Cholecystitis: Inflammation from large gallstones; may require surgical removal.

Stomach

Gastritis: Inflammation of stomach lining from drugs, stress, infection.
Peptic ulcers: Erosion by acid/enzymes; most caused by Helicobacter pylori.

Pancreas

Pancreatitis: Inflammation from gallstones, infection, or drugs; can be fatal if enzymes digest pancreas.

Intestines

Enteritis: Inflammation of small intestine; causes diarrhea.
Dysentery: Inflammation with bloody, mucus-laden diarrhea.
Gastroenteritis: Inflammation of stomach and intestines; often from infection.
Colitis: Inflammation of colon; can cause diarrhea or constipation.
Colorectal cancer: Often begins as polyps; risk factors include age, diet, genetics.

Summary Table: Major Digestive Enzymes and Their Functions

Enzyme	Source	Substrate	Product
Salivary amylase	Salivary glands	Starch	Maltose, dextrins
Lingual lipase	Tongue	Lipids	Fatty acids, monoglycerides
Pepsin	Stomach (chief cells)	Proteins	Peptides
Pancreatic amylase	Pancreas	Starch	Maltose, dextrins
Pancreatic lipase	Pancreas	Lipids	Fatty acids, monoglycerides
Trypsin, chymotrypsin, carboxypeptidase	Pancreas	Proteins, peptides	Peptides, amino acids
Nucleases	Pancreas	Nucleic acids	Nucleotides

Key Equations

General formula for carbohydrate digestion:

Protein digestion (stomach):

Lipid digestion (small intestine):

Additional info:

Most absorption occurs in the small intestine, especially the jejunum.
Accessory organs are essential for chemical digestion but do not directly contact ingested food (except teeth and tongue).
Defecation is a reflex action but can be voluntarily controlled.