The Digestive System: Structure, Function, and Processes

The Digestive System

Introduction to the Digestive System

The digestive system is responsible for acquiring nutrients from the environment, which are then used for synthesizing essential compounds (anabolism) or broken down to provide energy to cells (catabolism). It consists of the digestive tract (also called the gastrointestinal (GI) tract or alimentary canal) and various accessory organs.

• Digestive tract: Muscular tube extending from the oral cavity to the anus, including the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory organs: Teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

The GI tract acts as a 'disassembly line,' making nutrients increasingly available at each step.

Major Processes of Digestion

• Ingestion: Intake of food into the oral cavity.

• Mechanical digestion and propulsion: Physical breakdown (crushing, shearing) and movement of food along the tract.

• Chemical digestion: Enzymatic breakdown of food into absorbable molecules.

• Secretion: Release of water, acids, enzymes, buffers, and salts by the digestive tract and accessory organs.

• Absorption: Movement of nutrients, electrolytes, vitamins, minerals, and water across the digestive epithelium into the body.

• Defecation: Elimination of indigestible substances as feces.

Protection and Lining of the Digestive Tract

• The lining protects against corrosive acids/enzymes, mechanical stress, and bacteria.

Peritoneum and Mesenteries

• Peritoneum: Serous membrane lining the peritoneal cavity.

• Visceral peritoneum (serosa): Covers organs.

• Parietal peritoneum: Lines body wall.

• Peritoneal fluid: Lubricates surfaces.

• Retroperitoneal organs: Located outside the peritoneum.

• Mesenteries: Double sheets of peritoneal membrane that suspend and stabilize organs, and provide routes for vessels and nerves.

Histology of the Digestive Tract

Major Layers

• Mucosa: Inner lining; consists of epithelium, lamina propria (areolar tissue), and muscularis mucosae (smooth muscle).

• Submucosa: Dense irregular connective tissue with blood and lymphatic vessels.

• Muscular layer: Smooth muscle (inner circular and outer longitudinal layers) for mechanical digestion and propulsion.

• Serosa: Protective visceral peritoneum covering the muscular layer.

Digestive Epithelium

• Stratified squamous epithelium: Found in oral cavity, pharynx, esophagus, and anal canal (areas of abrasion).

• Simple columnar epithelium: Found in stomach, small intestine, and most of large intestine (areas of absorption/secretion).

The Oral Cavity

Functions and Structures

• Mechanical digestion: By teeth, tongue, and palatal surfaces.

• Lubrication: Mixing with mucus and saliva.

• Chemical digestion: Limited, mainly carbohydrates and lipids.

• Oral mucosa: Stratified squamous epithelium lining the cavity.

• Oral vestibule: Space between cheeks/lips and teeth.

• Gingivae (gums): Surround base of each tooth.

• Hard and soft palate, uvula: Separate oral and nasal cavities; uvula prevents premature entry of food into pharynx.

The Tongue

• Mechanical digestion (compression, abrasion, distortion).

• Manipulation of food for chewing and swallowing.

• Sensory analysis (touch, temperature, taste).

• Secretion of mucins and lingual lipase.

Teeth

• Dentin: Mineralized matrix, forms bulk of tooth, no cells.

• Pulp cavity: Contains blood vessels and nerves via root canal and apical foramen.

• Root: Embedded in jawbone, anchored by periodontal ligament (gomphosis joint).

• Crown: Exposed part above gum, covered by enamel (hardest substance in body).

Types of Teeth

• Incisors: Blade-shaped, for cutting, single root.

• Cuspids (canines): Fanglike, for tearing, single root.

• Premolars (bicuspids): Flattened crowns, for crushing/grinding, one or two roots.

• Molars: Large, flattened crowns, for grinding, two or three roots.

Sets of Teeth

• Deciduous (primary, milk, baby) teeth: 20, erupt between 6–24 months.

• Permanent (secondary) teeth: 32, replace deciduous teeth between ages 6–12; all but third molars erupt by end of adolescence.

Salivary Glands and Saliva

• Major pairs: Parotid (serous, amylase), sublingual (mucus), submandibular (buffers, mucins, amylase).

• Saliva: 1.0–1.5 L/day; 99.4% water, 0.6% solutes (electrolytes, buffers, glycoproteins, antibodies, enzymes, wastes).

• Functions: Cleans oral surfaces, moistens food, maintains pH, controls bacteria, dissolves chemicals for taste, initiates carbohydrate digestion.

Mastication (Chewing)

• Food is chewed and compacted into a bolus for swallowing.

• Muscles of mastication close jaws and move lower jaw side to side.

Pharynx and Esophagus

Pharynx

• Common passageway for food, liquid, and air.

• Regions: Nasopharynx, oropharynx, laryngopharynx.

• Lined with stratified squamous epithelium and mucus glands.

Esophagus

• Hollow muscular tube from laryngopharynx to stomach.

• Conveys food/liquids to stomach; anchored by adventitia.

Swallowing (Deglutition)

• Initiated voluntarily, then proceeds automatically.

• Phases: Buccal (bolus to oropharynx), pharyngeal, esophageal (peristalsis moves food to stomach).

Stomach

Functions and Regions

• Chemical digestion (acids, enzymes), mechanical digestion (muscular contractions), temporary storage.

• Regions: Fundus (superior), body (largest, mixing tank), pyloric part (to duodenum via pyloric sphincter).

• Rugae: Folds in mucosa that allow expansion.

Gastric Glands and Secretions

• Gastric pits: Openings on surface; connect to gastric glands.

• Mucous cells: Replace superficial cells.

• Parietal cells: Secrete HCl (via proton pumps, carbonic acid, alkaline tide) and intrinsic factor (for vitamin B12 absorption).

• Chief cells: Secrete pepsinogen (converted to pepsin by HCl).

• Enteroendocrine cells (G cells): Produce gastrin (stimulates parietal/chief cells and gastric wall contractions).

• Stomach of infants: Produce rennin and gastric lipase for milk digestion.

Chemical Digestion in the Stomach

• Some carbohydrate (salivary amylase) and lipid (lingual lipase) digestion continues.

• Protein digestion by pepsin increases as pH drops to ~2.0.

• Little nutrient absorption; some chemicals (alcohol, aspirin) can be absorbed.

Regulation of Gastric Activity

• Regulated by neural and hormonal mechanisms in three overlapping phases:

Phase	Trigger	Main Events
Cephalic	Sight/thought/taste/smell of food	Prepares stomach for food (neural stimulation)
Gastric	Food in stomach (distension, peptides, pH rise)	Gastrin release, increased secretion and motility
Intestinal	Chyme in duodenum (low pH, partially digested food)	Regulates gastric emptying and secretion

Accessory Organs

Pancreas

• Exocrine function: Secretes pancreatic juice (enzymes, water, ions) into duodenum via pancreatic duct.

• Endocrine function: Pancreatic islets secrete insulin and glucagon into bloodstream.

• Pancreatic acini: Secrete alkaline juice (~1000 mL/day), controlled by duodenal hormones.

Pancreatic Enzymes

• Alpha-amylase: Breaks down starches.

• Lipase: Breaks down complex lipids.

• Nucleases: Break down RNA/DNA.

• Proteolytic enzymes: Proteases (large proteins), peptidases (small peptides to amino acids).

Liver

• Largest visceral organ; divided into right, left, caudate, and quadrate lobes.

• Liver lobules: Hexagonal units with hepatocyte plates radiating from central vein; portal triads at corners (bile duct, hepatic artery, hepatic portal vein).

• Liver sinusoids: Leaky capillaries between plates; contain Kupffer cells (macrophages).

• Bile ducts: Bile leaves via common hepatic duct, joins cystic duct to form bile duct.

Liver Functions

• Metabolic regulation: Nutrient extraction/storage, waste removal, vitamin/mineral storage, drug inactivation.

• Hematological regulation

• Bile production: Bile salts emulsify fats for digestion.

Gallbladder

• Stores and concentrates bile; releases it via cystic duct into bile duct.

Small Intestine

Structure and Function

• Main site of nutrient absorption; extends from pyloric sphincter to ileocecal sphincter.

• Subdivisions: Duodenum, jejunum, ileum.

• Structural modifications (villi, microvilli, brush border) increase surface area for absorption.

• Goblet cells: Secrete mucus.

• Lacteal: Lymphatic vessel in villus for fat absorption.

• Intestinal juice: Aids in digestion and absorption.

Large Intestine

Structure and Function

• Subdivided into cecum (with ileocecal sphincter and appendix), colon, rectum, and anal canal (with sphincters).

• No further digestion except by enteric bacteria.

• Reabsorbs water and absorbs vitamins (K, biotin, B5).

• Propels and stores fecal material for defecation.