The Digestive System: Structure, Function, and Regulation

21.1 Anatomy of the Digestive System

Overview of the Gastrointestinal (GI) System

• Gastrointestinal system includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

• The oral cavity (mouth and pharynx) is the entry point for food and the site of initial mechanical and chemical digestion.

• The GI tract is a continuous tube from the esophagus to the anus, divided by sphincters that regulate passage of material.

• Accessory organs (salivary glands, pancreas, liver) secrete substances that aid in digestion.

Structure of the Digestive Tube

• Food is mixed with digestive secretions to form chyme.

• Digestion begins in the oral cavity (chewing, salivary enzymes), continues through the esophagus, stomach (fundus → body → antrum), small intestine (duodenum → jejunum → ileum), and large intestine (colon → rectum → anus).

• Products of digestion are absorbed across the epithelium into interstitial fluid, then into blood or lymph for distribution.

• The digestive system is exposed to the external environment and is home to commensal microorganisms.

Layers of the GI Tract Wall

• The GI tract wall has four main layers, each with specialized functions:

Layer	Main Features
Mucosa	Mucosal epithelium, lamina propria (connective tissue, contains GALT/Peyer's patches), muscularis mucosae
Submucosa	Connective tissue, contains submucosal plexus of the enteric nervous system
Muscularis externa	Two layers of smooth muscle, contains myenteric plexus of the enteric nervous system
Serosa	Outer covering, continuation of the peritoneal membrane (mesentery)

• Modifications such as rugae, plicae, villi, gastric glands, and crypts increase surface area for absorption.

21.2 Digestive Function and Processes

Major Processes of Digestion

• There are four main processes in the digestive system:

Process	Description
Secretion	Movement of material from cells into the lumen or ECF
Digestion	Chemical and mechanical breakdown of food into absorbable units
Absorption	Movement of material from GI lumen to ECF
Motility	Movement of material through the GI tract via muscle contractions

• Mass balance is maintained by matching fluid input and output in the GI tract.

21.3 GI Motility and Regulation

GI Smooth Muscle Contraction

• Motility serves two main purposes: moving food through the tract and mechanically mixing food to break it into small particles.

• Types of contractions:

  • Tonic contractions: Last minutes to hours (e.g., sphincters).

  • Phasic contractions: Last seconds (e.g., peristalsis, segmentation).

• Slow-wave potentials originate in interstitial cells of Cajal.

Patterns of Contraction

• Three main patterns:

  • Migrating motor complex: Sweeps remnants and bacteria between meals.

  • Peristalsis: Moves bolus forward through the tract.

  • Segmental contractions: Mix contents without net forward movement.

• Guanylate cyclase-C (GC-C) receptor regulates fluid secretion; overactivation can cause diarrhea, while agonists can treat constipation.

21.4 GI Hormones

Major GI Hormones and Their Actions

Hormone	Stimulus for Release	Primary Target(s)	Primary Effects	Other Information
Gastrin (G cells)	Peptides, amino acids, neural reflexes	ECL cells, parietal cells	Stimulates gastric acid secretion and mucosal growth	Somatostatin inhibits release
Cholecystokinin (CCK)	Fatty acids, amino acids	Gallbladder, pancreas, stomach	Stimulates gallbladder contraction, pancreatic enzyme secretion, inhibits gastric emptying and acid secretion	May act as neurotransmitter
Secretin	Acid in small intestine	Pancreas, stomach	Stimulates HCO3 secretion, inhibits gastric emptying and acid secretion
Motilin	Fasting: periodic release	Gastric and intestinal smooth muscle	Stimulates migrating motor complex	Inhibited by eating
GIP, GLP-1	Glucose, fatty acids, amino acids in small intestine	Endocrine pancreas	Stimulates insulin release, inhibits gastric emptying and acid secretion	Promote satiety

21.5 Integrated Function: The Cephalic Phase

Cephalic Phase of Digestion

• Triggered by smelling, seeing, or thinking about food; reflex begins in the brain (feedforward response).

• Salivary secretions have four main functions:

  1. Soften and lubricate food

  2. Digest starch (chemical digestion)

  3. Enable taste

  4. Provide defense (antimicrobial action)

• Mechanical digestion begins with chewing (mastication).

21.6 Integrated Function: The Gastric Phase

Gastric Phase of Digestion

• The stomach has three main functions:

  1. Storage (upper stomach)

  2. Digestion (lower stomach): Lipids and proteins are digested by acid, enzymes, paracrine signals, and hormones

  3. Defense: Against swallowed pathogens

• Digestive activity in the stomach is initiated by the cephalic phase and continues with short gastric reflexes.

21.7 Integrated Function: The Intestinal Phase

Intestinal Phase of Digestion

• Motility in the small intestine is controlled by segmental and peristaltic contractions to ensure efficient digestion and absorption.

• Surface area is maximized by villi, crypts, and the brush border.

• Absorbed nutrients enter the circulatory system (via the hepatic portal system) or lymphatic system (for fats).

Intestinal Secretions

• Digestive enzymes, bile, bicarbonate, mucus, and isotonic NaCl are secreted to promote digestion.

• The CFTR channel is involved in isotonic NaCl secretion; mutations cause cystic fibrosis.

Pancreatic and Liver Secretions

• The pancreas has endocrine (insulin, glucagon) and exocrine (digestive enzymes, bicarbonate) functions.

• Brush border enteropeptidase activates trypsinogen to trypsin.

• Bicarbonate neutralizes gastric acid in the small intestine.

• The liver secretes bile (bile salts, pigments, cholesterol) to aid fat digestion; bile is stored in the gallbladder and released into the duodenum.

Digestion and Absorption of Nutrients

• Fats: Digested by bile salts, lipases, and colipase; absorbed as chylomicrons via lymphatics.

• Carbohydrates: Digested to monosaccharides (glucose, galactose, fructose) by pancreatic and brush border enzymes; absorbed via SGLT, GLUT2, and GLUT5 transporters.

• Proteins: Digested by endopeptidases (pepsin, trypsin, chymotrypsin) and exopeptidases (aminopeptidases, carboxypeptidases); absorbed as amino acids, dipeptides, and tripeptides.

• Nucleic acids: Digested into bases and monosaccharides.

• Vitamins and minerals: Fat-soluble vitamins absorbed with fats; water-soluble vitamins by mediated transport; vitamin B12 requires intrinsic factor.

• Ions and water: Absorbed via solute and osmotic gradients; Na+ absorption is key for water uptake.

The Large Intestine

• Concentrates waste by absorbing water and ions; only about 0.1 L of water is lost in feces.

• Regions: Cecum (with appendix), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus.

Summary Table: Key Digestive Processes

Region	Main Function(s)	Key Secretions
Oral cavity	Mechanical and chemical digestion	Saliva (amylase, mucus)
Stomach	Storage, digestion, defense	HCl, pepsinogen, mucus, intrinsic factor
Small intestine	Digestion, absorption	Enzymes, bile, bicarbonate, mucus, NaCl
Large intestine	Water and ion absorption, waste concentration	Mucus

Additional info: This summary integrates textbook slide content with standard academic explanations for clarity and completeness. Figures referenced in the slides (e.g., GI tract wall, digestive processes, absorption mechanisms) are described in the text but not reproduced as images.