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, GI tract, and accessory organs.

• Oral cavity: Consists of the mouth and pharynx, where digestion begins.

• Gastrointestinal tract (GI tract): A continuous tube including the esophagus, stomach, small intestine, and large intestine. The term "gut" refers to the region from the stomach through the anus.

• The GI tract is divided into functional regions by sphincters, which regulate passage of material.

The Digestive System as a Tube

• Accessory glands (salivary glands, pancreas, liver) secrete substances aiding digestion.

• Chyme: The mixture of food and digestive secretions in the GI tract.

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

• Waste is excreted from the GI tract via the anus.

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

Major Regions of the GI Tract

• Digestion begins in the oral cavity (chewing, salivary glands).

• Esophagus transports food to the stomach.

• Stomach regions: fundus → body → antrum; pylorus with pyloric valve controls entry to small intestine.

• Small intestine: duodenum → jejunum → ileum.

• Large intestine: colon → rectum → anus (with external anal sphincter).

GI Tract Wall Structure

• Four main layers (from inner to outer):

  • Mucosa: Epithelium, lamina propria (connective tissue), muscularis mucosae; contains immune tissue (GALT).

  • Submucosa: Connective tissue with submucosal nerve plexus.

  • Muscularis externa: Smooth muscle layers (circular and longitudinal), with myenteric nerve plexus.

  • Serosa: Outermost connective tissue layer, forms part of the mesentery.

• Surface area is increased by rugae, plicae, villi, gastric glands, crypts, and submucosal glands.

21.2 Digestive Function and Processes

Basic Processes of the Digestive System

• Digestion: Chemical and mechanical breakdown of food into absorbable units.

• Absorption: Movement of nutrients, water, and electrolytes from the lumen into the body.

• Secretion: Transfer of water, ions, and digestive substances into the lumen or extracellular fluid.

• Motility: Movement of material through the GI tract via muscle contractions.

Challenges of Digestion

1. Avoiding autodigestion: Preventing self-digestion of GI tissues by enzymes and acid.

2. Mass balance: Matching fluid input and output to prevent dehydration or fluid overload.

3. Defense: Protecting the body from pathogens in ingested food.

Fluid Secretion and Mucus

• About 9 L/day of fluid passes through the GI tract (2 L ingested, 7 L secreted).

• Digestive enzymes are secreted into the mouth, stomach, and intestine, often as inactive precursors (zymogens).

• Mucus is produced by mucous cells, serous cells, and goblet cells for protection and lubrication.

21.2 Motility: GI Smooth Muscle Contraction

Types and Regulation of GI Motility

• GI smooth muscle contracts spontaneously for two main purposes:

  • Moving food through the tract

  • Mechanically mixing food to break it into small particles

• Contraction types:

  • 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

• Migrating motor complex: Sweeps residual contents through the GI tract between meals.

• Peristalsis: Propels bolus forward via coordinated muscle contractions.

• Segmental contractions: Mix contents by alternating contraction and relaxation.

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

21.3 Regulation of GI Function

Enteric Nervous System (ENS)

• The ENS can function independently to control motility, secretion, and growth of the digestive system.

• ENS shares features with the central nervous system (CNS):

  1. Intrinsic neurons (within ENS)

  2. Extrinsic neurons (from CNS to ENS)

  3. Neurotransmitters and neuromodulators

  4. Glial support cells

  5. Diffusion barrier

  6. Integrating centers

• Short reflexes are integrated within the ENS; long reflexes involve the CNS (e.g., cephalic reflexes).

GI Peptides: Hormones, Neuropeptides, and Cytokines

• GI peptides regulate motility and secretion.

• Major hormone families:

  • Gastrin family: Gastrin, cholecystokinin (CCK)

  • Secretin family: Secretin, vasoactive intestinal peptide (VIP), glucose-dependent insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1)

  • Others: Motilin

Hormone	Stimulus for Release	Primary Targets	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	Some effects may occur due to CCK as a neurotransmitter
Secretin	Acid in small intestine	Pancreas, stomach	Stimulates bicarbonate secretion; inhibits gastric emptying and acid secretion
Motilin	Fasting: periodic release every 1.5–2 hours	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 (feedforward); inhibits gastric function	Promotes satiety

21.4 Integrated Function: The Cephalic Phase

Cephalic Phase of Digestion

• Initiated by smelling, seeing, or thinking about food; reflex begins in the brain (medulla).

• Anticipation or presence of food in the oral cavity activates neural pathways, initiating the cephalic phase.

• Chemical and mechanical digestion begins in the mouth.

• Salivary secretions have four functions:

  1. Soften and lubricate food

  2. Digest starch (chemical digestion)

  3. Taste

  4. Defense (antimicrobial action)

• Mechanical digestion begins with chewing (mastication).

Saliva as an Exocrine Secretion

• Three pairs of salivary glands (parotid, sublingual, submandibular) secrete saliva via acini.

• Parotid: watery, enzyme-rich; sublingual: mucus-rich; submandibular: mixed.

• Salivary secretion is under autonomic control.

• Swallowing (deglutition) moves food from mouth to stomach; the epiglottis closes off airways during swallowing.

21.5 Integrated Function: The Gastric Phase

Functions of the Stomach

• Storage: Upper stomach stores food.

• Digestion: Lower stomach digests lipids and proteins using acid, enzymes, paracrine signals, and hormones.

• Defense: Protects against swallowed pathogens.

• Gastric activity is initiated by long vagal reflexes (cephalic phase) and short reflexes (gastric phase).

Gastric Secretions

• Gastric glands in gastric pits secrete:

  • Gastrin (from G cells, stimulated by gastrin-releasing peptide)

  • Acid (HCl) (from parietal cells)

  • Pepsinogen (from chief cells; activated to pepsin) and gastric lipase

  • Histamine (from ECL cells), intrinsic factor (from parietal cells), somatostatin (from D cells)

Protection of the Stomach

• The stomach is protected from autodigestion by a mucus-bicarbonate barrier, secreted by mucous cells on the luminal surface and neck of gastric glands.

21.6 Integrated Function: The Intestinal Phase

Motility and Absorption in the Small Intestine

• Motility is controlled by segmental and peristaltic contractions to ensure proper mixing and absorption.

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

• Most absorbed nutrients enter the circulatory system via the hepatic portal system; fats enter via lymphatic circulation.

Intestinal Secretions

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

• The CFTR channel (cystic fibrosis transmembrane conductance regulator) is involved in NaCl secretion.

Pancreatic and Liver Secretions

• Pancreas:

  • Endocrine portion (islets): secretes insulin and glucagon.

  • Exocrine portion: secretes digestive enzymes (acini) and sodium bicarbonate (duct cells).

  • Brush border enteropeptidase converts trypsinogen to trypsin.

  • Bicarbonate neutralizes gastric acid.

• Liver:

  • Secretes bile (bile salts, bile pigments, cholesterol) via hepatocytes.

  • Bile is stored in the gall bladder and released into the duodenum.

  • Bile aids in fat digestion and absorption.

Digestion and Absorption in the Small Intestine

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

• Carbohydrates: Digested by pancreatic and brush border enzymes to monosaccharides (glucose, galactose, fructose); 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 (e.g., vitamin B12 with intrinsic factor); iron and calcium absorption is regulated.

• Ions and water: Absorbed via solute and osmotic gradients; Na+ absorption helps create the gradient.

21.6 The Large Intestine

Function and Motility

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

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

• Motility: Segmental contractions mix chyme; mass movements propel feces and trigger defecation reflex.

• Bacteria in colon digest undigested material, producing fatty acids (energy for colonocytes) and vitamins.

• Diarrhea can cause dehydration due to excessive water loss.

21.7 Immune Functions of the GI Tract

GI Immune Defense

• M cells sample gut contents and deliver antigens to immune cells.

• Immune cells secrete cytokines, attracting more immune cells and triggering inflammation.

• Cytokines increase Cl-, fluid, and mucus secretion as a defense mechanism.

• Vomiting is a protective reflex to expel harmful substances.