The Digestive System

Anatomy of the Digestive System

The digestive system is composed of a series of organs that process food, absorb nutrients, and eliminate waste. The gut wall consists of four distinct layers, each with specialized functions.

• Mucosa: The innermost layer facing the lumen, composed of three sublayers:

  • Mucosal epithelium: Contains transporting cells, endocrine and exocrine cells, and stem cells. GI epithelial cells have a rapid turnover, lasting only a few days.

  • Lamina propria: Subepithelial connective tissue supporting the epithelium. Contains nerve cells, small blood vessels, lymphatic vessels, and wandering immune cells.

  • Muscularis mucosae: Thin layer of smooth muscle responsible for moving the villi.

• Submucosa: Connective tissue with larger blood and lymphatic vessels. Contains the submucosal plexus, a major nerve network of the enteric nervous system that innervates the epithelial and muscularis mucosae layers.

• Muscularis externa: Primarily two layers of smooth muscle (circular and longitudinal). The stomach has an additional oblique layer. The myenteric plexus lies between the muscle layers and coordinates movement.

• Serosa: Outermost connective tissue layer, continuous with peritoneal membranes.

Chyme is the soupy mixture of food and GI tract secretions.

Digestive Functions and Processes

The digestive system performs four basic processes to make food usable for the body:

• Digestion: Chemical and mechanical breakdown of food into smaller pieces for absorption.

  • Mechanical digestion: Physical breakdown increases surface area for enzymatic action.

  • Chemical digestion: Enzymatic breakdown into absorbable building blocks.

• Absorption: Movement of substances from the GI tract lumen into the extracellular fluid.

• Secretion: Movement of substances (water, ions, enzymes, mucus) into the GI tract lumen.

• Motility: Movement of substances in the GI tract via muscular contraction.

Approximately 9 L of fluid pass through the GI tract daily, with most reabsorbed and only about 100 mL released in feces.

• Digestive enzymes: Secreted by exocrine glands or epithelial cells, often in inactive forms (e.g., pepsinogen).

• Mucus: Viscous secretion of glycoproteins (mucins) for protection and lubrication.

Most absorption occurs in the small intestine; water and ions are also absorbed in the large intestine. Nutrients enter the blood or lymph after absorption.

Motility in the GI Tract

GI smooth muscle contracts spontaneously to move and mix food. Motility is determined by muscle properties and modified by nerves, hormones, and paracrine signals.

• Tonic contractions: Sustained for minutes or hours; found in sphincters and anterior stomach.

• Phasic contractions: Short contraction-relaxation cycles; occur in posterior stomach and small intestine.

• Slow wave potentials: Cycles of depolarization and repolarization associated with smooth muscle contractions. Originates from interstitial cells of Cajal located between muscle layers and nerve plexuses.

Patterns of contraction:

• Migrating motor complex: Occurs between meals, sweeps remnants and bacteria from stomach to large intestine (~90 min cycle).

• Peristalsis: Progressive waves move food forward.

• Segmental contractions: Mix contents and keep them in contact with intestinal walls.

Regulation of GI Function

The enteric nervous system (ENS) acts independently of the CNS, controlling motility, secretion, and growth. The ENS is sometimes called the "little brain" of the gut.

• Short reflexes: Originate and integrate within the ENS; submucosal plexus controls secretion, myenteric plexus controls motility.

• Long reflexes: Integrate in the CNS; include feedforward and emotional responses (cephalic reflexes). Parasympathetic input is excitatory, sympathetic is inhibitory.

GI peptides (hormones, neuropeptides, cytokines) regulate GI function. Some act outside the GI tract (e.g., CCK enhances satiety and acts as a neurotransmitter).

Integrated Function: Cephalic Phase

Digestion begins in the mouth with both chemical and mechanical processes.

• Saliva: Exocrine secretion with four functions:

  1. Softens and moistens food

  2. Digests starch (salivary amylase)

  3. Facilitates taste

  4. Provides defense (lysozyme, IgA)

• Chewing: Mechanical digestion

• Swallowing: Moves food to stomach; epiglottis blocks trachea

Integrated Function: Gastric Phase

The stomach performs storage, digestion, and defense. Gastric phase reflexes are triggered by stomach distension and presence of peptides/amino acids.

• Receptive relaxation: Stomach relaxes to accept food; regulates passage to small intestine.

• Dumping syndrome: Improper regulation leads to rapid passage of unabsorbed chyme to large intestine.

Gastric secretions:

• G cells: Secrete gastrin, promoting acid release by parietal cells.

• Parietal cells: Secrete gastric acid (HCl) and intrinsic factor.

  • Gastric acid: Activates pepsin, triggers somatostatin release, denatures proteins, kills pathogens, inactivates salivary amylase.

  • Intrinsic factor: Essential for vitamin B12 absorption in small intestine.

• Chief cells: Secrete pepsinogen and gastric lipase (minor fat digestion).

• Enterochromaffin-like (ECL) cells: Secrete histamine to stimulate acid secretion.

• D cells: Secrete somatostatin, inhibiting acid and pepsinogen secretion.

Integrated Function: Intestinal Phase

Begins when chyme enters the small intestine. Peristalsis and segmentation mix chyme with enzymes and facilitate absorption. Most digestion and absorption occur here.

• Surface area maximization: Folds (plicae), villi, and microvilli (brush border) increase surface area for absorption.

• Absorption: Most nutrients absorbed into capillaries; fats absorbed into lymphatic system.

• Hepatic portal system: Veins from digestive organs carry blood to liver for metabolism and detoxification before systemic circulation.

Pancreas: Secretes digestive enzymes and bicarbonate to neutralize acidic chyme.

Liver: Secretes bile (bile salts, pigments, cholesterol); bile stored in gall bladder.

Digestion in Small Intestine:

• Proteins: Digested by endopeptidases (proteases) and exopeptidases into amino acids and small peptides. Absorbed as free amino acids, dipeptides, tripeptides. Some larger peptides absorbed intact by transcytosis (may cause allergies).

• Carbohydrates: Digested to monosaccharides by brush border enzymes (maltase, sucrase, lactase). Lactose intolerance results from decreased lactase activity.

• Fats: Digested by pancreatic lipases and colipase; bile salts emulsify fats. Phospholipids digested by phospholipase.

• Nucleic acids: Digested into bases and monosaccharides by pancreatic and intestinal enzymes.

• Vitamins and minerals: Fat-soluble vitamins (A, D, K, E) absorbed with fat; water-soluble vitamins (C, B) absorbed with water. Vitamin B12 requires intrinsic factor. Iron and calcium absorption regulated by body needs.

• Ions and water: Most water absorbed in small intestine; >5 L absorbed in colon.

The Large Intestine

The large intestine concentrates waste and absorbs water and ions. Motility is primarily via mass movements, which trigger defecation.

• Mass movements: Occur 3-4 times daily, associated with eating and stomach distension (gastrocolic reflex).

• Defecation: Triggered by rectal distension.

• Bacterial fermentation: Colonic bacteria break down undigested carbs and proteins, produce vitamin K.

• Diarrhea: Can cause dehydration due to excessive water loss.

Summary Table: Digestive Processes by Region

Example: In lactose intolerance, decreased activity of lactase in the brush border prevents digestion of lactose, leading to gastrointestinal symptoms.

Additional info: The hepatic portal system is a unique circulatory route that ensures nutrients and toxins absorbed from the GI tract are processed by the liver before entering systemic circulation.