Digestive System: Structure, Function, and Regulation

Overview of the Digestive System

Alimentary Canal and Accessory Organs

The digestive system consists of the alimentary canal (also called the GI tract or gut) and accessory organs. The alimentary canal is a continuous tube open to the external environment at both ends, and its lumen is technically external to the body. Accessory organs contribute digestive enzymes and fluids to aid in digestion.

Basic Digestive Processes

• Ingestion: Taking food into the mouth.

• Propulsion: Moving food through the GI tract, primarily by peristalsis (alternating contraction and relaxation of smooth muscle).

• Mechanical Breakdown: Physically breaking food into smaller pieces.

• Digestion: Chemical breakdown of food into absorbable molecules.

• Absorption: Transport of digested nutrients into the blood or lymph.

• Defecation: Elimination of indigestible substances as feces.

Peristalsis moves food through the digestive tract, while segmentation mixes food and brings it into contact with the intestinal wall for absorption.

Histological Organization of the GI Tract

Layers of the GI Tract

• Mucosa: Secretes mucus, absorbs nutrients, and protects underlying tissues.

• Submucosa: Contains blood vessels, nerves, and lymphatics.

• Muscularis externa: Responsible for segmentation and peristalsis.

• Serosa/Adventitia: Outermost layer; provides a pathway for vessels and nerves and anchors the tract.

Regulation of Motility

The enteric nervous system (ENS) is a network of neurons in the GI tract wall, including the submucosal and myenteric plexuses. It optimizes absorption and digestion by controlling local conditions. Motility is regulated by short reflexes (ENS only) and long reflexes (involving the CNS).

Digestive Processes in the Oral Cavity

Oral Cavity and Tongue

The oral cavity is the entry point for food, where ingestion, mechanical breakdown, and limited chemical digestion (by salivary amylase) occur. The tongue is a skeletal muscle organ that mixes food with saliva, forms the bolus, and contains taste buds within papillae.

Salivary Glands and Saliva

• Parotid and submandibular glands: Mostly serous cells, secrete watery fluid with enzymes and ions.

• Sublingual gland: Mostly mucous cells, secrete thicker mucus.

• Saliva: 97–99.5% water, contains electrolytes, enzymes (amylase, lingual lipase), and antimicrobial agents (lysozyme, secretory IgA, defensins). Lubricates, cleanses, and moistens food, aiding in bolus formation.

Salivation and Mastication

• Salivation is stimulated by parasympathetic reflexes (cranial nerves VII and IX) in response to food-related stimuli.

• Mastication (chewing) is the first stage of mechanical breakdown, involving teeth, lips, cheeks, and tongue.

Swallowing and Esophageal Function

Phases of Swallowing (Deglutition)

Swallowing is divided into three phases:

1. Buccal phase: Voluntary; the tongue pushes the bolus into the oropharynx.

2. Pharyngeal-esophageal phase (beginning): Involuntary; the tongue blocks the mouth, the uvula blocks the nasal cavity, and the epiglottis blocks the trachea.

3. Pharyngeal-esophageal phase (continues): Pharyngeal muscles contract, pushing food into the esophagus.

4. Esophageal phase: Peristalsis moves the bolus through the esophagus to the stomach. The gastroesophageal sphincter relaxes to allow entry into the stomach.

Stomach Structure and Function

Stomach Mucosa and Gastric Glands

The stomach mucosa contains millions of gastric pits leading to gastric glands, which produce gastric juice. The mucosa is protected by a double layer of alkaline mucus (insoluble top layer and bicarbonate-rich layer below).

Cells of Gastric Glands

• Mucous neck cells: Secrete acidic mucus.

• Parietal cells: Secrete HCl and intrinsic factor.

• Chief cells: Secrete pepsinogen (inactive enzyme).

• Enteroendocrine cells: Secrete paracrines and hormones (serotonin, histamine, somatostatin, gastrin).

Mucosal Barrier and Ulcers

The stomach is protected from its acidic environment by tight junctions, a thick mucus/bicarbonate layer, and rapid epithelial renewal. Failure of this barrier can lead to gastric ulcers, often caused by Helicobacter pylori infection.

Stomach Digestive Processes

• Propulsion and mechanical breakdown: Churning mixes food with gastric juice.

• Chemical digestion: Protein digestion begins with pepsin and HCl.

• Absorption: Limited to alcohol and aspirin.

• Intrinsic factor: Essential for vitamin B12 absorption; deficiency leads to pernicious anemia.

Regulation of Gastric Secretion

Phases of Gastric Secretion

• Cephalic phase: Triggered by sight, smell, taste, or thought of food; mediated by the vagus nerve.

• Gastric phase: Triggered by stomach distension and partially digested proteins; involves neural and hormonal (gastrin) stimulation.

• Intestinal phase: Begins when chyme enters the duodenum; initially stimulates, then inhibits gastric secretion via enterogastric reflex and hormones (secretin, CCK).

Mechanism of HCl Production

Parietal cells generate HCl by the following reaction:

$\mathrm{CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+}$

Proton pumps move H+ into the lumen in exchange for K+, while HCO3- is exported to the blood (alkaline tide) and Cl- is swapped in.

Gastric Motility

• Gastric filling: Stomach stretches to accommodate food (receptive relaxation and gastric accommodation).

• Gastric churning: Peristaltic waves mix and propel chyme toward the pyloric sphincter; only small amounts enter the duodenum at a time.

Small Intestine Structure and Function

Surface Area Adaptations

The small intestine is the major site of digestion and absorption. Its surface area is increased by circular folds, villi, and microvilli (brush border).

Cells of the Small Intestine Mucosa

• Enterocytes: Absorptive cells with microvilli.

• Goblet cells: Secrete mucus.

• Enteroendocrine cells: Release hormones (enterogastrones).

• Paneth cells: Secrete antimicrobials.

• Stem cells: Rapidly divide to renew the epithelium.

Most digestive enzymes in the small intestine are imported from the pancreas, except for brush border enzymes.

Motility in the Small Intestine

• Post-meal: Segmentation mixes chyme and brings it into contact with the epithelium; weak peristaltic waves move food forward slowly.

• Between meals: The migrating motor complex (MMC) moves indigestible matter toward the ileocecal valve.

Large Intestine Structure and Function

Functions of the Large Intestine

• Absorbs water and electrolytes from indigestible food residues.

• Stores fecal material until defecation.

• Hosts bacterial flora that ferment plant fibers, produce vitamins, and outcompete pathogens.

Motility and Defecation

• Haustral contractions: Mix contents and aid absorption.

• Mass movements: Propel feces toward the rectum, stimulated by the gastrocolic reflex.

• Defecation: Involuntary spinal reflex contracts the rectum and relaxes the internal anal sphincter; voluntary control relaxes the external anal sphincter.

Accessory Digestive Organs

Pancreas

The exocrine pancreas contains acinar cells that secrete inactive digestive enzymes (proteases, lipases, amylase, nucleases) and duct cells that secrete bicarbonate and water. Pancreatic juice is delivered to the duodenum to aid digestion.

Liver and Gallbladder

• Liver: Metabolizes nutrients, detoxifies substances, stores vitamins, and produces bile for fat digestion.

• Bile: Contains bile salts (emulsify fats), phospholipids, bile pigments (bilirubin), cholesterol, and triglycerides.

• Gallbladder: Stores and concentrates bile; releases it into the duodenum when needed.

Enterohepatic Circulation of Bile Salts

Bile salts are reabsorbed in the ileum and returned to the liver via the hepatic portal vein, allowing for recycling and efficient fat digestion.

Regulation of Bile and Pancreatic Juice Secretion

• CCK (cholecystokinin): Released in response to fats and proteins in the duodenum; stimulates pancreatic enzyme secretion, gallbladder contraction, and relaxation of the hepatopancreatic sphincter.

• Secretin: Released in response to acidic chyme; stimulates pancreatic duct cells to release bicarbonate-rich juice.

Additional info: The digestive system is regulated by a complex interplay of neural and hormonal signals to optimize digestion and absorption while protecting the body from harsh luminal conditions.