Chapter 24

Digestive System Study Notes

Chapter 24 Overview

• Source: © McGraw Hill, LLC

• Contributors: ChatGPT & Andrew Seely

Anatomy of the Digestive System

1. Digestive Tract (Gastrointestinal/GI Tract, Alimentary Canal)

   • Definition: A continuous tube that extends from the mouth to the anus.

   • Components Include:

     • Oral Cavity (Mouth)

     • Pharynx

     • Esophagus

     • Stomach

     • Small Intestine

     • Large Intestine

2. Accessory Organs

   • Tongue

   • Teeth

   • Salivary Glands

   • Liver

   • Gallbladder

   • Pancreas

   • Function: Glands that secrete substances into the digestive tract.

Major Digestive Organs

Functions of the Digestive System

1. Ingestion & Mastication

2. Propulsion & Mixing

   • Swallowing (Deglutition): Moves food/liquid (bolus) from the oral cavity to the esophagus.

   • Peristalsis: Moves material throughout the digestive tract via muscular contractions.

     • Mechanism: Circular smooth muscles relax in front of the bolus/chyme; contractions behind propel it.

   • Mixing: Carried out by mixing waves in the stomach and segmental contractions in the small intestine and mass movements in the large intestine.

3. Secretion:

   • Function: Lubricate, liquefy, digest.

   • Components:

     • Mucus: Lubricates food, coats lining, protects from mechanical digestion, acids, and enzymes.

     • Water: Liquefies food, eases digestion/absorption.

     • Enzymes: Facilitate chemical digestion.

4. Digestion: Mechanical and chemical processes that break down food into absorbable units.

5. Absorption: Movement of nutrients from the digestive tract into the blood or lymph.

6. Elimination: Removal of waste products from the body through defecation (feces).

Peristalsis

1. Definition: Muscular contractions with waves of circular muscle relaxation ahead of the bolus.

2. Process:

   • Strong contraction of circular muscles behind the bolus, forcing it through the digestive tract.

   • Travel Time: About 10 seconds through the esophagus; short distances in small and large intestines.

Segmental Contractions

1. Process: As the bolus mass begins at one location, segments of the digestive tract alternate between contraction and relaxation, spreading the bolus in both directions.

Digestive Functions by Organ System

Oral Cavity

• Ingestion and Mastication: Solid food and fluids introduced into the digestive tract.

  • Teeth break food into smaller pieces.

• Propulsion and Mixing: Tongue forms bolus and initiates swallowing.

• Digestion and Secretion: Mechanical digestion via mastication; salivary amylase begins carbohydrate digestion.

• Absorption: No nutrient absorption; however, some drugs can be absorbed across the oral mucosa.

Pharynx

• Propulsion and Mixing: Swallowing moves bolus from the oral cavity to the esophagus; prevents material from entering the nasal cavity with the aid of the soft palate and epiglottis.

• Secretion: Mucus lubricates food.

Esophagus

• Propulsion and Mixing: Peristaltic contractions move bolus to the stomach; the lower esophageal sphincter prevents reflux.

• Secretion: Mucus lubricates and protects the esophagus from stomach acid.

Stomach

• Propulsion and Mixing: Mixing waves churn ingested materials into chyme; rugae allow expansion and storage; peristaltic waves move chyme into the small intestine.

• Secretion:

  • Hydrochloric acid (HCl) creates an acidic environment, kills microorganisms, and activates pepsinogen to pepsin (digests proteins).

  • Mucus lubricates the stomach and prevents self-digestion.

• Digestion: Mechanical digestion occurs, and protein digestion begins with HCl and pepsin.

• Absorption: Water, alcohol, and aspirin are absorbed in small quantities.

Small Intestine

• Propulsion and Mixing: Segmental contractions mix chyme; peristalsis moves it toward the large intestine.

• Secretion: Bicarbonate ions and bile neutralize stomach acid; mucus protects the intestinal wall.

• Digestion: Enzymes from the pancreas and intestinal lining complete food digestion; bile salts emulsify lipids.

• Absorption: Circular folds, villi, and microvilli greatly increase surface area for absorption.

Large Intestine

• Propulsion and Mixing: Mild segmental mixing occurs; mass movements drive feces toward the anus.

• Secretion: Mucus provides lubrication, and bicarbonate ions protect against bacteria-produced acids.

• Absorption: Absorbs salts, water, and vitamins produced by bacteria.

• Elimination: Stores feces until defecated.

Histology of the Digestive Tract

1. Mucosa: The innermost layer consists of:

   • Mucous Epithelium: Nonkeratinized stratified squamous in mouth, oropharynx, esophagus, anal canal; simple columnar in rest of GI tract.

   • Lamina Propria: Loose connective tissue.

   • Muscularis Mucosae: Smooth muscle, extends into the lamina propria to form intestinal glands or crypts.

   • Specialized Cells: Mechanoreceptors and chemoreceptors.

2. Submucosa: Thick connective tissue layer containing nerves, blood vessels, and lymphatics.

   • Submucosal Plexus (Meissner Plexus): Network of neurons that control intestinal secretion.

   • Note: Esophagus and stomach lack submucosal plexus.

3. Muscularis: Composed of 2-3 layers of smooth muscle (circular and longitudinal layers) and contains Myenteric Plexus (Auerbach Plexus) which controls GI tract movements.

4. Serosa or Adventitia: Connective tissue layer; serosa along visceral peritoneum and adventitia along the outer layer.

Regulation of the Digestive System

1. Nervous Regulation:

   • Local Regulation: Enteric nervous system coordinates peristalsis.

   • General Regulation: CNS input affects digestive reflexes; primarily parasympathetic (vagus nerves) enhances function.

   • Sympathetic Input: Inhibits muscle contraction, secretion, and blood flow.

2. Chemical Regulation:

   • Neurotransmitters: Over 30 associated with the enteric nervous system including acetylcholine (stimulates) and norepinephrine (inhibits).

   • Hormones: Gastrin and secretin influence motility and secretions.

   • Paracrine Chemicals: Such as histamine, which aid local reflexes controlling digestion.

Key Gastrointestinal Reflexes

Mastication & Salivary Glands

• Mastication: Cutting and grinding with three pairs of salivary glands:

  • Parotid: Largest gland, secretes serous saliva.

  • Submandibular: Produces saliva that is more viscous.

  • Sublingual: Smallest gland, secretes mucus saliva.

• Other glands include those in the palate, cheeks, and lips.

Swallowing Phases

1. Voluntary Phase: Tongue moves food bolus from the oral cavity to the pharynx.

2. Pharyngeal Phase: Reflex action controlled by medulla oblongata; the soft palate elevates, and the upper esophageal sphincter relaxes.

3. Esophageal Phase: Stretching of the esophagus triggers peristalsis controlled by the enteric nervous system.

Stomach Anatomy

• Location of Lower Esophageal Sphincter: Structure preventing reflux.

• Parts of Stomach:

  • Cardia

  • Fundus

  • Body

  • Pylorus (with pyloric sphincter controlling passage to the duodenum).

• Mucosa structure: Simple columnar epithelium with specialized cells in gastric pits that produce gastric juice.

Hydrochloric Acid Production

1. Main Steps in HCl Production in Parietal Cells:

   • CO2 uptake, carbonic acid formation, dissociation into H+ and bicarbonate.

   • H+ ions are exchanged for potassium ions, and Cl- enters the gastric duct.

Regulation of Stomach Secretions

1. Cephalic Phase: Triggered by taste, smell, or thought of food, stimulates secretion via vagus nerve and activates hormones like gastrin.

2. Gastric Phase: Triggered by stomach distension, further stimulating secretions through local reflexes and gastrin.

3. Intestinal Phase: Presence of chyme in the duodenum inhibits gastric secretions via hormones like secretin and cholecystokinin.

Functions of Major Gastrointestinal Hormones

Movements in the Stomach

• Types of Movements:

  1. Weak Contractions: Mix food to form chyme.

  2. Peristaltic Waves: Stronger contractions occur at a rate of three per minute, pushing chyme towards the pyloric sphincter.

• The pyloric sphincter typically remains closed but permits a small amount of chyme to pass into the duodenum with each contraction.

Regulation of Stomach Emptying

• Stomach typically empties in 3-4 hours after a meal, but rapid emptying can reduce absorption efficiency and harm the duodenum.

• Various hormonal and neural mechanisms govern the process, including enterogastric reflex and cholecystokinin which inhibit gastric motility and secretions.

Small Intestine Structure

• Primary Site for digestion and absorption of nutrients:

  • Segments:

    • Duodenum: First 25 cm beyond pyloric sphincter.

      • where most digestion occurs

    • Jejunum: 2.5 m long.

    • Ileum: 3.5 m long.

      • peyers patches: lymphatic nodules of mucosa and submucosa

    • ileocecal junction: cotnains ring of smooth muscle (ileocecal sphincter) and one-way oleocecal

  • Major site of nutrient absorption

Small Intestine Histology

1. Circular Folds/Plicae: Folds of mucosa and submucosa aid in absorption.

2. Villi: Finger-like projections covered by simple columnar epithelium, contain blood capillaries and lacteals.

3. Microvilli: Cytoplasmic extensions of epithelium that increase surface area for absorption.

Functions of the Liver

1. Bile Production: For digestion and excretion, neutralizes stomach acid, emulsifies lipids.

2. Nutrient Storage: Stores glycogen, fats, vitamins, and minerals.

3. Processing of Nutrients: Converts amino acids and vitamins to usable forms.

4. Detoxification: Converts ammonia to urea and removes debris from blood.

5. Synthesis of New Molecules: Produces essential proteins like albumin and clotting factors.

Secretions:

• fluid primarily composed of water, electrolytes and mucus

• mucus secreted by duodenal glands, intestinal glands, and goblet cells

  • protects against digestive enzymes and stomach acids

• electrolytes and water from epithelium keep chyme in aqueous solution to facicilate chemical digestion

• digestive enzymes bound to membranes of absorptive cells

  • disaccharidases

  • peptidases

Gallbladder Function

• Bile storage, concentration, and release regulated by hormones (e.g. CCK).

• Common issues include gallstones that can form from cholesterol.

Bile Secretion Control

1. Flow of Bile: Path from the liver through ducts and gallbladder to the duodenum for fat digestion.

2. Bile Production Mechanism: Interactions between hormones and neural inputs regulate flow.

Pancreatic Secretions

• Enzymes secreted play critical roles in digestion:

  • Trypsinogen, Chymotrypsinogen, Pancreatic Amylase, Lipase, among others.

• Secreted in response to hormones like secretin and CCK.

Large Intestine Structure & Function

• Sections include cecum, colon, rectum, and anal canal, serving to absorb water and salts and convert chyme to feces.

• Secretions: Primarily mucus for lubrication; interaction with microbiota aids digestion.

• Motility: Mass movements initiated by stretch receptors and reflexes, including the gastrocolic reflex.

Representative Diseases and Disorders of the Digestive System