Digestive System Notes

Chapter 24: Digestive System

Lecture Outline

• The digestive tract in humans includes structures for:

  • Ingestion of food.

  • Breakdown of food into nutrients.

  • Absorption of nutrients.

  • Removal of wastes.

• Major digestive processes in different organs:

  • Oral Cavity: Ingestion, mastication.

  • Pharynx: Swallowing.

  • Esophagus: Swallowing.

  • Stomach: Storage, mixing, digestion.

  • Small Intestine: Mixing, digestion, absorption.

  • Large Intestine: Storage, elimination.

24.1 Anatomy of the Digestive System

• The digestive system consists of the digestive tract and accessory organs.

• Digestive Tract (Gastrointestinal or GI tract, alimentary canal): A tube from the mouth to the anus.

• Accessory Organs: Glands that secrete substances into the digestive tract.

• Major Digestive Organs:

  • Oral cavity (mouth)

  • Pharynx (throat)

  • Esophagus

  • Stomach

  • Small intestine

  • Large intestine

  • Rectum

  • Anus

• Accessory Digestive Organs:

  • Salivary glands

  • Liver

  • Gallbladder

  • Pancreas

24.2 Functions of the Digestive System

1. Ingestion and Mastication:

   • Ingestion: Introduction of food into the oral cavity.

   • Mastication: Chewing to mechanically break down foods.

2. Propulsion and Mixing:

   • Propulsion: Movement of food through the GI tract.

   • Mixing: Movement of food back and forth.

   • Swallowing (Deglutition): moves food/liquid called a bolus from oral cavity into esophagus.

   • Peristalsis: Moves material through the digestive tract.

     • A wave of circular smooth muscle relaxation moves ahead of the bolus of food or chyme, allowing the digestive tract to expand.

     • A wave of contraction of the circular smooth muscles behind the bolus of food or chyme propels it through the digestive tract.

   • Mass movements in the large intestine.

   • Two major types of mixing:

     • Mixing waves in the stomach.

     • Segmental contractions in the small intestine.

3. Secretions: Lubricate, liquefy, digest.

   • Mucus: Secreted along the entire digestive tract.

     • Lubricates food and lining.

     • Coats lining and protects from mechanical digestion, acid, and digestive enzymes.

   • Water: Liquefaction makes food easier to digest and absorb.

   • Enzymes: Chemical digestion.

4. Digestion: Mechanical and chemical.

5. Absorption: Movement from tract into blood or lymph.

6. Elimination: Waste products (feces) removed from the body through the process of defecation.

Peristalsis

1. Peristaltic waves are muscular contractions consisting of a wave of relaxation of the circular muscles in front of the bolus.

2. This is followed by a wave of strong contraction of the circular muscles behind the bolus, which force the bolus along the digestive tract. Each peristaltic wave travels the length of the esophagus in about 10 seconds. Peristaltic waves in the small and large intestines usually travel only short distances.

Segmental Contractions

1. A mass of chyme within the tract begins at one location.

2. Segments of the digestive tract alternate between contraction and relaxation.

3. The mass of chyme spreads out in both directions.

Digestive Functions of the Digestive Tract

• Oral Cavity

  • Ingestion and Mastication: Solid food and fluids are taken into the digestive tract through the oral cavity. The teeth break food into smaller pieces.

  • Propulsion and Mixing: The tongue forms food into a bolus and pushes the bolus into the pharynx to begin the swallowing reflex.

  • Digestion and Secretion: Mastication begins mechanical digestion of food. Amylase in saliva begins carbohydrate (starch) digestion.

  • Absorption: There is no absorption of nutrients in the mouth, although some drugs can be absorbed across the oral mucosa.

• Pharynx

  • Propulsion and Mixing: Swallowing moves the bolus from the oral cavity to the esophagus. Materials are prevented from entering the nasal cavity by the soft palate and kept out of the lower respiratory tract by the epiglottis and vestibular folds.

  • Secretion: Mucus provides lubrication.

• Esophagus

  • Propulsion and Mixing: Peristaltic contractions move the bolus from the pharynx to the stomach. The lower esophageal sphincter limits reflux of the stomach contents into the esophagus.

  • Secretion: Mucus provides lubrication and protects the inferior esophagus from stomach acid.

• Stomach

  • Propulsion and Mixing: Mixing waves churn ingested materials and stomach secretions into chyme. Rugae allow the stomach to expand and store food. Peristaltic waves move the chyme into the small intestine.

  • Secretion: Release of hydrochloric acid creates the acidic stomach environment. The acid kills most microorganisms and activates the precursor of the proteolytic enzyme pepsin. Mucus provides lubrication and prevents digestion of the stomach wall.

  • Digestion: Mechanical digestion occurs as food is churned in the stomach by mixing waves. Protein digestion begins as a result of the actions of hydrochloric acid and pepsin.

  • Absorption: Absorption of only a few substances (e.g, water, alcohol, aspirin) takes place in the stomach.

• Small Intestine

  • Propulsion and Mixing: Segmental contractions mix the chyme, and peristaltic contractions move the chyme into the large intestine.

  • Secretion: Bicarbonate ions from the pancreas and bile from the liver neutralize stomach acid to form a pH environment suitable for pancreatic and intestinal enzymes. Mucus provides lubrication, prevents digestion of the intestinal wall, and protects the small intestine from stomach acid.

  • Digestion: Segmental contractions aid mechanical digestion. Enzymes from the pancreas and the lining of the small intestine complete the breakdown of food molecules. Bile salts from the liver emulsify lipids to allow lipid digestion.

  • Absorption: The circular folds, villi, and microvilli increase surface area. Most nutrients are actively or passively absorbed. Most of the ingested water or the water in digestive tract secretions is absorbed.

• Large Intestine

  • Propulsion and Mixing: Slight segmental mixing occurs. Mass movements propel feces toward the anus, and defecation eliminates the feces.

  • Secretion: Mucus provides lubrication; mucus and bicarbonate ions protect against acids produced by bacteria.

  • Absorption: The first half of the colon absorbs salts (e.g., NaCl), water, and vitamins (e.g., K) produced by bacteria.

  • Elimination: The second half of the colon holds feces until they are eliminated.

24.3 Histology of the Digestive Tract

The digestive tract has four main layers:

1. Mucosa: Innermost layer.

   • Epithelium:

     • Moist stratified squamous epithelium in the mouth, oropharynx, esophagus, and anal canal.

     • Simple columnar epithelium elsewhere. Extends into lamina propria to form glands or crypts.

   • Lamina Propria: Loose connective tissue.

   • Muscularis Mucosae: Smooth muscle.

2. Submucosa: Connective tissue with nerves, blood vessels, lymphatics, and small glands.

   • Submucosal plexus: Network of neurons that control intestinal gland secretions.

3. Muscularis: Includes circular and longitudinal layers of smooth muscle.

   • Myenteric plexus: Controls GI movements.

   • Enteric nervous system: Composed of submucosal and myenteric plexuses.

4. Serosa or Adventitia:

   • Serosa: Where the visceral peritoneum is present.

   • Adventitia: Where the outer layer is derived from adjacent connective tissue.

24.4 Regulation of the Digestive System

• Nervous Regulation:

  • Local: Enteric Nervous System (ENS).

    • Sensory, motor, and interneurons within the wall of the digestive tract.

    • Myenteric and submucosal plexuses.

    • Controls motility and secretions via local reflexes.

  • CNS modulates ENS: Sight, smell, and taste of food.

    • Parasympathetic primarily (via the vagus nerve).

    • Sympathetic inhibits motility, secretions, and decreases blood flow to the GI tract.

  • Major ENS Neurotransmitters:

    • Acetylcholine (stimulatory).

    • Norepinephrine (inhibitory).

    • Serotonin: stimulates gut motility (95% of body's serotonin is found in the GI tract).

• Chemical Regulation:

  • Hormones: Gastrin, secretin (discussed later).

  • Paracrine Chemicals: Help control the internal environment of the GI tract.

    • Histamine: Helps regulate pH.

24.5 Peritoneum and Mesenteries

• Peritoneum: Serous membrane.

  • Visceral: Covers organs.

  • Parietal: Covers the interior surface of the body wall.

• Mesenteries: Two layers of peritoneum with a thin layer of loose connective tissue between them.

  • Lesser Omentum: Connects the lesser curvature of the stomach to the liver and diaphragm.

  • Greater Omentum: Connects the greater curvature of the stomach to the transverse colon.

    • Omental bursa: Cavity between the two layers of the greater omentum.

  • Coronary Ligament: Attaches the liver to the diaphragm.

  • Falciform Ligament: Attaches the liver to the anterior abdominal wall.

• Retroperitoneal: Organs behind the peritoneum (e.g., kidneys, pancreas, duodenum).

• Peritonitis: Inflammation of the peritoneum.

• Ascites: Accumulation of serous fluid in the peritoneal cavity.

24.6 Oral Cavity

• Moist stratified squamous epithelium (non-keratinized).

• Vestibule: Between lips/cheeks and alveolar processes.

• Oral Cavity Proper: Medial to alveolar processes.

• Lips (Labia): Orbicularis oris muscle within. Keratinized stratified squamous epithelium is thin; blood in the dermis gives the red/pink color.

  • Labial Frenulum: Fold of tissue anchors lips to alveolar processes of the maxilla and mandible.

• Cheeks: Lateral walls of the oral cavity with buccinator muscle and buccal fat pad.

• Lips, cheeks - important for mastication and speech.

• Palate: Separates nasal and oral cavities.

  • Hard Palate: Anterior (maxilla, palatine bone).

  • Soft Palate: Posterior; skeletal muscle, connective tissue.

    • Uvula: Projects from the posterior of the soft palate.

  • Fauces: Posterior boundary of the oral cavity.

    • Palatine tonsils in lateral walls.

Tongue

• Muscular structure covered with moist stratified squamous epithelium.

• Moves food in the mouth and participates in speech and swallowing.

• Intrinsic Muscles: Change shape.

• Extrinsic Muscles: Protrude/retract tongue, move side to side.

• Lingual Frenulum: Attaches tongue inferiorly to the floor of the oral cavity.

• Terminal Sulcus: Groove divides the tongue into anterior 2/3 and posterior 1/3.

  • Anterior part: papillae with taste buds.

  • Posterior part: no papillae, few scattered taste buds, lymphoid tissue - lingual tonsil.

Teeth

• Dentition: Involved in mastication and speech.

• Two sets of teeth:

  • Primary/Deciduous: Childhood (20).

  • Permanent/Secondary: Adult (32).

• Distributed in two dental arches.

• Each quadrant has:

  • 2 incisors

  • 1 canine

  • 2 premolars

  • 3 molars

• Incisors and canines cut off food; premolars and molars grind.

Teeth Structure

• Crown: Enamel-covered part.

  • Cusps: Points.

• Neck: Below gum line.

• Root: Anchors tooth in bone.

• Enamel: Outermost layer of the crown. Non-living; acellular. Protective.

• Dentin: Living, cellular, calcified tissue.

• Pulp Cavity: Filled with blood vessels, nerves, and connective tissue.

  • Pulp of the root called the root canal.

• In the root, dentin is covered by cementum - a bone-like structure that helps hold the tooth in the socket.

• Periodontal Ligaments: Hold tooth in socket.

• Gingiva: Dense, fibrous connective tissue covered by stratified squamous epithelium.

Mastication

• Mastication: Chewing.

• Mastication Reflex: Controlled by the medulla oblongata, but descending pathways from the cerebrum provide conscious control.

  • Controls basic movements involved in chewing.

  • Muscles involved: masseter, temporalis, medial and lateral pterygoids.

Salivary Glands

• Salivary glands produce saliva.

• Three large paired multicellular glands:

  • Parotid: Largest. (Serous.)

    • Anterior to the ear. The duct crosses over the masseter, penetrates the buccinator, and enters the oral cavity adjacent to the 2nd upper molar.

  • Submandibular: (Mixed.)

    • The duct enters the oral cavity on either side of the lingual frenulum.

  • Sublingual: Smallest. (Mucous.)

    • Each has 10 to 12 ducts that enter the floor of the oral cavity.

• Other small salivary glands, including lingual glands on the surface of the tongue.

• Parasympathetic input stimulates saliva production (facial and glossopharyngeal CN).

• Tactile stimulation; thoughts, smell, and sight of food

Functions of Saliva

• Mucus lubricates and helps form a bolus for swallowing.

• Digestive Functions:

  • Salivary amylase breaks down starch.

  • Lingual lipase for lipid digestion.

• Protective Functions:

  • Saliva continually washes the oral surface.

  • Bicarbonate (HCO_3) neutralizes acid produced by oral bacteria.

  • Lysozyme is an enzyme that has antibacterial action.

  • Immunoglobulin A (IgA) antibody.

• Any reduction in salivary gland secretion increases the risk for infection of the oral mucosa and dental caries (cavities).

Functions of Major Digestive Systems Secretions

Oral Cavity

• Serous Saliva: Composed mostly of water, HCO_3

  • Moistens food and mucous membrane; neutralizes bacterial acids; flushes bacteria from the oral cavity; has weak antibacterial activity.

• Salivary Amylase: Digests carbohydrates.

• Mucus:

  • Lubricates food.

  • Protects the digestive tract from digestion.

• Lingual Lipase: Digests a minor amount of lipids.

Pharynx

• Mucus:

  • Lubricates the esophagus.

  • Protects the lining of the esophagus from abrasion and allows food to move more smoothly through the esophagus.

Gastric

• Hydrochloric Acid (HCl):

  • Antibacterial.

  • Decreases stomach pH to activate pepsinogen to pepsin conversion.

• Pepsin: Secreted in an inactive form and then activated by HCl; digests protein into smaller peptide chains; activates additional pepsinogen.

• Mucus: Protects the stomach lining from acids and digestion.

• Intrinsic Factor: Binds to vitamin B12 and aids in its absorption in the small intestine.

• Gastric Lipase: Digests a minor amount of lipids.

Pancreas

• Zymogens

  • Trypsin: Digests proteins (breaks bonds at amino acids lysine or arginine); activates additional trypsinogen as well as other digestive enzymes

  • Chymotrypsin: Digests proteins (breaks bonds at hydrophobic amino acids)

  • Carboxypeptidase: Digests proteins (breaks bonds between amino acids at carboxyl end of proteins)

• Other Enzymes and Components

  • Pancreatic Amylase: Digests carbohydrates (hydrolyzes starches and glycogen to form maltose and isomaltose

  • Pancreatic Lipase: Digests lipids (breaks down triglycerides into monoglycerides and free fatty acids)

  • Cholesterol Esterase: Digests cholesteryl esters (breaks them down into cholesterol and a free fatty acid)

  • Ribonuclease: Digests ribonucleic acid (breaks phosphodiester bonds)

  • Deoxyribonuclease: Digests deoxyribonucleic acid (breaks phosphodiester bonds)

  • HCO_3: Neutralizes acid from stomach to provide optimal pH for pancreatic enzymes

Liver

• Bile: Bile salts in bile emulsify lipids, making them available to lipases, and help make end products soluble and available for absorption by the intestinal mucosa; many of the other bile contents are waste products, such as bile pigments, that are transported to the intestine for disposal

Small Intestine

• Mucus: Protects duodenum from acidic chyme and intestinal wall from digestive enzymes

• Brush Border Enzymes

  • Peptidases: Break peptide bonds between amino acids of polypeptides

  • Enterokinase: Converts trypsinogen to trypsin

  • Sucrase: Breaks apart sucrose into glucose and fructose

  • Maltase: Breaks apart maltose into two glucose molecules

  • Isomaltase: Breaks apart isomaltose into two glucose molecules

  • Lactase: Breaks apart lactose into glucose and galactose

Large Intestine

• Mucus: Provides adhesion for fecal matter; protects intestinal wall from bacterial acids and actions

24.7 Swallowing

• Three phases of swallowing:

  1. Voluntary: bolus of food moved by tongue from oral cavity to oropharynx.

  2. Pharyngeal: reflex. Controlled by swallowing center in medulla oblongata.

     • The soft palate elevates, and the epiglottis tips posteriorly due to the pressure of the bolus; the larynx elevates to prevent food from entering.

     • Upper esophageal sphincter relaxes.

     • Food is pushed into the esophagus by successive contraction of pharyngeal constrictor muscles (superior to inferior) in the posterior wall of the oropharynx and laryngopharynx.

  3. Esophageal: reflex. Stretching of the esophagus causes the enteric NS to initiate peristalsis of esophageal muscles.

• Esophagus:

  • Transports food from the pharynx to the stomach.

  • Passes through the esophageal hiatus (opening) of the diaphragm.

  • Sphincters: Upper and lower.

  • The mucosa is moist stratified squamous epithelium.

  • Produces a thick layer of mucus.

24.8 Stomach

• Enlarged part of the GI tract functions as a storage and mixing chamber.

• Regions:

  • Cardia: The esophagus opens into this part.

  • Fundus: Above the cardia.

  • Body: Largest part with greater and lesser curvatures.

  • Pylorus: Pyloric antrum (widest part) and pyloric canal - opens to the small intestine through the pyloric orifice.

• Sphincters:

  • Cardiac (lower esophageal).

  • Pyloric.

Anatomy and Histology of the Stomach

• Layers:

  • Mucosa

  • Submucosa

  • Muscularis: Three layers.

    • Outer longitudinal.

    • Middle circular.

    • Inner oblique.

  • Serosa - visceral peritoneum (simple squamous epithelium overlying CT).

  • Rugae: Folds in mucosa and submucosa of the stomach when empty. Allow stretching as the stomach fills.

Histology of the Stomach: Gastric Glands and Cell Types

• Gastric Pits: Openings for gastric glands.

  • Lined with simple columnar epithelium.

• Cells:

  • Surface Mucous: Mucus that protects the stomach lining from acid and digestive enzymes.

  • Mucous Neck: Mucus.

  • Parietal: Hydrochloric acid and intrinsic factor.

  • Chief: Pepsinogen and gastric lipase.

  • Endocrine: Regulatory hormones.

    • Gastrin - stimulates enzyme and acid secretion

    • Histamine - stimulates acid secretion.

    • Somatostatin - inhibits gastrin and insulin secretion.

    • (Different types of endocrine cells produce each.)

Secretions of the Stomach

• HCl: Parietal Cells

  • Kills bacteria.

  • Denatures proteins.

  • Helps convert pepsinogen to pepsin.

  • Stops carbohydrate digestion by inactivating salivary amylase.

• Intrinsic Factor: Parietal Cells

  • Binds vitamin B12 and helps it to be absorbed. B12 is necessary for DNA synthesis.

• Mucus: Surface and Neck Mucous Cells

  • Viscous and alkaline.

  • Protects from acidic chyme and enzyme pepsin.

  • Irritation of stomach mucosa causes greater mucus secretion.

• Digestive Enzymes: Chief Cells

  • Pepsinogen: Zymogen granules released by exocytosis.

    • Pepsin catalyzes the breaking of covalent bonds in proteins.

  • Gastric lipase.

• Chyme: Ingested food plus stomach secretions.

Regulation of Stomach Secretions: Cephalic Phase (