The Digestive System

Chapter 23: The Digestive System

Part 1: Overview

Definition of Digestion

  • Digestion: The process by which food substances are changed into forms that can be absorbed through cell membranes.

Digestive System Structures

  • Gastrointestinal (GI) Tract:

    • Also known as the alimentary canal.

    • Continuous tube from mouth to anus through which food passes, considered "outside" the body.

    • Digestive Structures (in order):

    1. Mouth

    2. Pharynx

    3. Esophagus

    4. Stomach

    5. Small intestine

    6. Large intestine

    7. Anus

Overview of the Digestive System

  • Accessory Digestive Structures:

    • Assist in digestion but are not part of the GI tract itself.

    • Includes:

    • Teeth

    • Tongue

    • Gallbladder

    • Salivary glands

    • Liver

    • Pancreas

Digestive Processes (7 Steps)

  1. Ingestion: Taking food into the digestive tract, typically through the mouth.

  2. Propulsion: Movement of food through the GI tract.

    • Includes:

      • Swallowing

      • Peristalsis

  3. Secretion: Release of water, acid, buffers, and enzymes into the lumen of the GI tract.

  4. Mechanical Digestion: Physical breakdown of food.

    • Includes:

      • Chewing (mastication)

      • Mixing with saliva

      • Churning food in the stomach

      • Segmentation (in intestines)

  5. Chemical Digestion: Catabolism of complex to simple forms.

    • Enzymatic breakdown of food; hydrolysis.

  6. Absorption: Uptake of broken down foods from intestinal cells to the bloodstream, to the liver, and finally to the cells.

  7. Defecation: Elimination of indigestible substances (solid wastes; feces).

Digestive Process Overview

Digestive System: Histology

  1. Mucosa:

    • Innermost lining of GI organs.

    • Composed of:

      • Epithelium

      • Lamina propria (areolar connective tissue, MALT, blood vessels, lymph vessels).

      • Thin smooth muscle.

  2. Submucosa:

    • Areolar connective tissue.

    • Contains blood vessels and autonomic nerve plexuses.

  3. Muscularis Externa:

    • Composed of inner circular smooth muscle and outer longitudinal smooth muscle.

  4. Serosa:

    • Connective tissue covered by simple squamous epithelium.

    • Peritoneum:

      • Visceral: Mesothelium on the organ surface.

      • Parietal: Mesothelium on the abdominal wall.

Part 2: Primary Organs of the Digestive System (Mouth to Small Intestine)

Organs of the Digestive System: The Mouth

  • Functions:

    1. Receives food.

    2. Mechanical digestion (chewing).

    3. Chemical digestion (via saliva).

  • Structure: Surrounded by cheeks, lips, tongue, palates, and uvula.

Organs of the Digestive System: Teeth

  • Types of Teeth:

    • 20 Primary (Deciduous) teeth.

    • 32 Secondary (Permanent) teeth.

Organs of the Digestive System: Salivary Glands

  • Salivary Glands: Release saliva controlled by the autonomic nervous system.

    • Types:

    • Parotid

    • Submandibular

    • Sublingual

  • Functions of Saliva:

    1. Lubrication.

    2. Binding food together.

    3. Begins digestion of carbohydrates via salivary amylase.

    4. Cleans teeth.

Organs of the Digestive System: Tongue

  • Structure:

    • Intrinsic muscles change the tongue's shape; extrinsic muscles alter its position.

    • Lingual Frenulum: Attachment to the floor of the mouth.

  • Functions:

    1. Repositioning and mixing food during chewing.

    2. Formation of the bolus.

    3. Initiation of swallowing, speech, and taste.

Tongue Surface Papillae

  1. Filiform: Whitish, give roughness, provide friction.

  2. Fungiform: Reddish, scattered across the tongue.

  3. Circumvallate (Vallate): V-shaped row in the back of the tongue.

  4. Foliate: Located on the lateral aspects of the posterior tongue.

Organs of the Digestive System: Pharynx

  • Structure:

    • Pharynx = throat; passageway for food into esophagus and air into larynx.

  • Functions: Swallowing:

    • Chew food, mix with saliva into a bolus at the back of the pharynx, then involuntary swallowing reflex is triggered.

4 Steps of Swallowing (Deglutition)

  1. Buccal Phase:

    • Compression of the bolus against the hard palate.

    • Tongue forces bolus against the soft palate, sealing off the nasopharynx.

    • Bolus enters the oropharynx.

  2. Pharyngeal Phase:

    • Swallowing reflex stimulated by the uvula.

    • Swallowing center in medulla triggers muscular contractions.

    • Bolus passes over epiglottis and into the esophagus.

    • Process takes less than 1 second; during this, breathing stops.

  3. Esophageal Phase:

    • Bolus pushed toward stomach via peristalsis.

    • Approach of bolus triggers the opening of the lower esophageal sphincter.

  4. Bolus Enters Stomach:

Organs of the Digestive System: Esophagus

  • Structure: Located behind the trachea; serves as a passageway for food bolus from the pharynx to the stomach.

  • Function: Move food to the stomach via:

    1. Food pushed from mouth to esophagus by the tongue.

    2. Food moves toward the stomach by gravity and peristaltic waves from the esophagus meeting gastroesophageal sphincter muscle.

    3. Sphincter relaxes, allowing bolus to move into the stomach.

Organs of the Digestive System: The Stomach

  • General Structure:

    • J-shaped pouch located under diaphragm [left side]; capacity ~ 1 liter.

    • Named Structures:

    1. Cardia: Smallest portion with abundant mucous glands.

    2. Fundus: Contains gastric glands.

    3. Body: Mixing tank for food and secretions with gastric glands for digestion.

    4. Pyloric Region: Contains pyloric canal and pyloric sphincter muscle.

  • Functions:

    1. Storage of ingested food.

    2. Mechanical breakdown of ingested food.

    3. Disruption of chemical bonds in food material through acids and enzymes.

    4. Production of “intrinsic factor,” a glycoprotein required for the absorption of vitamin B12.

Histology of the Stomach

  • Components:

    • Gastric Pit: Opening to gastric glands.

    • Components of Gastric Glands (secrete ~1500 ml gastric juice daily):

    1. Mucous Cells: Produce mucous.

    2. Chief Cells: Produce pepsin.

    3. Parietal Cells: Produce hydrochloric acid (HCl) and intrinsic factor.

    4. G-Cells: Produce gastrin.

Regulation of Gastric Activity

  • Major Actions in the Stomach:

    1. Secretion of gastric juice.

    2. Contraction of smooth muscles.

  • Three Phases:

    1. Cephalic Phase:

    • Occurs before food enters the stomach.

    • Duration = minutes.

    • Involves cortical activation of medulla and triggers the parasympathetic (PNS) via the vagus nerve stimulating gastric juice secretion and muscle churning.

    1. Gastric Phase:

    • Food arrives at stomach.

    • Duration = 3-4 hours.

    • Responses:

      • Neural Response: Triggered by food mass and chemicals, stimulating a parasympathetic reflex.

      • Local Response: Histogram released locally to stimulate acid secretion.

      • Hormonal Response: Partially digested proteins and caffeine trigger gastrin release, which stimulates gastric gland secretion and increases stomach motility, affecting the gastroesophageal and pyloric sphincters.

    1. Intestinal Phase:

    • Chyme enters small intestine.

    • Duration = hours to days.

    • Neural Response: Enterogastric reflex that inhibits stomach activity and gastrin release.

    • Hormonal Response: GIP (gastric inhibitory peptide) responds to lipids, reducing contractions and protecting the intestinal lining; Secretin triggers pancreas bicarbonate secretion and liver bile secretion; CCK stimulates more acid and enzyme production.

Part 3: Accessory Organs

Accessory Organs: The Pancreas

  • Location: Left side of the stomach, posterior.

  • Structure: Composed of tail, body, and head, with the pancreatic duct leading into the small intestine.

  • Function: Secretion of enzymes and buffers, collectively known as "pancreatic juice."

Enzymes in Pancreatic Juice

  1. Pancreatic Amylase: Converts complex carbohydrates to disaccharides.

  2. Pancreatic Lipase: Converts fats/triglycerides to fatty acids and monoglycerides.

  3. Proteolytic Enzymes: Convert proteins to peptides.

  4. Nucleases: Convert nucleic acids to nucleotides.

Activation of Pancreas

  1. Duodenum fills with acidic chyme.

  2. Hormones CCK and secretin are released.

  3. Secretin stimulates release of pancreatic juice into the duodenum.

Accessory Organs: The Liver

  • Location: Inferior to the diaphragm, with a right lobe (large) and left lobe (small), and two minor lobes (quadrate and caudate).

  • Hepatic Lobules: Functional unit of the liver, hexagon-shaped around a central vein.

  • Functions of the Liver:

    1. Metabolic Regulation:

    • Metabolism of carbohydrates, lipids, amino acids.

    • Vitamin storage (A, B12, D, iron).

    • Breakdown of drugs.

    1. Hematological Regulation:

    • Blood filtration.

    • Phagocytosis and antigen presentation.

    • Removal of hormones and toxins.

    1. Bile Production:

    • Composition: bile salts, bile pigments, cholesterol, electrolytes.

    • Function: Emulsification of fat molecules.

Liver Dysfunction

  • Cirrhosis: Caused by chronic alcoholism or viral infection, leading to inflammation, fat accumulation, and fibrous tissue formation.

  • Hepatitis: Chronic liver infections such as HVC, leading to inflammation.

Accessory Organs: The Gall Bladder

  • Location: Under the liver, connected via the cystic duct.

  • Function: Stores bile between meals.

Regulation of Bile Release

  1. Small intestine fills with fatty chyme.

  2. Hormones CCK and secretin are released into the blood.

  3. CCK stimulates gallbladder contraction; secretin increases bile production from the liver.

  4. Bile passes through the cystic duct and common bile duct, opening a sphincter to release bile into the duodenum for fat emulsification.

Gallstones

  • Definition: Biliary calculi caused by too much cholesterol or too few bile salts, forming crystallized cholesterol.

  • Treatments: Medications to dissolve, lithotripsy (ultrasound pulverization), lasers, or surgery.

Part 4: Primary Organs from the Small Intestine to Outside

Organs of the Digestive System: The Small Intestine

  • Main Site: Chemical digestion and absorption.

  • Structures:

    • Duodenum: Pyloric sphincter → jejunum, ~25 cm (10 in).

    • Jejunum: ~1 m (3 ft), main portion.

    • Ileum: Joins large intestine, ~2 m (6 ft).

Functions of the Small Intestine

  • Major Site of Absorption: ~90% of nutrients absorbed here.

    • Forms of Absorption:

    • Monosaccharides

    • Amino acids

    • Fatty acids and monoglycerides.