Digestive System Flashcards

Digestive System

Learning Objectives

• Identify the major components of the alimentary canal:
  • Oral cavity
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anal canal
• Identify and describe the histological layers of the gastrointestinal tract:
  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa/adventitia
• Differentiate the histological characteristics of each part of the alimentary canal:
  • Variations in cell types
  • Glandular structures
  • Specialized features (e.g., villi in the small intestine)

Components of the Human Digestive System

• Alimentary canal (gastrointestinal tract):
  • Oral cavity
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anal canal
• Associated organs:
  • Tongue
  • Teeth
  • Salivary glands
  • Liver
  • Gall bladder
  • Pancreas

Major Functions of the Digestive System

• Breaking down food into nutrients for the circulatory system.
• Stages of food processing:
  • Ingestion: Taking in food or drink through the mouth.
  • Food Preparation: Chewing and mixing food with saliva to form a bolus.
  • Transport: Moving food and water through the digestive system.
  • Storage: Temporary holding of food in the stomach to mix with digestive juices.
  • Digestion and Absorption: Breaking down food and absorbing nutrients.
  • Secretion: Releasing liquids, salts, hormones, and digestive juices.
  • Excretion: Eliminating undigested waste.

Layers of the Digestive Tract

• Mucosa:
  • Epithelium: Varies based on location (e.g., stratified squamous in the oral cavity and simple columnar in the intestines).
  • Lamina Propria: Connective tissue layer containing blood vessels and immune cells.
  • Muscularis Mucosae: Thin layer of smooth muscle for local movement.
• Submucosa:
  • Dense connective tissue with larger blood vessels, nerves (submucosal plexus), and glands.
• Muscularis Externa:
  • Two layers of muscle (inner circular and outer longitudinal) for peristalsis and segmentation.
  • Myenteric plexus (network of nerve fibers) located between the muscle layers.
• Serosa/Adventitia:
  • Outer layer; serosa for organs in the abdominal cavity, adventitia for those outside it.

Oral Cavity

• Vestibule: Space between the lips, cheeks, and teeth.
• Oral Cavity Proper: Area enclosed by the teeth, palate, and floor of the mouth.
• Lining: Both the vestibule and oral cavity proper are lined with a mucous membrane.

Histology of the Oral Cavity

• Epithelium:
  • Stratified squamous epithelium (keratinized or non-keratinized).
    • Keratinized Epithelium: Areas subject to abrasion (e.g., gums and hard palate).
    • Non-Keratinized Epithelium: Softer areas (e.g., inner cheeks and floor of the mouth).
• Lamina Propria:
  • Connective tissue beneath the epithelium with blood vessels, nerves, and immune cells.
• Submucosa:
  • Contains larger blood vessels, nerves, and salivary glands.

Components of the Oral Cavity

• Tongue:
  • Muscular organ covered by mucous membrane with specialized structures called papillae (filiform, fungiform, foliate, and circumvallate).
  • Involved in tasting, manipulating food, and swallowing.
• Teeth:
  • Composed of enamel, dentin, cementum, and pulp.
  • Anchored in the alveolar bone.
  • Essential for mechanical digestion (chewing).
• Salivary Glands:
  • Major glands: parotid, submandibular, and sublingual glands.
  • Produce saliva containing enzymes (like amylase), mucus, and antibacterial compounds.
• Hard Palate:
  • Bony anterior part of the roof of the mouth, covered by keratinized epithelium.
  • Provides a surface against which the tongue can press food during chewing.
• Soft Palate:
  • Muscular, flexible posterior part of the roof of the mouth, covered by non-keratinized epithelium.
  • Plays a role in swallowing and separating the oral cavity from the nasal cavity.
• Floor of the Mouth:
  • Contains the sublingual glands and is lined by non-keratinized epithelium.
  • Supports the tongue and aids in the movement of food.

Tongue Papillae

• Small, nipple-like projections on the surface of the tongue that play a key role in taste, texture, and sensation.

Types of Papillae

1. Fungiform Papillae
   • Shape: Mushroom-shaped.
   • Location: Scattered across the anterior two-thirds of the tongue.
   • Function: Contain taste buds, providing the sensation of taste.
2. Foliate Papillae
   • Shape: Leaf-like folds.
   • Location: Located on the sides of the tongue, mainly in children.
   • Function: Contain taste buds, though they are less prominent in adults.
3. Circumvallate Papillae
   • Shape: Large and round, surrounded by a trench.
   • Location: Arranged in a V-shape at the back of the tongue.
   • Function: Contain numerous taste buds; they are important for the taste of bitterness.
4. Filiform Papillae
   • Shape: Thread-like and conical.
   • Location: Cover most of the tongue's surface.
   • Function: Do not contain taste buds; primarily provide texture and grip for food.

Taste Buds

• Located within the fungiform and circumvallate papillae.
• Structure: Oval shape and central pore, often with multiple cell types.

Teeth

• Function: Mastication (chewing) of food.
• Assisted by the hard palate.

Esophagus

Layers of the Esophagus

• Mucosa:
  • Epithelium: Nonkeratinized stratified squamous epithelium.
  • Basement Membrane: Provides support for the epithelium.
  • Lamina Propria: Composed of loose areolar connective tissue.
  • Muscularis Mucosae: Contains smooth muscle.
• Submucosa:
  • Tissue Type: Dense irregular connective tissue.
  • Nerve Plexus: Contains the submucosal nerve plexus (Meissner’s plexus).
  • Glands: Houses submucosal esophageal glands that produce mucus.
• Muscularis Externa:
  • Comprised of two layers of smooth muscle:
    • Inner Layer: Circular muscle.
    • Outer Layer: Longitudinal muscle.
  • The upper half of the esophagus also contains skeletal muscle.
  • Nerve Plexus: Contains the myenteric nerve plexus (Auerbach’s plexus) situated between the two muscle layers.
• Adventitia:

Sphincters

• Pharyngoesophageal Sphincter: Also known as the superior esophageal sphincter.
• Inferior Esophageal Sphincter: Located at the level of the diaphragm.
• Function: Effectively transport food and liquids while maintaining proper function and coordination.
• Epithelium: Stratified squamous epithelium for protection against abrasion.
• Muscularis Externa: Transitions from skeletal to smooth muscle along its length.

Stomach

• The transition from the esophagus to the stomach involves a change in epithelial type, muscle composition, and the presence of a sphincter mechanism.
• Epithelial Lining:
  • Esophagus: Stratified squamous epithelium for protection against abrasion from food.
  • Stomach: Simple columnar epithelium, specialized for secretion and absorption.
• The innermost layer of the stomach is the mucosa, which is specialized to produce gastric juices and protect the stomach lining.
• Gastric Pits: Small invaginations in the stomach lining that lead to gastric glands; lined with epithelial cells and responsible for secreting various substances into the gastric lumen.
• Gastric Glands: Located at the base of the gastric pits, made up of specialized cells that contribute to gastric secretion.

Gastric Gland Cells

• Parietal Cells:
  • Function: Secrete hydrochloric acid (HCl) and intrinsic factor.
  • Role of HCl: Creates an acidic environment (pH 1.5 to 3.5) that is essential for activating digestive enzymes (pepsinogen to pepsin) and killing harmful bacteria.
  • $pH = 1.5 - 3.5$
• Chief Cells:
  • Function: Secrete pepsinogen, the inactive precursor of the enzyme pepsin.
  • Role of Pepsin: Digestion of proteins into smaller peptides once activated by the acidic environment.
• Mucous Cells:
  • Function: Produce mucus that coats and protects the stomach lining.
  • Role of Mucus: Barrier against the corrosive effects of gastric acid and helps to lubricate the stomach contents.

Pyloric Glands

• Located in the pyloric region of the stomach (connects to the duodenum).
• Pyloric region: end of the stomach, where contents are prepared to exit into the small intestine.

Isthmus

• Dividing cells in the isthmus of a pyloric gland are primarily stem cells that play a crucial role in the regeneration of the glandular epithelium.
• Their ability to divide and differentiate ensures the continuous supply of various cell types necessary for the stomach's digestive functions, while also contributing to the overall health of the gastric mucosa.

Small Intestine

• A crucial component of the digestive system, primarily responsible for the absorption of nutrients.
• Circular folds (plicae circulares) enhance its surface area and aid in nutrient absorption.
• Circular folds are permanent ridges in the mucosa and submucosa that enhance nutrient absorption by slowing the passage of food and increasing surface area.
• Mucosa contains specialized cells for digestion and absorption, the submucosa provides structural support, the muscularis externa facilitates movement through peristalsis, and lymphatic nodules help protect against pathogens.

Small Intestine Layers:

• Mucosa:
  • Villi and microvilli increase surface area for absorption.
  • Intestinal glands (crypts of Lieberkühn) produce digestive enzymes.
• Submucosa:
  • Peyer’s patches (lymphoid tissue) in the ileum for immune function.
• Small intestine villi are tiny, finger-like projections that line the inner surface of the small intestine, increasing its surface area to enhance nutrient absorption.
• Each villus contains a network of blood vessels and a lymphatic vessel called a lacteal, which help transport absorbed nutrients into the bloodstream and lymphatic system

Large Intestine

• The final section of the digestive system, primarily responsible for absorbing water and electrolytes from indigestible food matter and forming solid waste (feces) for excretion.
• Consists of several parts, including the cecum, colon, rectum, and anal canal.
• Home to a diverse community of beneficial bacteria that play a role in digestion and overall gut health.

Large Intestine Layers

• Mucosa:
  • Simple columnar epithelium specialized for water and electrolyte absorption.
  • Numerous goblet cells secrete mucus, aiding in lubrication and movement of intestinal contents.
• Submucosa:
  • Connective Tissue, Blood Vessels, and Lymphatic Nodules
• Muscularis Externa:
  • A layer of smooth muscle responsible for the movement of food through the digestive tract.
  • Outer Longitudinal Layer: Forms the teniae coli, which are three distinct bands that aid in shortening and contracting the colon.
  • Inner Circular Layer: Constricts the lumen of the intestine, facilitating segmentation and further mixing of digestive contents.
• Serosa/Adventitia: Outermost layer and its role in protecting and anchoring the colon.
• Colonic Mucosa:
  • Parallel test tube-like crypts composed of a single layer of the absorptive columnar cells and the goblet cells supported by surrounding lamina propria with the bases sitting on the muscularis mucosae separating mucosa from submucosa.
• Crypt and Lamina Propria:
  • The cross section of crypts showing regular tubular shape, surrounded by supportive mesenchymal cells and inflammatory cells including lymphocytes, plasma cells, macrophages, lymphocytes and eosinophils.

Cecum

• The first part of the large intestine.
• Begins at the ileocecal valve, where the small intestine meets the large intestine, and extends down to the right iliac fossa, terminating in a blind pouch.
• This configuration is important for its role in digestion and absorption processes.
• The cecum exhibits similar distinguishing features like the rest of the colon (taeniae, haustra, semilunar folds), except appendices epiploicae.

Vermiform Appendix

• The vermiform appendix has a structure similar to the colon but with a higher concentration of lymphoid tissue.
• Its mucosa is lined by simple columnar epithelium with numerous goblet cells, while its lamina propria and submucosa are rich in lymphatic nodules, suggesting its role in immune function

Rectum

• The final section of the large intestine and serves as a storage site for feces before elimination from the body.
• Mucosa: Lined with simple columnar epithelium, which is specialized for absorption and secretion. The mucosa contains numerous goblet cells that produce mucus to facilitate the passage of feces.
• Muscular Layer: Composed of smooth muscle, allowing for peristaltic movements to help propel fecal matter toward the anal canal.
• Submucosa: Contains blood vessels, lymphatics, and nerves, providing support and nourishment to the rectal wall.

Rectum and Anal Canal

• Rectum: The rectum's mucosa is lined with simple columnar epithelium similar to the colon, with abundant goblet cells for mucus secretion. The lamina propria contains lymphoid tissue, and the muscularis externa facilitates peristalsis for the movement of feces.
• Anal Canal: The anal canal has a transitional epithelium.
  • Its upper part is lined by simple columnar epithelium, which transitions to stratified squamous non- keratinized epithelium in the middle and stratified squamous keratinized epithelium near the anal orifice.
  • This adaptation helps the anal canal withstand mechanical stress during defecation.
  • The mucosa also contains longitudinal folds called anal columns, and the muscularis externa includes the internal and external anal sphincters for control.

Selected Histological Features of the Alimentary Canal

• Esophagus

  • Epithelium: Stratified squamous
  • Lamina Propria: Esophageal cardiac glands
  • Submucosa: Collagenous CT, esophageal glands proper
  • Muscularis Externa: Longitudinal
  • Muscularis Mucosae: Inner circular, outer longitudinal

• Stomach

  • Epithelium: Simple columnar, no goblet cells
  • Lamina Propria: Gastric glands
  • Submucosa: Collagenous CT, no glands
  • Muscularis Externa: Inner oblique, middle circular, outer longitudinal
  • Muscularis Mucosae: Inner circular, outer longitudinal, sometimes circular

• Small intestine

  • Epithelium: Simple columnar with goblet cells
  • Lamina Propria: Villi, crypts of Lieberkühn, Peyer patches in ileum (extend into submucosa), lymphoid nodules
  • Submucosa: Fibro elastic CT, Brunner glands in duodenum
  • Muscularis Externa: Inner circular, outer longitudinal
  • Muscularis Mucosae: Inner circular, outer longitudinal

• Large intestine, cecum, colon

  • Epithelium: Simple columnar with goblet cells
  • Lamina Propria: Crypts of Lieberkühn (lack Paneth cells), lymphoid nodules
  • Submucosa: Fibroelastic CT, no glands
  • Muscularis Externa: Inner circular, outer longitudinal (modified to form teniae coli)
  • Muscularis Mucosae: Inner circular, outer longitudinal

• Rectum

  • Epithelium: Simple columnar with goblet cells
  • Lamina Propria: Crypts of Lieberkühn (fewer but deeper than in colon), lymphoid nodules
  • Submucosa: Fibro elastic CT, no glands
  • Muscularis Externa: Inner circular (forms internal anal sphincter), outer longitudinal
  • Muscularis Mucosae: Inner circular, outer longitudinal

• Anal canal

  • Epithelium: Simple columnar cuboidal Appendix (proximal), stratified squamous nonkeratinized (distal to anal valves), stratified squamous keratinized (anus)
  • Lamina Propria: Sebaceous glands, circumanal glands, lymphoid nodules, rectal columns or Morgagni (involve entire mucosa), hair follicles (anus)
  • Submucosa: Fibro elastic CT with large veins, no glands
  • Muscularis Externa: Inner circular, outer longitudinal
  • Muscularis Mucosae: Two layers: inner circular, outer longitudinal

• Appendix

  • Epithelium: Simple columnar with goblet cells
  • Lamina Propria: Crypts of Lieberkühn (shallow), lymphoid nodules (large, numerous, and may extend into the submucosa)
  • Submucosa: Fibro elastic CT, confluent lymphoid nodules, no glands, fat tissue (sometimes)
  • Muscularis Externa: Inner circular, outer longitudinal
  • Muscularis Mucosae: Inner circular, outer longitudinal