AVS 3011 - Lab 3 - Pig GI Tract (Monogastric)

Overview of the Monogastric Digestive Tract

• The digestive tract is essentially a hollow tube that extends from the mouth to the anus.
  • It consists of a lumen, the cavity within the hollow structure.

Digestion and Absorption

• Digestion: Process of breaking down food into smaller components.
• Absorption: The process through which digested nutrients cross the intestinal epithelium and enter the bloodstream.

Principal Parts of the Digestive System

• Main components: Mouth, teeth, tongue, pharynx, esophagus, stomach, small intestine, cecum, and large intestine.
• Accessory organs: Salivary glands, liver, gallbladder, pancreas.

Oral Cavity (Mouth)

• Food is received and mechanically reduced in size (initial digestion).
  • Prehension: The act of seizing and bringing food to the mouth.
  • Major structures: Lips, teeth, and tongue.
  • Mastication: Mechanical breakdown of food particles into smaller sizes for easier digestion.
  • Tongue: Muscular organ that assists in moving food mass around in the mouth and into the esophagus.

Salivary Glands

• Three pairs of glands: Parotid, mandibular, and sublingual.
  • Secretions can be serous (watery), mucous (viscous), or mixed.
  • Serous secretion: Clear, watery fluid.
  • Mucous secretion: Tenacious material that protects the digestive tract.

Pharynx

• Common passageway for food and air located caudal to oral and nasal cavities.
  • Essential for deglutition (swallowing), which occurs in three stages (the first being voluntary).
  • The epiglottis prevents food from entering the glottis and nasal cavity by covering the glottis during swallowing.

Esophagus

• Muscular tube connecting the mouth to the stomach.
• Food is moved by peristaltic waves (contraction waves of the muscular wall).
  • Layers: Circular and longitudinal muscle layers for movement.
• Submucosa contains glands; mucosa consists of stratified squamous epithelium.

Stomach

• The pig has a monogastric (simple stomach) with one compartment.
• Parts of the stomach:
  • Cardia: Entrance area where the esophagus connects.
  • Cardiac sphincter: Prevents reflux to the esophagus.
  • Fundus: Dome-shaped area where gastric glands produce gastric juice.
  • Body: Large middle portion that continues gastric juice production.
  • Antrum: Constricted part connecting to the small intestine through the pylorus (pyloric sphincter prevents premature release).
  • Curvatures: Lesser (short side) and Greater (long side).
  • Rugae: Ridges that increase surface area for enzyme and acid secretion.

Gastric Secretions

• Gastric glands and mucus-secreting regions help reduce ulcers.
• Chyme: Partially digested, liquefied food entering the intestine, has fluid to semi-fluid consistency, is acidic, and varies in composition based on diet.

Small Intestine (SI)

1. Pancreas

   • Functions include both endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions.
     • Insulin (produced by beta cells): Breaks down sugars.
     • Glucagon (produced by alpha cells).
     • Secretes HCO3- to neutralize stomach acid.

2. Liver: Detoxifies blood and produces bile.

   • Bile stores in the gallbladder (absent in horses).
   • Bile consists of bile salts, bilirubin, cholesterol, lecithin, and electrolytes.

3. Intestinal Absorption: Nutrients are absorbed as they pass through the small intestine.

   • Involves the ileocecal valve, marking the junction to the large intestine.

Histology of the Small Intestine

• Mucosa: Composed of epithelial cells in contact with lumen content.
• Submucosa: Contains connective tissue and nerve networks.
• Muscularis externa: Smooth muscle layers for movement (circular and longitudinal).
• Serosa: Outer layer of the intestine, continuous with the mesentery.

Large Intestine (LI)

• Comprised of cecum, colon, rectum.
• Fermentation occurs primarily in the cecum and colon, especially in herbivores.
• Cecum: Blind-ended pouch at SI and LI junction, site of fermentation.
• Bacteria metabolize undigested foodstuff, producing gases (flatulence).
• Colon parts: Include ascending, transverse, descending colon, each with a specific function in water absorption and feces storage.

Anus

• The terminal portion of the digestive tract that closes via a muscular sphincter.К