Bio 15.1 Alimentary Canal Components and Functions

Alimentary Canal Components and Functions

• The alimentary canal, also known as the gastrointestinal tract, is a continuous tube that begins at the mouth and ends at the anus.
• Components of the alimentary canal include:
  • Oral Cavity: Initial site of mechanical digestion (mastication), lubricates food with saliva, and begins chemical digestion of some macromolecules.
  • Esophagus: A passageway for food to move from the mouth to the stomach.
  • Stomach: Storage site, mixes food into chyme, and performs both chemical and mechanical digestion.
  • Small Intestine: Completes macromolecular digestion and is the primary site for nutrient absorption.
  • Large Intestine: Reabsorbs electrolytes and water from indigestible material and compacts it into feces.

Oral Cavity and Esophagus

• Mechanical digestion in the oral cavity occurs through chewing (mastication).
• Saliva, secreted from salivary glands, lubricates food contains enzymes for initial chemical digestion:
  • Lingual Lipase: Hydrolyzes triglycerides into free fatty acids, glycerol, monoglycerides, and diglycerides.
  • Salivary Amylase: Hydrolyzes starch into maltose.
• The tongue and cheeks compress the mechanically digested food into a bolus.
• The bolus passes through the pharynx and the upper esophageal sphincter into the esophagus.

Stomach

• The lower esophageal sphincter (cardiac sphincter) regulates food entry from the esophagus into the stomach.
• The stomach converts the food bolus into chyme, a semifluid mixture of digested and undigested materials.
• The stomach lining contains specialized cells that secrete different products:
  • G cells: Produce the hormone gastrin, which stimulates parietal cells to secrete hydrochloric acid.
  • Parietal cells: Produce hydrochloric acid and intrinsic factor (essential for vitamin B12 absorption).
  • Chief cells: Produce pepsinogen (inactive form of pepsin), which breaks down proteins into smaller peptides, and also secrete gastric lipase for lipid digestion.
  • Mucous cells: Produce mucus and bicarbonate ions, which protect the stomach lining from autodigestion by acidic gastric juice.

Small Intestine

• The pyloric sphincter controls the flow of chyme from the stomach into the small intestine.
• Three subdivisions of the small intestine:
  • Duodenum: Primary site of chemical digestion.
  • Jejunum: Continues digestion and absorbs nutrients.
  • Ileum: Absorbs nutrients.
• The duodenum receives bile and pancreatic secretions:
  • Bile: Produced by the liver and stored in the gallbladder, it aids in fat digestion and neutralizes acidic chyme.
  • Pancreatic Secretions: Include enzymes for digesting fats, proteins, and carbohydrates, as well as bicarbonate for neutralizing chyme.
• Surface modifications of the small intestine increase surface area and promote absorption:
  • Circular folds: Increase surface area and slow chyme movement.
  • Villi: Fingerlike projections from the lining.
  • Microvilli: Smaller projections on individual absorptive cells, forming the brush border.
• Intestinal enzymes complement pancreatic enzymes:
  • Lipases: Further digest fats.
  • Proteolytic enzymes: Hydrolyze polypeptides.
  • Dipeptidase: Digests proteins.
  • Enteropeptidase: Activates trypsinogen into trypsin.
  • Disaccharidases: Hydrolyze disaccharides.
• Hormones produced by the small intestine regulate digestive processes:
  • Secretin: Promotes pancreatic secretions and bicarbonate release, inhibits gastric acid secretion, and slows digestive tract motility.
  • Cholecystokinin (CCK): Promotes pancreatic enzyme and bile release.
• The small intestine absorbs nutrients and water.

Large Intestine

• The large intestine is wider and shorter than the small intestine.
• Three subdivisions:
  • Cecum: A small pouch connected to the appendix.
  • Colon: Reabsorbs electrolytes and water, forming feces.
  • Rectum: Stores feces before elimination.
• Gut flora (bacteria) residing in the large intestine participate in digestion:
  • Breakdown of indigestible carbohydrates into short-chain fatty acids, which serve as an energy source for the body.
  • Synthesis of certain vitamins.

Peritoneum

• The peritoneum lines the abdominal cavity.
• Two layers of tissue:
  • Parietal peritoneum: Lines the abdominal wall.
  • Visceral peritoneum: Covers the organs.
• The peritoneal cavity is the space enclosed by the peritoneum.