Digestion and Absorption of Food Nutrients

Learning Outcomes

  • By the end of the session, students will be able to:
    • Detail the gross anatomy and physiology of the gastrointestinal (GI) tract.
    • Understand mechanical and chemical digestion.
    • Explain how and where nutrients are digested and absorbed.
    • Identify 3 medications acting on the GI tract.

Functions of the GI Tract

  • The GI tract has multiple functions:
    • Ingestion: The intake of food and liquids.
    • Propulsion: Includes both voluntary (swallowing) and involuntary (peristalsis) activities that move food through the GI tract.
    • Digestion: Both mechanical (physical breakdown) and chemical (enzymatic breakdown) processes.
    • Absorption: The uptake of nutrients into the bloodstream.
    • Secretion: The release of digestive juices and enzymes.
    • Excretion: The elimination of waste through defecation.
    • Storage: Holding undigested food until it can be processed.

Gross Anatomy of the GI Tract

  • The GI tract comprises:
    • Mouth: The entry point for food.
    • Oesophagus: Connects the mouth to the stomach.
    • Stomach: A temporary holding area for food that undergoes further digestion.
    • Small Intestine: Site of most nutrient digestion and absorption.
    • Large Intestine: Absorbs water and electrolytes, forming waste.
    • Rectum and Anus: Excretory pathway.
    • Accessory Organs:
    • Lips, Teeth, Tongue, Soft & Hard Palate: Assist in digestion through mechanical breakdown.
    • Salivary Glands: Produce saliva containing enzymes like salivary amylase.
    • Liver: Produces bile, which aids in fat digestion.
    • Gall Bladder: Stores bile until needed.
    • Pancreas: Produces digestive enzymes and bicarbonate to neutralize stomach acid.
  • The GI tract is supported by a mesentery, which is a connective tissue structure that maintains its position and support.

Four Layers of the GI Tract

  1. Mucosa:

    • Innermost layer composed of a mucous membrane.
    • Contains epithelial cells that secrete digestive juices.
    • Stratified in areas like the oesophagus and anal canal for protection.

  2. Submucosa:

    • Contains blood vessels, nerves, collagen, and elastic fibers.
    • Responsible for regulating digestive secretions and binding mucosa to the muscularis externa.

  3. Muscularis Externa:

    • Contains two layers of smooth muscle: outer longitudinal and inner circular layers.
    • Responsible for peristalsis and segmentation.

  4. Serosa:

    • Part of the visceral peritoneum, protecting the GI tract.
    • Parietal peritoneum lines the abdominal cavity, with the peritoneal cavity in between, filled with fluid for cushioning.

Digestion in the Mouth & Accessory Organs

  • Ingestion of Food: Initiated in the mouth.
  • Propulsion:
    • Voluntary action via swallowing.
  • Mechanical Digestion:
    • Chewing: Alters food to make it easier to swallow and increases enzyme accessibility.
  • Chemical Digestion:
    • Salivary Amylase: Degrades starch into simpler disaccharides.
    • Lingual Lipase: Begins lipid digestion.

Pharynx & Oesophagus

  • Role:
    • Primary conduits for food.
    • Secretion of Mucus: Lubricates the food for easier passage.
    • Propulsion: Involuntary movement via peristalsis.

The Stomach

  • Storage: Acts as a temporary holding tank for food.P
  • Mechanical Digestion: Churning mixes food with gastric juices to create chyme.
  • Chemical Digestion:
    • Proteins: Pepsinogen is converted to pepsin for protein digestion.
    • Gastric Lipase: Continues lipid breakdown.
  • Secretion:
    • Parietal Cells: Secrete hydrochloric acid, stimulated by gastrin and acetylcholine.
    • Mucus: Provides lubrication and protection.
    • Intrinsic Factor: Essential for vitamin B12 absorption.
  • Limited Absorption: Minor absorption of water, alcohol, and aspirin occurs in the stomach.

The Small Intestine

  • Composed of three sections:
    • Duodenum:
    • Jejunum:
    • Ileum:
  • Mechanical Digestion: Achieved via segmentation movements.
  • Primary Site for Digestion and Absorption:
    • Absorption occurs mainly through villi, with active transport mechanisms.
    • Transporting Nutrients:
    • Glucose: Absorbed through a coupled transport process with sodium, utilizing ATP.

Digestion in the Small Intestine

  • Carbohydrates: Continued hydrolysis by pancreatic amylase.
  • Final carbohydrate digestion occurs at the brush border of villi.
  • Lipid Digestion: Emulsification by bile enhances lipid digestibility.
  • Pancreatic Juice: Neutralizes stomach acids for optimal enzyme activity.
    • Enzymes break down larger molecules into smaller units.
    • Long-chain fatty acids absorbed by mucosal cells.

Absorption of Nutrients

  • Proteins: Final digestion occurs via TRYPSIN, with amino acids absorbed through active transport.
  • Vitamins:
    • Fat-soluble vitamins absorbed along with dietary lipids.
    • Water-soluble vitamins absorbed mainly by diffusion, except for vitamin B12, which requires intrinsic factor for absorption through endocytosis.
  • Water: Passively absorbed; excess water-soluble vitamins are excreted in urine.

The Large Intestine

  • Also called the colon, consists of anatomical sections:
    • Ascending colon
    • Transverse colon
    • Descending colon
    • Sigmoid colon
    • Rectum
    • Anal canal
  • Lacks villi compared to the small intestine.
  • Absorption: Remaining water, electrolytes, and vitamins are absorbed.
  • Functions:
    • Propulsion via peristalsis, haustral churning, and mass movement.
    • Secretion of mucus for lubrication.
    • Bacterial fermentation of undigested residue producing flatus (gas).
    • Storage and eventual defecation of faeces.

Hormones Controlling Digestion

  • Four key hormones:
    • Cholecystokinin (CCK):
    • Released by the duodenum; regulates stomach motility, gallbladder contraction, bile flow, and pancreatic secretion.
    • Secretin:
    • Released in response to acidic chyme; stimulates bicarbonate secretion from pancreas.
    • Gastric Inhibitory Peptide:
    • Decreases gastric motility in response to high lipid content in the stomach.
    • Gastrin:
    • Stimulates hydrochloric acid secretion, which activates pepsin, kills pathogenic organisms, and enhances nutrient absorption.

Peristalsis

  • Definition: Involves recurring, progressive contractions of smooth muscle in the GI tract, aiding in food movement.
  • Sphincters: Act as valves, controlling food passage; respond to nerve signals, hormones, and pressure changes.

Drugs Acting on the GI Tract

  1. Lactulose:

    • Usage: Treats chronic constipation and prevents complications of hepatic encephalopathy.
    • Side effects: Diarrhea, nausea, vomiting, bloating, belching, and stomach pain.

  2. Magnesium Trisilicate:

    • Classification: Antacid, neutralizes stomach acid.
    • Usage: Alleviates acid reflux, dry cough, heartburn, and bloating.
    • Side effects: Rarely causes constipation or allergic reactions.

  3. Loperamide:

    • Classification: Antidiarrheal, inhibits peristalsis.
    • Usage: Treats acute diarrhea and manages discharge in ileostomy patients.
    • Side effects: Dizziness, tiredness, gastric upset, and potential allergic reactions.

Conclusion

  • The session covered the structure and function of the GI tract, the mechanisms of digestion and absorption, and the pharmacological agents influencing gastrointestinal processes.