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Overview of Gastrointestinal Tract Physiology

The gastrointestinal (GI) tract is responsible for the digestion and absorption of nutrients, as well as the elimination of waste products. It includes the mouth, oesophagus, stomach, small intestine, large intestine, and accessory organs such as the liver, pancreas, and gallbladder.

Key Concepts

• Anatomy of the Gastrointestinal Tract:
  • Mouth: Begins mechanical digestion through chewing and chemical digestion with saliva.
  • Oesophagus: Transports food from the mouth to the stomach via peristalsis.
  • Stomach: Mixes food with gastric juices, initiating protein digestion.
  • Small Intestine: Major site of digestion and absorption of nutrients.
  • Large Intestine: Absorbs water and electrolytes, forms and stores feces.
  • Accessory Organs:
    • Liver: Produces bile, processes nutrients absorbed from the small intestine.
    • Pancreas: Produces digestive enzymes and bicarbonate, secretes insulin and glucagon.
    • Gallbladder: Stores and concentrates bile.
• Digestive Processes:
  • Ingestion: Taking in food and liquids.
  • Propulsion: Movement of food through the GI tract, including swallowing and peristalsis.
  • Mechanical Digestion: Physical breakdown of food (chewing, churning in the stomach).
  • Chemical Digestion: Enzymatic breakdown of macromolecules into absorbable units.
  • Absorption: Transfer of nutrients from the GI tract into the bloodstream or lymphatic system.
  • Defecation: Elimination of indigestible substances and waste products as feces.

Detailed Processes

• Mouth:
  • Saliva: Contains enzymes (amylase for starch digestion) and lubricates food for easier swallowing.
  • Mastication: Chewing increases the surface area of food for enzymatic action.
• Oesophagus:
  • Peristalsis: Rhythmic contractions of smooth muscle to propel food toward the stomach.
  • Lower oesophageal sphincter prevents reflux of stomach contents.
• Stomach:
  • Gastric Juices: Contain hydrochloric acid (HCl), pepsinogen (converted to pepsin), and mucus.
  • HCl denatures proteins and activates pepsinogen.
  • Pepsin begins the digestion of proteins into peptides.
  • Chyme: Semi-liquid mixture of partially digested food and gastric juices.
• Small Intestine:
  • Divided into three parts: duodenum, jejunum, and ileum.
  • Duodenum: Receives chyme from the stomach, bile from the liver, and pancreatic juice from the pancreas.
  • Jejunum and Ileum: Primary sites for nutrient absorption.
  • Villi and Microvilli: Increase surface area for absorption; contain capillaries and lacteals (lymphatic vessels).
  • Enzymes: Brush border enzymes complete the digestion of carbohydrates and proteins.
• Large Intestine:
  • Includes the caecum, colon, rectum, and anal canal.
  • Absorbs water and electrolytes to form solid faeces.
  • Harbors beneficial bacteria that aid in the fermentation of undigested carbohydrates and production of vitamins (e.g., vitamin K).
  • Defaecation: The expulsion of feces from the rectum through the anus.

Regulation of Digestive Activity

• Neural Regulation:
  • Enteric Nervous System: Intrinsic nervous system of the GI tract that controls local functions.
  • Autonomic Nervous System: Parasympathetic stimulation enhances digestive activity; sympathetic stimulation inhibits it.
• Hormonal Regulation:
  • Gastrin: Secreted by the stomach; stimulates acid secretion and gastric motility.
  • Secretin: Released by the small intestine in response to acidic chyme; stimulates bicarbonate secretion from the pancreas.
  • Cholecystokinin (CCK): Released by the small intestine in response to fats and proteins; stimulates the release of digestive enzymes from the pancreas and bile from the gallbladder.
  • Gastric Inhibitory Peptide (GIP): Inhibits gastric motility and secretion; stimulates insulin release.

Clinical Relevance

• Gastr oesophageal Reflux Disease (GERD):
  • Condition where stomach acid frequently flows back into the oesophagus, causing irritation.
  • Symptoms: Heartburn, regurgitation, and difficulty swallowing.
  • Management: Lifestyle changes, medications, and surgery in severe cases.
• Peptic Ulcer Disease:
  • Sores that develop on the lining of the stomach, small intestine, or oesophagus, usually due to H. pylori infection or NSAID use.
  • Symptoms: Burning stomach pain, bloating, and indigestion.
  • Management: Antibiotics, proton pump inhibitors, and lifestyle changes.
• Inflammatory Bowel Disease (IBD):
  • Includes Crohn's disease and ulcerative colitis; characterized by chronic inflammation of the GI tract.
  • Symptoms: Diarrhea, abdominal pain, weight loss, and fatigue.
  • Management: Medications, dietary changes, and sometimes surgery.
• Irritable Bowel Syndrome (IBS):
  • Functional GI disorder causing symptoms like abdominal pain, bloating, and altered bowel habits.
  • Management: Dietary modifications, stress management, and medications to alleviate symptoms.
• Coeliac Disease:
  • Autoimmune disorder triggered by ingestion of gluten, leading to damage in the small intestine.
  • Symptoms: Diarrhoea, abdominal pain, bloating, and malabsorption.
  • Management: Strict gluten-free diet.

Summary

The gastrointestinal tract is essential for digestion, absorption, and elimination. It involves complex processes regulated by neural and hormonal signals. Understanding the physiology of the GI tract is crucial for diagnosing and managing various digestive disorders.