Digestive System

• Digestive System


  Specialized organ system to breakdown and absorb food and eliminate undigested food in faeces
• Digestive System


  • Includes the gastrointestinal (GI) tract, which is a continuous tube from mouth to anus
  • Length of GI tract is about 16.5-23 ft in a living person
  • Teeth, tongue, salivary glands, liver, gallbladder, and pancreas serve as accessory digestive organs
• Organs of the gastrointestinal tract

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
• Functions of the Digestive System

  1. Ingestion - materials enter digestive tract via mouth

  2. Secretion - release of water, acids, enzymes, buffers, and salts

  3. Mixing and propulsion - crushing and shearing, propelling materials along digestive tract

  4. Digestion - chemical breakdown of food into small organic fragments

  5. Absorption - movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium

  6. Excretion - removal of waste products through defecation
• Layers of the gastrointestinal tract

  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa and peritoneum
• Peritoneum


  Continuous membrane lining the abdominal cavity and covering the abdominal organs, supporting the viscera and providing pathways for blood vessels and lymph
• Parietal peritoneum


  Lines the internal surface of the abdominopelvic wall, sensitive to pressure, pain, laceration and temperature
• Visceral peritoneum


  Covers the majority of the abdominal viscera, only sensitive to stretch and chemical irritation
• Peristalsis


  1. Waves of muscular contractions that move nutrients and waste through the intestines

  2. Circular muscles contract behind bolus while relaxing ahead of it

  3. Longitudinal muscles ahead of bolus contract, shortening adjacent segments

  4. Wave of contraction in circular muscles forces bolus forward
• Mouth/Oral cavity


  • Formed by cheeks, hard and soft palates, lips, and tongue
  • Tongue is composed of skeletal muscle covered in mucous membrane, with papillae containing taste buds
  • Salivary glands (parotid, submandibular, sublingual) secrete saliva to lubricate food and start carbohydrate digestion
• Pharynx


  Funnel-shaped tube composed of skeletal muscle and lined by mucous membrane, connects mouth to esophagus
• Esophagus


  Muscular tube connecting pharynx to stomach, peristalsis moves food down through involuntary contractions
• Stomach


  • 4 main regions: cardia, fundus, body, pylorus
  • Stomach wall has 4 layers: mucosa, submucosa, muscularis, serosa
  • Produces mucus, hydrochloric acid, pepsin, intrinsic factor, and gastrin
• Gastrin


  Peptide hormone that stimulates gastric acid secretion and aids gastric motility
• Pepsin


  Extracellular enzyme released by chief cells, breaks down proteins in the acidic environment
• Parietal cells


  Secrete hydrochloric acid, bicarbonates, and intrinsic factor
• Mucous neck cells


  Secrete mucus in the stomach
• Small Intestine


  • Divided into duodenum, jejunum, and ileum
  • Length is about 10 ft in a living person
  • Major site of digestion and absorption
• Duodenum


  • First and shortest portion of small intestine, begins after pyloric sphincter
  • Contains Brunner's glands that secrete alkaline juices and mucus
• Gastrin


  Peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility
• G cells


  Cells in the pyloric antrum of the stomach, duodenum, and the pancreas that release gastrin
• Pepsin


  Extracellular enzyme released by the chief cells of the stomach into gastric juice, released in an inactive form called pepsinogen and converted to an active enzyme by hydrochloric acid
• Chief cells


  Cells in the stomach that release pepsinogen
• Parietal cells


  Cells responsible for the secretion of hydrochloric acid (HCl), bicarbonates, and intrinsic factor
• Hydrochloric acid (HCl)


  Gastric juice secreted by parietal cells
• Mucous neck cells


  Cells located within gastric glands, interspersed between parietal cells, that secrete mucus
• Mucus


  Secreted by mucous neck cells
• Small intestine


  • Length is about 10 ft. in a living person, extends from the pyloric sphincter to the ileocecal sphincter, divided into the duodenum, jejunum, and ileum
• Digestion and absorption in the small intestine

  Within 2 to 4 hours after eating a meal, the stomach has emptied its contents into the small intestine, where the major events of digestion and absorption occur
• Ileocecal sphincter


  Sphincter that marks the end of the small intestine and the beginning of the large intestine
• Divisions of the duodenum

  • Superior
  • Descending
  • Horizontal
  • Ascending
• Brunner's glands


  Glands located in the wall of the duodenum that secrete alkaline juices and mucus
• Chemical digestion


  Takes place mostly in the duodenum
• Duodenal papilla


  Openings through which the liver and pancreas secrete their digestive juices into the duodenum
• Jejunum


  Starts from the duodeno-jejunal flexure, approximately 2/5 the length of the small intestine, characterized by plicae circulares (valves of Kerckring) that increase absorption
• Ileum


  Approximately 3/5 the length of the small intestine, where bile acids and vitamin B12 get absorbed, characterized by Peyer's patches (lymphoid tissue monitoring intestinal bacteria populations)
• Structure of the small intestine

  • Highly adapted for digestion and absorption, with glands producing enzymes, bile, and mucus, and microvilli, villi, and circular folds providing a large surface area
• Digestion in the small intestine

  Protein digestion completed by peptidases, triglycerides broken down into fatty acids and monoglycerides by pancreatic lipase, nucleic acids broken down by nucleases
• About 90% of all absorption takes place in the small intestine, the other 10% occurs in the stomach and large intestine
• Absorption


  Passage of nutrients from digested food in the gastrointestinal tract into the blood or lymph, occurs mostly in the small intestine by simple diffusion, facilitated diffusion, osmosis, and active transport