Digestive systems

• 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
• Ingestion


  Occurs when materials enter digestive tract via the mouth
• Secretion


  Release of water, acids, enzymes, buffers, and salts by epithelium of digestive tract and by glandular organs
• Mixing and propulsion


  Crushing and shearing, makes materials easier to propel along digestive tract
• Digestion


  Chemical breakdown of food into small organic fragments for absorption
• Absorption


  Movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium tissue into the interstitial fluid of digestive tract
• Excretion


  Removal of waste products from the body through defecation, which removes faeces
• Layers of the gastrointestinal tract wall

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


  • Outer coat which includes a membrane and connective tissue
• Muscle layer


  • Includes longitudinal muscle tissue in the outermost layer with a layer of circular muscle tissue on the inside
• Submucosa


  • Connective tissue containing blood and lymph vessels
• Mucosa


  • Lining of the digestive tract, composed of epithelium which is folded to form the villi
• Peritoneum


  Continuous membrane which lines the abdominal cavity and covers the abdominal organs, acting to support the viscera and provide pathways for blood vessels and lymph
• Parietal peritoneum


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


  • Invaginates to cover the majority of the abdominal viscera and is only sensitive to stretch and chemical irritation
• Peristalsis


  Waves of muscular contractions that move nutrients and waste through the intestines
• Peristaltic motion


  1. Circular muscles contract behind bolus, while circular muscles ahead of bolus relax

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

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


  Formed by the cheeks, hard and soft palates, lips, and tongue
• Tongue


  • Forms the floor of the oral cavity, composed of skeletal muscle covered with mucous membrane, with papillae containing taste buds
• Salivary glands


  Three pairs (parotid, submandibular, sublingual) that secrete saliva to lubricate food and start carbohydrate digestion
• Functions of the oral cavity

  • Sensory analysis of material before swallowing
  • Mechanical processing through actions of teeth, tongue, and palatal surfaces
  • Lubrication by mixing with mucus and salivary secretions
  • Limited digestion of carbohydrates and lipids
• Mastication


  Mixing of food with saliva and shaping into a bolus
• Salivary amylase


  Enzyme that begins the digestion of starches in the mouth
• Pharynx


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


  Muscular tube that connects the pharynx to the stomach, using peristalsis to push food down
• Stomach


  • Has 4 main regions: cardia, fundus, body, and pylorus
  • Wall composed of the same 4 layers as the rest of the GI tract
  • Contains glands that produce mucus, hydrochloric acid, pepsin, intrinsic factor, and gastrin
• Mechanical digestion in the stomach

  Mixing waves that macerate food and mix it with gastric juice, forming chyme
• Chemical digestion in the stomach

  Beginning of protein digestion by the enzyme pepsin
• Mucus in the stomach

  Secreted by mucous cells to coat the mucosa and form a barrier between the cells and gastric juice
• Lingual lipase


  Enzyme produced by the tongue that digests triglycerides in the acid environment of the stomach
• Gastrin


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


  Extracellular enzyme released by chief cells of the stomach that breaks down proteins
• Hydrochloric acid


  Secreted by parietal cells, creates the acidic environment of the stomach
• Intrinsic factor


  Glycoprotein secreted by parietal cells that aids in vitamin B12 absorption
• Mucous neck cells


  Secrete mucus within the gastric glands
• Small intestine


  • Extends from the pyloric sphincter to the ileocecal sphincter, divided into duodenum, jejunum, and ileum
  • Major site of digestion and absorption
• Duodenum


  • Begins immediately after the pyloric sphincter, shortest portion at approximately 20-25cm, divided into superior, descending, horizontal, and ascending sections
• Brunner's glands


  Located in the wall of the duodenum, secrete alkaline juices and mucus