Digestive system

• Digestive system


  One of the most clearly defined systems in the body, consisting of a long passageway (the digestive tract) and associated glands like the liver and pancreas

  
• Six functions of the digestive system

  • Ingestion
  • Mechanical processing
  • Digestion
  • Secretion
  • Absorption
  • Excretion

  
• Ingestion


  Materials enter the digestive tract via the mouth

  
• Mechanical processing


  Crushing and shearing to make materials easier to propel along the digestive tract
• Digestion


  The chemical breakdown of food into small organic fragments for absorption
• Secretion


  The release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and glandular organs

  
• Absorption


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


  The removal of waste products from body fluids, with the process of defecation removing feces
• Digestive tract


  Also called the gastrointestinal (GI) tract or alimentary canal, a muscular tube extending from the mouth to the anus

  
• Major organs of the digestive tract

  • Oral cavity (mouth)
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anus

  
• Accessory organs of the digestive system

  Teeth, tongue, salivary glands, liver, gallbladder, pancreas

  
• Teeth


  Mechanical processing through chewing (mastication)
• Tongue


  Assists mechanical processing with teeth, sensory analysis
• Salivary glands


  Secretion of lubricating fluid containing enzymes that break down carbohydrates
• Liver


  Secretion of bile (important for lipid digestion), storage of nutrients, many other vital functions

  
• Gallbladder


  Storage and concentration of bile
• Pancreas


  Exocrine cells secrete buffers and digestive enzymes, endocrine cells secrete hormones

  
• The digestive system is vital for our survival, as it is our nutrient feed, main defense system, main elimination route for waste, and the location for a million different vital biochemical processes
• Oral cavity


  Functions include sensory analysis, mechanical processing, lubrication, and limited digestion

  
• Saliva


  Moistens food and begins the process of chemical digestion

  
• Saliva is about 99% water and not only moistens food but cleanses the mouth, dissolves food chemicals so they can be tasted, and contains enzymes that start the chemical breakdown of starchy foods
• Salivary glands


  Three pairs: parotid, sublingual, and submandibular (submaxillary)

  
• Tongue


  Creates a bolus of food so it can travel down the pharynx and esophagus

  
• Peristalsis


  Waves of muscular contractions that move a bolus along the length of the digestive tract
• Stomach


  A muscular-walled, J-shaped sac where food is stored, churned, and mixed with gastric juices
• Gastric juices


  Include digestive enzymes and hydrochloric acid, which break down food and kill potentially harmful microbes
• Liver


  Secretes bile to emulsify fats in the small intestine

  
• Gallbladder


  Stores and concentrates bile secreted by the liver
• Pancreas


  Secretes pancreatic juice, a mix of digestive enzymes, water, buffers, and electrolytes
• Villi


  Line the walls of the small intestine and absorb nutrients into capillaries and lacteals

  
• Ileocecal valve


  Allows chyme to pass from the small intestine into the cecum of the large intestine
• Defecation


  The process of expelling waste products from digestion through the rectum and anus, involving contraction of rectal muscles, relaxation of the internal anal sphincter, and contraction of the external anal sphincter