Digestive system

• Digestive system


  System that breaks down food into molecular fragments that cells can use for energy production
• Parts of the digestive system

  • Alimentary canal (gastrointestinal tract)
  • Accessory digestive organs (teeth, tongue, gallbladder, salivary glands, liver, pancreas)
• Sequence of actions in the digestive system

  1. Ingestion

  2. Propulsion

  3. Mechanical breakdown

  4. Digestion

  5. Absorption

  6. Defecation
• Alimentary canal (gastrointestinal tract)


  • Continuous tube starting at the mouth and ending at the anus
  • Food is broken down and absorbed through the lining
• Accessory digestive organs


  • Not part of the GI tract but have a considerable role in digestion
• Peritoneum


  Membrane lining the abdominal cavity, divided into visceral and parietal peritoneum
• Structure of the GI tract

  • Innermost layer is the mucosa
  • Submucosa contains blood vessels, lymphatic vessels, lymphoid follicles, and nerve fibers
  • Muscularis externa performs segmentation and peristalsis
  • Outermost layer is the serosa
• Intrinsic nerve plexuses


  Submucosal nerve plexus and myenteric nerve plexus that regulate digestive system activity
• Oral cavity


  • Lined with thick stratified squamous epithelium
  • Produces antimicrobial peptides
  • Contains hard and soft palate, uvula, palatine tonsils
• Tongue


  Made of skeletal muscle fibers, secured by the lingual frenulum
• Salivary glands


  Produce saliva that cleans the mouth, moistens and dissolves food, and contains enzymes that begin breaking down food
• Teeth


  Help in the mechanical breakdown of food through mastication (chewing)
• Esophagus


  • Muscular tube joining the stomach
  • Stratified squamous epithelium changes to simple columnar epithelium at the stomach junction
• Stomach


  • Converts food into a paste called chyme
  • Gastric pits lead to gastric glands that produce very acidic gastric juice
  • Mucosal barrier protects the stomach from the acidic conditions
• Small intestine


  • Site of most digestion and absorption
  • Duodenum, jejunum, and ileum
  • Circular folds, villi, and microvilli increase absorptive surface area
  • Crypt epithelial cells secrete intestinal juice
• Liver


  • Produces bile to break down fats
  • Liver lobules with a central vein and portal triad (bile duct, arteriole, venule)
• Gallbladder


  • Stores bile produced by the liver
• Pancreas


  • Produces pancreatic juice containing enzymes to break down food
• Large intestine


  • Absorbs more water from food residues and compacts them into fecal matter
  • Cecum, appendix, colon, rectum, anal canal
• There is plenty more to be said regarding the mechanism of digestion, including the specific enzymes required to break down different biomolecules
• Digestive system


  1. Ingestion

  2. Propulsion

  3. Mechanical breakdown

  4. Chemical digestion

  5. Absorption

  6. Defecation
• Enzymes


  Catalysts that speed up chemical reactions
• Macromolecules


  • Carbohydrates
  • Lipids
  • Proteins
  • Nucleic acids
• Small intestine


  • Duodenum absorbs iron
  • Jejunum absorbs folate
  • Ileum absorbs vitamin B12
• GERD is a burning sensation
• Salivary glands


  • Parotid
  • Sublingual
  • Submandibular
• Bile


  Made up of bile salts, cholesterol, and lipids
• Major sphincters

  • Swallowing reflex
  • Lower esophageal sphincter
  • Upper esophageal sphincter
  • Pyloric sphincter
  • Anal sphincter
• Large intestine absorbs salt and water
• Layers of digestive tract

  • Lumen
  • Endothelial lining
  • Submucosa
• Liver


  • Stores fat-soluble vitamins
  • Filters blood and dangerous substances
  • Involved in cholesterol homeostasis
• Bile


  Aids in digestion of fats into fatty acids
• Bile is excreted into the duodenum
• Bile helps absorb vitamin B12
• Gallbladder stores and concentrates bile, then releases it into the duodenum in response to food
• Calories obtained from food provide the energy we need to stay alive and the raw materials required for building all of our tissues
• We measure the energy stored in the chemical bonds in food using calories
• Accessory organs of the digestive system

  • Liver
  • Pancreas
  • Gallbladder
  • Salivary glands
• The digestive system converts the foods we eat into their simplest forms, like glucose, amino acids, and fatty acids, which are then absorbed
• The digestive system includes the esophagus, stomach, small intestine, and large intestine