Digestive system

• Digestive canal


  A continuous tube that extends through the thoracic and abdominal cavities from the esophagus to the anus
• Organs of the digestive canal

  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anal canal

  
• Accessory digestive organs


  • Mouth
  • Teeth
  • Tongue
  • Salivary glands
  • Pharynx
  • Liver
  • Gallbladder
  • Pancreas
• Digestive canal


  • Length is about 5-7 meters in a living person
  • Length is about 7-9 meters in a cadaver

  
• Functions of the digestive system

  1. Ingestion

  2. Secretion

  3. Mixing and propulsion

  4. Digestion

  5. Absorption

  6. Defecation
• Ingestion


  Taking food and liquids into the mouth (eating)
• Secretion


  Cells within the walls of the digestive canal and accessory digestive organs secrete water, acid, buffers, and enzymes into the lumen of the digestive canal
• Motility


  The capability of the digestive canal to mix and move material along its length
• Mechanical digestion


  The teeth cut and grind food before it is swallowed, and smooth muscles of the stomach and small intestine churn the food
• Chemical digestion


  Large carbohydrate, lipid, protein, and nucleic acid molecules in food are split into smaller molecules by hydrolysis catalyzed by digestive enzymes
• Absorption


  The movement of the products of digestion from the lumen of the digestive canal into blood plasma or lymph plasma
• Defecation


  Wastes, indigestible substances, bacteria, cells, and unabsorbed digested materials leave the body through the anus
• The wall of the digestive canal has four layers: mucosa, submucosa, muscular layer, and serosa/adventitia

  
• Mucosa


  The inner lining of the digestive canal, composed of epithelium, lamina propria, and muscularis mucosae
• Submucosa


  Areolar connective tissue that binds the mucosa to the muscularis, containing blood/lymphatic vessels and the submucosal neural plexus
• Muscular layer


  Contains an inner circular muscle layer and an outer longitudinal muscle layer, responsible for involuntary contractions that break down food and propel it along the canal
• Serosa


  The superficial layer of the digestive canal suspended in the abdominal cavity, composed of areolar connective tissue and simple squamous epithelium

  
• Enteric nervous system (ENS)


  The intrinsic set of nerves that regulate the digestive canal, consisting of the myenteric and submucosal neural plexuses
• Myenteric neural plexus


  • Located between the longitudinal and circular smooth muscle layers, controls digestive canal motility
  • Contains motor, sensory, and interneurons
• Submucosal neural plexus


  • Located within the submucosa, controls secretions of the digestive canal organs
  • Contains motor, sensory, and interneurons
• Muscular layer of the mouth, pharynx, and superior and middle parts of the esophagus


  • Contains skeletal muscle that produces voluntary swallowing
  • Skeletal muscle also forms the external anal sphincter, which permits voluntary control of defecation
• Muscular layer of the rest of the digestive canal


  • Consists of smooth muscle that is generally found in two sheets: an inner sheet of circular fibers and an outer sheet of longitudinal fibers
  • Involuntary contractions of the smooth muscle help break down food, mix it with digestive secretions, and propel it along the digestive canal
• Serosa


  • A superficial layer on portions of the digestive canal that are suspended in the abdominal cavity
  • Serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium)
  • Also called the visceral peritoneum

  
• The esophagus lacks a serosa; instead, only a single layer of areolar connective tissue called the adventitia forms the superficial layer of this organ
• Enteric nervous system


  Consists of neurons arranged into the myenteric and submucosal neural plexuses
• Myenteric neural plexus


  One of the neural plexuses of the enteric nervous system
• Submucosal neural plexus


  One of the neural plexuses of the enteric nervous system
• The enteric nervous system is regulated by the autonomic nervous system
• Digestive canal reflex pathway


  Involves sensory receptors, sensory neurons, and neurons in the enteric, autonomic, and central nervous systems that regulate digestive canal secretion and motility
• Peritoneum


  The largest serous membrane of the body, consisting of a layer of simple squamous epithelium (mesothelium) with an underlying supporting layer of areolar connective tissue
• Parietal peritoneum


  Lines the wall of the abdominal cavity
• Visceral peritoneum


  Covers some of the organs in the abdominal cavity and is their serosa
• Peritoneal cavity


  The slim space containing lubricating serous fluid that is between the parietal and visceral portions of the peritoneum
• Ascites


  Condition where the peritoneal cavity becomes distended by the accumulation of several liters of fluid
• Retroperitoneal


  Organs that lie on the posterior abdominal wall and are covered by peritoneum only on their anterior surfaces, not in the peritoneal cavity
• Major peritoneal folds


  • Greater omentum
  • Falciform ligament
  • Lesser omentum
  • Mesentery
  • Mesocolon
• Greater omentum


  • The longest peritoneal fold, drapes over the transverse colon and coils of the small intestine “fatty apron”
  • A double sheet that folds back on itself, giving it a total of four layers
  • Contains a considerable amount of adipose tissue
• Falciform ligament


  Attaches the liver to the anterior abdominal wall and diaphragm
• Lesser omentum


  Arises as an anterior fold in the serosa of the stomach and duodenum, and connects the stomach and duodenum to the liver
• Mesentery


  A fan-shaped fold of the peritoneum that binds the jejunum and ileum of the small intestine to the posterior abdominal wall