digestive

• The food we eat contains a variety of nutrients, which are used for building new body tissues and repairing damaged tissues
• Food is also vital to life because it is our only source of chemical energy
• Most of the food we eat consists of molecules that are too large to be used by body cells
• Digestive system


  Breaks down food into molecules small enough to enter body cells
• The digestive system forms an extensive surface area in contact with the external environment, and is closely associated with the cardiovascular system

  
• The combination of extensive environmental exposure and close association with blood vessels is essential for processing the food that we eat
• Digestive system


  Contributes to homeostasis by breaking down food into forms that can be absorbed and used by body cells, absorbing water, vitamins, and minerals, and eliminating wastes
• Organs of the digestive system

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine

  
• Accessory digestive organs


  • Teeth
  • Tongue
  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas
• Teeth


  Aid in the physical breakdown of food
• Tongue


  Assists in chewing and swallowing
• Accessory digestive organs never come into direct contact with food

  
• Accessory digestive organs produce or store secretions that flow into the GI tract through ducts to aid in the chemical breakdown of food
• GI tract


  1. Contains food from the time it is eaten until it is digested and absorbed or eliminated

  2. Muscular contractions physically break down food and propel it along the tract

  3. Enzymes secreted by accessory digestive organs and cells that line the tract break down food chemically
• Processes performed by the digestive system

  • Ingestion
  • Secretion
  • Mixing and propulsion
  • Digestion
  • Absorption
  • Defecation
• Ingestion


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


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


  Alternating contractions and relaxations of smooth muscle in the walls of the GI tract that mix food and secretions and move them toward the anus
• Mechanical digestion


  Teeth cut and grind food before swallowing, 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


  Movement of the products of digestion from the lumen of the GI tract into blood or lymph
• Defecation


  Wastes, indigestible substances, bacteria, cells sloughed from the lining of the GI tract, and digested materials that were not absorbed leave the body through the anus
• The wall of the GI tract from the lower esophagus to the anal canal has the same basic, four-layered arrangement of tissues: mucosa, submucosa, muscularis, and serosa/adventitia
• Mucosa


  The inner lining of the GI tract, composed of epithelium, lamina propria, and muscularis mucosae
• Epithelium of mucosa


  • Nonkeratinized stratified squamous epithelium in mouth, pharynx, esophagus, and anal canal
  • Simple columnar epithelium in stomach and intestines
• Lamina propria


  Areolar connective tissue containing blood and lymphatic vessels, supports the epithelium and binds it to the muscularis mucosae
• Muscularis mucosae


  Thin layer of smooth muscle that throws the mucous membrane of the stomach and small intestine into folds
• Submucosa


  Areolar connective tissue that binds the mucosa to the muscularis, contains blood and lymphatic vessels, and may contain glands and lymphatic tissue
• Muscularis


  Contains skeletal muscle in mouth, pharynx, and upper esophagus, and smooth muscle in the rest of the tract
• Serosa


  Serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium), forms the visceral peritoneum
• Adventitia


  Single layer of areolar connective tissue that forms the superficial layer of the esophagus
• The skeletal muscle in the muscularis of the mouth, pharynx, and superior and middle parts of the esophagus produces voluntary swallowing
• The skeletal muscle also forms the external anal sphincter, which permits voluntary control of defecation
• The rest of the muscularis contains smooth muscle, generally found in two sheets: an inner circular layer and an outer longitudinal layer
• The involuntary contractions of the smooth muscle help break down food, mix it with digestive secretions, and propel it along the tract
• Enteric nervous system (ENS)


  Intrinsic set of nerves that regulate GI tract motility and secretions
• Enteric nervous system


  • Contains motor neurons that supply the muscularis and secretory cells
  • Contains interneurons that interconnect the neurons
  • Contains sensory neurons with receptors that detect stimuli in the lumen
• Autonomic nervous system


  Extrinsic set of nerves that regulate the ENS, with parasympathetic nerves generally stimulating GI tract activity and sympathetic nerves generally inhibiting it
• Serosa


  Serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium)
• The esophagus lacks a serosa; instead, only a single layer of areolar connective tissue called the adventitia forms the superficial layer of this organ