chapter 5

• Ingestion


  The oral cavity allows food to enter the digestive tract and have mastication (chewing) occurs, and the resulting food bolus is swallowed.
• Mechanical digestion


  Muscular movement of the digestive tract (mainly in the oral cavity and stomach) physically break down food into smaller particles.
• Chemical digestion


  Hydrolysis reactions aided by enzymes (mainly in the stomach and small intestine) chemically break down food particles into nutrient molecules, small enough to be absorbed.
• Secretion


  Enzymes and digestive fluids secreted by the digestive tract and its accessory organs facilitate chemical digestion.
• Absorption


  Passage of the end-products (nutrients) of chemical digestion from the digestive tract into blood or lymph for distribution to tissue cells.
• Elimination


  Undigested material will be released through the rectum and anus by defecation.
• Organs of the digestive system

  • Mouth, oral cavity, oropharynx, esophagus, stomach, small intestine, large intestine, rectum, anus
• Accessory structures of the digestive system


  • Teeth, tongue (in oral cavity), salivary glands, liver, gallbladder, pancreas
• Peristalsis


  Wavelike movement that occurs from the oropharynx to the rectum, allowing the GI tract to push food particles toward the anus.
• Mixing


  Mixing motion in the oral cavity and stomach that allows the GI tract to repeatedly break down food into smaller particles, using mechanical digestion.
• Segmentation


  Regions of the small intestine contracting and relaxing independently, allowing the small intestine to digest and absorb more efficiently.
• Parasympathetic nerves


  Stimulate GI tract activities.
• Sympathetic nerves

  Inhibit GI tract activities.
• Hormones


  From endocrine glands and from the GI tract itself help regulate GI tract activities.
• Reflex mechanism

  Regions of the GI tract (especially the stomach and small intestine) use reflexes to stimulate or inhibit one another.
• Ingestion


  Food enters the GI tract.
• Mechanical digestion


  Mastication (chewing) in the oral cavity.
• Chemical digestion

  Amylase enzyme in saliva breaks down polysaccharide into disaccharides.
• Tongue


  Made of skeletal muscle, manipulates the food during mastication and contains taste buds to detect taste sensations (intrinsic).
• Bolus


  Food particles mixed with saliva during mastication, resulting in a moist lump for easier passage into oropharynx.
• Teeth


  Adapted for mechanical digestion (mastication) in the oral cavity. 20 deciduous or primary teeth before the age of 6, then 32 permanent or secondary teeth divided into 4 types: incisors, canines, premolars, and molars.
• Salivary glands


  • Parotid, submandibular, and sublingual glands
• Saliva

  Helps moisten the food during mastication, dissolve the food in forming the bolus, and help cleanse the teeth. Consists of 99.5% water, the remaining 0.5% is dissolved substances including amylase enzyme, bicarbonate ion, and many electrolytes.
• Stomach


  A pouch-like organ primarily designed for food storage (for 2-4 hours), some mechanical and chemical digestion also occur. Contains two sphincters at both ends to regulate food movement: cardiac sphincter near the esophagus, and pyloric sphincter near the small intestine. Divided into 4 regions: cardiac, fundic, body, and pyloric. Contains thick folds called rugae for providing larger surface area.
• Gastric secretory cells


  • Chief cells, parietal cells, mucous cells, G cells
• Chief cells


  Secrete pepsinogen (an inactive enzyme).
• Parietal cells

  Secrete hydrochloric acid (HCl) and "intrinsic factor" (which helps absorption of vitamin B12 in the intestines).
• Mucous cells

  Secrete mucus and alkaline substances to help neutralize HCl in the gastric juice.
• G cells


  Secrete a hormone called gastrin, which stimulates the parietal cells and overall gastric secretion.
• Chemical digestion and absorption in the stomach

  Carbohydrate digestion continued with gastric amylase, protein digestion begins with pepsin, lipid digestion begins with gastric lipases. Absorption in the stomach is limited, where only small and fat-soluble substances can be absorbed.
• Chyme


  The yellowish paste resulting from all the mixing, chemical digestion, secretion, and absorption in the stomach, which will be passed on to the small intestine.
• Regulation of gastric secretion

  Involves both nervous and hormonal mechanisms, including the cephalic phase, gastric phase, and intestinal phase.
• Cephalic phase


  Involves special senses detecting food and using parasympathetic nerves in the vagus nerve to stimulate gastric activities.
• Gastric phase


  Involves the distension of stomach and stimulates its own activities by the vagus nerve. Amino acids and peptides in stomach lumen also stimulate acid secretion.
• Intestinal phase


  Involves acidic chyme passing into the small intestine which secretes intestinal gastrin hormone to inhibit gastric activities.
• Pancreas


  Most of the pancreas mass is devoted to its exocrine function: the secretion of pancreatic juice by the pancreatic acini and their ductile cells. Ductile cells produce sodium bicarbonate which helps neutralize the acidic gastric contents. Acinar cells produce a variety of digestive enzymes.
• Major pancreatic enzymes

  • Pancreatic amylase, pancreatic lipases, pancreatic nucleases
• Pancreas


  • Most pancreatic enzymes are produced as inactivate molecules, or zymogens, so that the risk of self-digestion within the pancreas is minimized
  • More than 98% of the pancreas mass is devoted to its exocrine function: the secretion of pancreatic juice by the pancreatic acini and their ductile cells
  • Ductile cells produce sodium bicarbonate which helps neutralize the acidic gastric contents
  • Acinar cells of the exocrine pancreas produce a variety of digestive enzymes to break down food substances into smaller absorbable molecules
  • Only 2% of pancreas mass is devoted to the islets of langerham, which produce insulin and glucagon, hormones that regulate blood sugar and carbohydrate metabolism (they have opposite effects)
• Major pancreatic enzymes

  • Pancreatic amylase: digest polysaccharides into disaccharides
  • Pancreatic lipases digest triglycerides into fatty acids
  • Pancreatic nucleases digest nucleic acids into nucleotides
  • Pancreatic proteinases (all secreted in their inactive forms) digest peptides into amino acids
• Activation of pancreatic proteases in the small intestine

  Trypsinogen is activated by enterokinase (secreted by duodenum) into trypsin, which in turn activates the other 3 enzymes - chymo-trypsinogen becomes chymotrypisn, proaminopeptidase becomes aminopeptidase, and procarboxypeptidase becomes carboxypeptidase