Digestive

• Digestive System


  System responsible for the digestion and absorption of food
• Villi of the Small Intestine

  • Increase the surface area for absorption of nutrients
• Function of the Digestive System

  1. Ingestion

  2. Break down the food (Mechanical digestion: chewing, mixing, and churning food; Chemical digestion: digestive enzymes breakdown food)

  3. Absorb nutrients

  4. Movement of nutrients from the GI tract to the blood or lymph

  5. Release of waste
• Organs of the Digestive Tract

  • Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, anus
• Accessory Digestive Organs


  • Teeth, tongue, gallbladder, salivary glands, liver, pancreas
• Tunics of the Digestive Tract

  • Mucosa (secretion of mucus, absorption of end products of digestion, protection against infectious disease)
  • Submucosa (connective tissue layer)
  • Muscularis (inner circular smooth muscle, outer longitudinal smooth muscle)
  • Serosa or adventitia (outermost layer)
• Enteric Nervous System


  • Submucosal nerve plexus (regulates glands and smooth muscle in the mucosa)
  • Myenteric nerve plexus (major nerve supply that controls GI tract mobility)
  • Sensory neurons (detect changes in chemical composition)
  • Motor neurons (stimulate or inhibit smooth muscle contraction and glandular secretion)
  • Interneurons (connect sensory to motor neurons)
• Peritoneum


  Serous membrane that lines the abdominopelvic cavity and organs
• Mesenteries


  Peritoneum that extend from the body wall to many of the abdominopelvic organs
• Retroperitoneal organs


  Organs located behind the peritoneum
• Oral Cavity


  • Bounded by lips, cheeks, palate, and tongue
  • Divided into vestibule and oral cavity proper
  • Lined with stratified squamous epithelium to withstand abrasions
• Tongue


  • Grips and repositions food during chewing
  • Mixes food with saliva and forms the bolus
  • Initiates swallowing and speech
  • Frenulum secures tongue to floor of mouth
• Types of Teeth


  • Incisors
  • Canines
  • Premolars
• Tooth Structure


  • Crown (exposed part above gum)
  • Dentin (covered by enamel)
  • Root (embedded in jawbone, contains pulp cavity)
  • Periodontal ligaments hold teeth in alveoli
• Enamel


  Acellular material composed of calcium salts and hydroxyapatite crystals, the hardest substance in the body
• Salivary Glands


  • Produce and secrete saliva that cleanses the mouth, moistens and dissolves food chemicals, aids in bolus formation, and contains enzymes that break down starch
  • Three pairs of large salivary glands: parotid, submandibular, and sublingual
• Saliva


  97-99.5% water and slightly acidic solution containing electrolytes, digestive enzyme (salivary amylase), proteins (mucin, lysozyme, defensins, IgA), and metabolic wastes (urea, uric acid)
• Control of Salivation


  • Primarily by the parasympathetic division of the autonomic nervous system
  • Stimulated by ingested food and the thought of food
  • Inhibited by strong sympathetic stimulation resulting in dry mouth
• Sialorrhea is excessive drooling
• Pharynx


  • Consists of the nasopharynx, oropharynx, and laryngopharynx
  • Pathway for food and fluids
• Saliva


  Secreted from serous and mucous cells of salivary glands, 97-99.5% water and slightly acidic solution containing electrolytes, digestive enzyme salivary amylase, proteins like mucin, lysozyme, defensins, and IgA, and metabolic wastes like urea and uric acid
• Serous saliva


  Contains enzymes, ions, and mucin
• Mucous saliva


  Produces mucous
• Salivation control


  • Primarily by the parasympathetic division of the autonomic nervous system
  • Salivary glands secrete serous, enzyme-rich saliva in response to ingested food, the thought of food, and strong sympathetic stimulation inhibits salivation and results in dry mouth
• Sialorrhea


  Excessive drooling
• Pharynx


  Consists of the nasopharynx, oropharynx, and laryngopharynx, transports food and fluids to the esophagus and air to the trachea
• Esophagus


  Connects the pharynx to the stomach, upper and lower esophageal sphincters regulate movement, mucous glands produce lubricating mucus
• Swallowing/Deglutition


  1. Voluntary phase: tongue moves bolus of food from oral cavity to pharynx

  2. Pharyngeal phase: reflex caused by stretch receptors in pharynx, soft palate closes nasopharynx, epiglottis/vestibular folds/vocal folds close larynx, pharyngeal muscles move bolus to esophagus

  3. Esophageal phase: reflex initiated by stretch receptors in esophagus, peristalsis moves food to stomach
• Peristalsis


  Wave of contraction that moves food through the esophagus to the stomach
• Stomach


  Temporary "storage tank" where chemical breakdown of proteins begins and food is converted from a bolus to chyme, openings are the gastroesophageal opening to the esophagus and pyloric orifice to the duodenum, major regions are the cardiac part, fundus, body, and pyloric part
• Stomach wall


  • External serosa, muscle layer, submucosa, simple columnar epithelium with surface mucous cells that produce alkaline mucus to protect the lining, empty stomach has 50mL volume but can hold about a gallon of food, folds called rugae when empty
• Stomach gland cell types

  • Mucous neck cells: secrete acid mucus
  • Parietal cells: secrete HCl and intrinsic factor
  • Chief cells: produce pepsinogen
  • Endocrine cells: produce regulatory hormones like gastrin, histamine, endorphins, serotonin, CCK, and somatostatin
• Chyme


  Ingested food mixed with gastric juice
• Gastric juice


  Contains mucus to protect stomach lining, pepsinogen converted to pepsin to digest proteins, HCl to promote pepsin activity and kill microorganisms, intrinsic factor necessary for vitamin B12 absorption
• Proton pump


  H+-K+ exchange pump that moves H+ out of parietal cells (proton pump inhibitor)
• Regulation of stomach secretion

  1. Cephalic phase: initiated by sight, smell, taste, or thought of food, stimulates HCl, pepsinogen, gastrin, and histamine secretion

  2. Gastric phase: initiated by stomach distension, stimulates gastrin secretion and activates CNS and local reflexes

  3. Gastrointestinal phase: acidic chyme entering duodenum stimulates neuronal reflexes and secretion of hormones like secretin and CCK that inhibit gastric secretions
• Stomach movements


  Waves mix stomach contents with secretions to form chyme, peristaltic waves move chyme into duodenum, hunger contractions (borborygmus), vomiting involves reverse peristalsis
• Small intestine


  Major digestive organ where digestion is completed and absorption occurs, subdivisions are duodenum, jejunum, and ileum
• Small intestine anatomy


  • Circular folds, villi, microvilli increase surface area 600-fold, epithelium contains absorptive cells, goblet cells, Paneth cells, and endocrine cells, Peyer's patches in submucosa
• Small intestine secretions


  Mucus protects against digestive enzymes and gastric acids, digestive enzymes like disaccharidases and peptidases are bound to intestinal wall, secretion stimulated by chemical/tactile irritation, vagal stimulation, and secretin