M18-20

• Digestive system


  Consists of the digestive tract / gastrointestinal (GI) tract (oral cavity, pharynx, esophagus, stomach, small & large intestines, & anus), plus the specific associated organs (salivary glands, liver, & pancreas)

  
• Functions of digestive system

  • Ingestion
  • Digestion
  • Absorption
  • Elimination
• Major tunics of digestive tract

  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa/Adventitia

  
• Mucosa


  Innermost; consists of (inner to outer) mucous epithelium, lamina propria, & muscularis mucosae
• Submucosa


  Contains plexus where autonomic nerves innervate
• Muscularis


  Consists of circular smooth muscle (inner) & longitudinal smooth muscle (outer)
• Serosa/Adventitia


  Consists of peritoneum / Regions not covered by peritoneum
• Peritoneum


  Comprised of 2 layers: the superficial parietal layer and the deep visceral layer. The peritoneal cavity contains the omentum, ligaments, and mesentery. Intraperitoneal organs include the stomach, spleen, liver, first and fourth parts of the duodenum, jejunum, ileum, transverse, and sigmoid colon.

  
• Oral cavity/Mouth


  First part of GI tract; bounded by lips & cheeks and contains the teeth & tongue
• Frenulum


  Thin fold of tissue that serves as anterior attachment of tongue to the floor of mouth
• Types of teeth

  • Central & lateral incisors
  • Canine
  • 1st & 2nd premolars
  • 1st, 2nd, & 3rd molars

  
• Permanent/Secondary teeth


  Adult teeth, normally 32 in number
• Primary/Deciduous/Milk/Baby teeth


  Lost & replaced during childhood, 20 in number
• Wisdom teeth


  The 3rd molar that appears in early 20's
• Regions of tooth


  • Crown (visible portion)
  • Neck
  • Root (largest & anchored in jawbone)

  
• Pulp cavity


  Center of tooth that contains pulp & is surrounded by dentin (cellular, calcified tissue)
• Enamel


  Extremely hard, acellular substance that protects tooth against abrasion & acids
• Alveoli


  Where the teeth are held in place along alveolar processes which are covered by gingiva / gums & secured by periodontal ligaments through embedding into cementum

  
• Palate


  Roof of oral cavity that prevents foods from passing into nasal cavity during chewing & swallowing; consists of hard palate (anterior) & soft palate (posterior)
• Uvula


  Posterior extension of soft palate
• Tonsils


  Located in the oral cavity
• Major salivary glands


  • Parotid glands
  • Submandibular glands
  • Sublingual glands
• Saliva


  Mixture of serous & mucous fluids that is secreted at rate of approximately 1L/day which is regulated by ANS, primarily the parasympathetic nervous system
• Functions of saliva


  • Helps keep the oral cavity moist
  • Contains enzymes that begin the process of digestion
  • Prevents bacterial infections in mouth with a mildly antibacterial enzyme called lysozyme
  • Neutralize pH
• Mucin


  A proteoglycan from mucous secretion that gives a lubricating quality to the secretions of glands
• Salivary amylase


  Breaks down starch
• Mastication/Chewing


  Begins the process of mechanical digestion, breaking down large particles into smaller ones
• Phases of swallowing/deglutition

  1. Bolus formed in mouth

  2. Tongue pushes bolus against hard palate & toward posterior part of mouth, into the oropharynx (Voluntary Phase)

  3. Pharyngeal Phase - bolus stimulates receptors to elevate soft palate (closing off the nasopharynx)

  4. Pharynx elevates to receive bolus

  5. Pharyngeal constrictor muscles contract while upper esophageal sphincter relaxes

  6. Food is pushed into esophagus

  7. Esophageal Phase - peristaltic waves activate & propel bolus through

  8. Cardiac sphincter relaxes as peristaltic waves approach the stomach
• Stomach


  Houses food for mixing with hydrochloric acid & other secretions
• Chyme


  Semifluid mixture of food & stomach secretions
• Secretions of stomach

  • Hydrochloric acid
  • Pepsin
  • Mucus
  • Intrinsic factor
• Hydrochloric acid


  Produces pH of about 2.0 (acidic) that kills microorganisms & activates the pepsin
• Pepsin


  Active form of pepsinogen that breaks down protein
• Mucus


  Forms thick layer that lubricates & protects epithelial cells from damaging effects of acidic chyme & pepsin; secretions can be stimulated by stomach irritation
• Intrinsic factor


  Binds with vitamin B12 to be readily absorbed in small intestine
• Phases of regulation of stomach secretions

  • Cephalic phase
  • Gastric phase
  • Intestinal phase
• Cephalic phase


  "Get started"; stomach secretions are increased in anticipation of incoming food
• Gastric phase


  "Go for it"; greatest volume of gastric secretion occurs due to presence of food in stomach
• Intestinal phase


  "Slow down"; inhibits gastric acid secretions as the acidic chyme enters into duodenum
• Parts of small intestine


  • Duodenum
  • Jejunum
  • Ileum