Digestive System

• Digestive System


  The system responsible for taking in food, breaking it down into nutrient molecules, absorbing the molecules into the bloodstream, and ridding the body of any indigestible remains
• Main functions of the digestive system

  • Take in food
  • Break it down into nutrient molecules
  • Absorb molecules into the bloodstream
  • Rid body of any indigestible remains
• Organs of the digestive system

  • Alimentary canal (gastrointestinal tract or gut): mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus
  • Accessory digestive organs: teeth, tongue, gallbladder, digestive glands (salivary glands, liver, pancreas)
• Digestive Processes


  • Ingestion: eating
  • Propulsion: swallowing, peristalsis
  • Mechanical breakdown: chewing, mixing with saliva, churning in stomach, segmentation
  • Digestion: enzymatic breakdown of complex molecules
  • Absorption: passage of digested fragments into blood or lymph
  • Defecation: elimination of indigestible substances via anus
• Peristalsis


  Adjacent segments of the alimentary canal organs alternately contract and relax, moving food distally along the tract
• Segmentation


  Nonadjacent segments of the alimentary canal organs contract and relax, moving food forward and backward to mix it and break it down mechanically
• Layers of the alimentary canal

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa
• Mucosa


  Tunic layer that lines the lumen, responsible for secretion, absorption, and protection
• Submucosa


  Consists of areolar connective tissue containing blood and lymphatic vessels, lymphoid follicles, and nerves
• Muscularis externa


  Muscle layer responsible for segmentation and peristalsis, containing an inner circular layer and an outer longitudinal layer
• Serosa


  Outermost layer made up of visceral peritoneum, or adventitia in some organs
• Components of the splanchnic circulation

  • Arteries branching off the aorta to serve digestive organs (hepatic, splenic, left gastric, inferior and superior mesenteric)
  • Hepatic portal circulation that drains nutrient-rich blood from digestive organs to the liver
• Enteric nervous system


  The nervous system intrinsic to the gastrointestinal tract, containing more neurons than the spinal cord and controlling motility
• Organs and structures of the mouth

  • Mouth (oral cavity)
  • Tongue
  • Salivary glands (parotid, submandibular, sublingual)
  • Teeth
• Mouth (oral cavity)


  Where food is chewed and mixed with saliva, and the swallowing process is initiated
• Tongue


  Occupies the floor of the mouth, composed of skeletal muscle, functions in gripping, repositioning, and mixing food, formation of the bolus, initiation of swallowing, speech, and taste
• Types of papillae on the tongue

  • Filiform papillae
  • Fungiform papillae
  • Vallate (circumvallate) papillae
  • Foliate papillae
• Salivary glands


  Produce saliva, which cleanses the mouth, dissolves food chemicals for taste, moistens food, compacts it into a bolus, and begins the breakdown of starch
• Major salivary glands


  • Parotid
  • Submandibular
  • Sublingual
• Saliva


  Mostly water, slightly acidic, contains electrolytes, salivary amylase and lingual lipase, proteins like mucin and lysozyme, and metabolic wastes
• Starch


  Broken down by enzyme amylase
• Saliva


  Produced by major (extrinsic) salivary glands located outside oral cavity
• Minor salivary glands


  Scattered throughout oral cavity; augment slightly
• Major salivary glands


  • Parotid: anterior to ear and external to masseter muscle, duct opens into oral vestibule next to second upper molar
  • Submandibular: medial to body of mandible, duct opens at base of lingual frenulum
  • Sublingual: anterior to submandibular gland under tongue, opens via 10-12 ducts into floor of mouth
• Saliva is mostly water (97-99.5%), so hypo-osmotic
• Saliva is slightly acidic (pH 6.75 to 7.00)
• Electrolytes in saliva


  • Na+, K+, Cl-, PO42-, HCO3-
• Saliva contains salivary amylase and lingual lipase
• Proteins in saliva


  • Mucin, lysozyme, and IgA
• Saliva contains metabolic wastes: urea and uric acid
• Lysozyme, IgA, defensins, and nitric oxide from nitrates in food protect against microorganisms
• Salivation


  1500 ml/day can be produced, minor glands continuously keep mouth moist
• Control of salivation


  1. Ingested food stimulates chemoreceptors and mechanoreceptors in mouth, sending signals to salivatory nuclei in brain stem that stimulate parasympathetic impulses along cranial nerves VII and IX to glands

  2. Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia)

  3. Smell/sight of food or upset GI can act as stimuli
• Teeth


  Lie in sockets in gum-covered margins of mandible and maxilla
• Mastication


  Process of chewing that tears and grinds food into smaller fragments
• Deciduous (milk) teeth


  • Incisors: central (6-8 mo), lateral (8-10 mo)
  • Canine (eyetooth) (16-20 mo)
  • Molars: first (10-15 mo), second (about 2 yr)
• Permanent teeth


  • Incisors: central (7 yr), lateral (8 yr)
  • Canine (eyetooth) (11 yr)
  • Premolars (bicuspids): first (11 yr), second (12-13 yr)
  • Molars: first (6-7 yr), second (12-13 yr), third (wisdom tooth) (17-25 yr)
• Pharynx


  Allows passage of food, fluids, and air, lined with stratified squamous epithelium with mucus-producing glands, has external muscle layers consisting of two skeletal muscle layers (inner longitudinal, outer pharyngeal constrictors)
• Esophagus


  Flat muscular tube that runs from laryngopharynx to stomach, collapsed when not involved in food propulsion, joins stomach at cardial orifice surrounded by gastroesophageal (cardiac) sphincter that keeps orifice closed when food is not being swallowed
• Esophagus has all four alimentary canal layers