Chapter 16: Digestive System

• Digestive System


  is a complex set of organs, glands, and ducts that work together to transform food into nutrients for cells.
• Food is taken into the digestive system, where it is broken down into smaller and smaller particles.
• Enzymes


  break the particles down into small molecules, which are absorbed into the blood and transported all over the body.
• Digestive System Functions

  Ingestion and mastication

  Propulsion and mixing

  Digestion and secretion

  Absorption

  Elimination
• digestive tract is also referred to as the GIT (gastrointestinal tract)
• The digestive tract consists of the

  oral cavity (mouth)

  pharynx

  esophagus

  stomach

  small intestines

  large intestines

  anus
• Associated organs of the digestive system

  salivary glands

  liver

  gallbladder

  pancreas
• Lips


  muscular structures, formed by the orbicularis oris muscle and covered by skin.
• Cheeks


  lateral walls of the oral cavity, within the cheeks are the buccinator muscles which flatten the cheeks against the teeth.
• Tongue


  large, muscular organ that occupies most of the oral cavity.
• Salivary Glands


  Produce saliva which is a mixture of serous (watery) and mucous fluids
• Parotid


  serous glands anterior to each ear
• Submandibular


  produce more serous than mucous secretions, found along the inferior border of the mandible.
• Sublingual


  produce primarily mucous and lie below the mucous membrane in the floor of the oral cavity
• Serous portion of saliva contains enzymes
• Amylase


  Digestive enzyme that breaks down carbohydrates
• Lysozyme


  Enzymes that are active against bacteria
• Mucous portion contains mucin for lubrication
• Pharynx


  Throat

  Connects the mouth to the esophagus
• Three parts of pharynx

  nasopharynx

  oropharynx

  laryngopharynx
• Esophagus


  Tube that connects the pharynx to the stomach

  Transports food to the stomach

  Joins stomach at cardiac opening
• Heartburn


  occurs when gastric juices regurgitate into esophagus

  caused by caffeine, smoking, or eating or drinking in excess
• Three phases of swallowing

  Voluntary phase

  Pharyngeal phase

  Esophageal phase
• Voluntary phase


  bolus (mass of food) formed in mouth and pushed into oropharynx
• Pharyngeal phase


  swallowing reflex initiated when bolus stimulates receptors in oropharynx
• Esophageal phase


  moves food from pharynx to stomach
• Peristalsis


  wave-like contractions moves food through digestive tract
• Stomach


  Located in abdomen

  Storage tank for food

  Can hold up to 2 liters of food

  Produces mucus, hydrochloric acid, protein digesting enzymes

  Contains a thick mucus layer that lubricates and protects epithelial cells on stomach wall from acidic pH (3)
• Rugae


  large folds that allow stomach to stretch
• Chyme


  paste-like substance that forms when food begins to be broken down
• Muscularis has 3 layers

  outer longitudinal, middle circular, and inner oblique to produce churning action
• Pyloric opening


  opening between stomach and small intestine
• Pyloric sphincter


  thick, ring of smooth muscle around pyloric opening which regulates movement of food into the small intestine
• Hydrochloric acid


  Produces a pH of about 2.0 in the stomach.

  Kills microorganisms, activates pepsin.
• Pepsin


  breaks covalent bonds of proteins to form smaller peptide chains
• Mucus


  A thick layer, which lubricates the mucosa of the stomach

  Protects mucosa from acidic chyme and pepsin
• Intrinsic factor


  Binds with vitamin B12 making it more readily absorbed by small intestine Vitamin B12 is important in DNA synthesis and red blood cell production
• Small Intestine


  Measures 6 meters in length

  Major absorptive organ of the gastrointestinal tract

  Contains enzymes to further breakdown food

  Contains secretions for protection against the acidity of chyme
• Duodenum Small Intestine


  first part

  25 cm long

  contains absorptive cells, goblet cells, granular cells, endocrine cells

  contains microvilli and many folds

  contains bile and pancreatic ducts
• Jejunum of Small Intestine


  second part

  2.5 meters long

  Primary site of nutrient absorption