Digestive System

• Digestive System


  Brings nutrients into the body
• Nutrients


  Substances in food that are required for growth, reproduction and health maintenance
• Organs of the gastrointestinal (GI) tract

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anus
• Lumen


  The space within the hollow tube where food and liquids travel
• Sphincters


  Thick rings of circular smooth muscle that close off passageways between some organs
• This video shows the pathway of food through the digestive tract
• Layers of the digestive tract

  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa
• Mucosa


  The innermost layer, where nutrients cross to enter the blood. This layer also secretes mucus, which helps to lubricate the movement of food.
• Submucosa


  A layer of connective tissue containing blood, lymph vessels and nerves
• Muscularis


  Responsible for movement of food through the GI tract. It is made up of two to three sublayers of smooth muscle.
• Serosa


  The outermost layer of the GI tract. It has a thin connective tissue sheath surrounding and protecting the other three layers and attaches the GI tract to the walls of the body cavities.
• Processes of digestion


  • Ingestion
  • Digestion
  • Movement
  • Absorption
  • Elimination
• Ingestion


  When the mouth takes in food
• Digestion


  The breakdown of food, either by chemical or mechanical means
• Mechanical digestion


  Takes place primarily in the mouth and stomach
• Chemical digestion


  Secretion of fluid, digestive enzymes, acid, alkali, bile and mucus are all important
• Movement


  Food moves from one organ to another in the digestive system
• Types of motility


  • Peristalsis
  • Segmentation
• Peristalsis


  Propels food forward as lumps (bolus). The bolus of food stretches the smooth muscle in front, and smooth muscles contract behind.
• Segmentation


  Mixes food by contraction and relaxation of short segments of smooth muscle. Food particles are pressed against the mucosa, enabling the body to obtain nutrients.
• Absorption


  Nutrient molecules pass across the mucosal layer into the blood or lymph
• Elimination


  Undigested materials are eliminated from the body
• The mouth is important as mechanical and chemical digestion begin here
• Teeth


  Chew food into small pieces. Children have 20 teeth by around two years old, and adults have 32 permanent teeth.
• More and more people are not developing wisdom teeth
• In our past history, we didn't have dentistry. When archeologists look at our ancestor's remains, they note that many people who were in their early teens had lost some of their teeth (due to rot, abscesses, etc.). The wisdom teeth were meant to come in and replace missing teeth.
• Now that we do have good dentistry care, we often don't lose any of our teeth, and so we don't need these replacements
• Scientists state this is a sign that the human species has not stopped evolving, we are still adapting to an every changing environment
• Tongue


  Positions and tastes food. It consists of skeletal muscle enclosed in mucous membrane.
• Salivary glands


  • Parotid gland (near back of jaw)
  • Submandibular gland (below lower jaw)
  • Sublingual gland (below tongue)
• Saliva


  Moistens food and contains four ingredients: mucin, salivary amylase, bicarbonate, and lysozyme
• An average adult makes between one and two liters of saliva per day, which amounts to around around 23,000 liters (6,075 gallons) of saliva in a lifetime, which is enough to fill two swimming pools
• Swallowing


  A voluntary movement that becomes involuntary as food reaches the far back of the throat
• The tongue and jaw push a food bolus into the pharynx to trigger a swallowing reflex in which the soft palate rises to close off the passageway into the nasal cavities and the larynx rises. The epiglottis bends to close the airway to the trachea so we don't inhale the food into the lungs and the tongue pushes the food back into the esophagus.
• Esophagus


  A muscular tube connecting the pharynx and stomach. The lining makes mucus and peristaltic contractions move food to the stomach.
• Lower esophageal sphincter


  Opens briefly to allow food in and then closes to prevent reflux
• Important jobs of the stomach

  • Food storage
  • Digestion
  • Regulation of delivery
• Food storage


  The stomach shrinks when empty and expands to 1-3 liters with food
• Digestion


  Strong acids and protein-degrading enzymes digest food. Acid kills bacteria and muscle contractions mix food with secretions and pushes the food into the small intestine.
• Regulation of delivery


  The stomach regulates the amount of food delivered to the intestine. Stomach contractions mix food and push it forward by peristalsis.