Introducing the digestive system

• Digestion

  1. Ingestion (taking in nutrients)

  2. Digestion (physical and chemical breakdown of complex food molecules into smaller molecules)

  3. Absorption (transfer digested nutrients from the digestive system to the bloodstream)

  4. Egestion (the removal of waste food materials from the body)
• Animal species

  • Each animal species has unique adaptations that enable it to obtain and digest food (ex. Mosquito's mouth)
• Gastrovascular cavity


  A simple, sac-like cavity that has only one opening to take in food and expel wastes (Ex. Jellyfish, flatworms)
• Gastrointestinal tract (GI tract)


  A passageway or tube that extends from the mouth to the anus (complete digestion)
• Every organ in the body


  Depends on the digestive system for nutrients
• The digestive system

  Relies on the muscles and bones of animals to catch and ingest food, the circulatory system transports oxygen to the digestive organs, the nervous system regulates the digestive organs
• Digestion in the Mouth and Stomach

  1. Physical and Chemical Digestion in the Mouth

  2. Food is broken down into small pieces by the teeth

  3. Saliva: salivary glands in the mouth secrete a watery fluid. Contains amylase, an enzyme that breaks down starch, and mucus which acts as a lubricant and aids swallowing

  4. Tongue: helps to roll the food into a ball, or bolus so that it can be swallowed

  5. Pharynx: food is pushed into the pharynx, the soft palate is raised to prevent food from entering the nasal passages

  6. Larynx: Raised against a flap called the epiglottis, it covers the entrance to the trachea to prevent food from entering the lungs
• Ingestion

  1. Food moves from the mouth to the pharynx, to the stomach by the esophagus

  2. The esophagus is a long muscular tube, food stretches the walls of the esophagus activating smooth muscles to undergo rhythmic contractions called peristalsis, this forces the food into the stomach
• The Structure of the Stomach


  • J shaped muscular organ
  • Can store up to 2L of food
  • Proteins are partly digested in the stomach and continue digestion in the small intestine
  • Lipids and carbs are not digested in the stomach
  • Sphincters control the movement of food into and out of the stomach
  • The gastroesophageal sphincter is located where the esophagus joins the stomach, when relaxed it allows food into the stomach, when contracted it prevents food from moving back into the esophagus
• The stomach has 4 layers

  • Mucosa: Secretes gastric juice (digestive enzymes, acid, and mucus)
  • Submucosa: Connective tissue containing nerves and blood vessels
  • Muscularis: Muscle layer containing smooth muscle (muscle contract with the food and acids creating a semi-liquid material called chyme)
  • Serosa: Holds the stomach in place, secretes lubricating fluid that eliminates friction between organs
• Gastrin


  A hormone that stimulates the release of hydrochloric acid and pepsinogen in the stomach
• Pepsin


  Protein digesting enzyme produced in the stomach
• HCl


  pH of 2-3, kills harmful microorganisms that are ingested with food. Stops the action of amylase, but activates pepsinogen
• Stomach Ulcers: When the lining of the stomach is exposed to strong stomach acid, an open sore called an ulcer can develop. Ulcers can bleed and can be painful. They are dangerous because bacteria can enter the blood stream
• Acid Reflux (heartburn): when the sphincters do not close properly, and acid from the stomach enters the esophagus. Causes a burning sensation in the lower throat