9.2.1 The Digestive System Overview

• Alimentary canal


  The digestive system

  
• Ingestion


  Taking of substances, e.g. food and drink, into the body through the mouth
• Physical digestion


  Breakdown of food into smaller pieces without chemical change to the food molecules. This happens in the mouth and stomach.
• Chemical digestion


  Breakdown of large, insoluble molecules into small, soluble molecules. ENZYMES help with this process.
• Absorption


  Movement of small food molecules and ions through the wall of the intestine into the blood. The majority of absorption happens in the small intestine.

  
• Assimilation


  Movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.
• Egestion


  Passing out of food that has not been digested or absorbed, as faeces, through the anus
• Mouth


  • Ingestion: Taking in of food substances through the mouth
  • The teeth and tongue physically digest the food by grinding it up to increase the surface area
  • The salivary glands produce saliva containing the enzyme amylase which helps to break down starch

  
• Oesophagus


  • A long muscular tube
  • Food moves down due to the muscles contracting and relaxing, a process called peristalsis

  
• Stomach


  • The muscular walls help to physically digest the food
  • The stomach contains gastric juice which includes hydrochloric acid and the enzyme pepsin to chemically digest food

  
• Liver and gallbladder


  • The liver produces an alkaline substance called bile which is stored in the gallbladder and secreted into the small intestine
• Pancreas


  • Produces digestive enzymes like lipase, protease and amylase which are secreted into the small intestine

  
• Small intestine


  • The site of absorption where small molecules and water move from the small intestine into the blood
  • Made up of the duodenum where food is mixed with digestive enzymes, and the ileum where most digested food and water is absorbed

  
• Large intestine


  • The site of some water absorption to ensure faeces is solid
  • Peristalsis occurs to move food along
  • Fibre cannot be digested but helps food move through the colon

  
• Rectum and anus


  • Faeces is stored in the rectum
  • Egestion occurs at the anus where undigested food is passed out as faeces

  
• Digestive enzymes


  • Amylase
  • Lipase
  • Protease
• Food does not flow through the pancreas

  
• Most water is absorbed into the blood in the small intestine
• Lack of fibre in the diet can cause colon cancer
• Peristalsis occurs in the oesophagus and small and large intestine