The digestive system

• Main structures and associated organs of the human alimentary canal


  • Mouth
  • Oesophagus
  • Stomach
  • Pancreas
  • Liver
  • Gall bladder
  • Bile duct
  • Small intestine
  • Large intestine
  • Anus
• Mouth


  Where food enters the alimentary canal and digestion begins by amylase enzyme in the saliva starting the digestion of starch
• Oesophagus


  Muscular tube which moves ingested food to the stomach
• Stomach


  Muscular organ where digestion continues. Protease is secreted to begin the digestion of proteins
• Pancreas


  Produces the digestive enzymes carbohydrase, protease and lipase. These enzymes are secreted from here into the small intestine
• Liver


  Produces bile and secretes it to the gall bladder for storage
• Gall bladder


  Stores bile before releasing it into the first section of the small intestine
• Bile duct


  Takes bile from the gall bladder to the first section of the small intestine
• Small intestine


  Where food is mixed with digestive enzymes and bile and digested food is absorbed into the blood
• Large intestine


  Where water is reabsorbed
• Anus


  Where faeces leave the alimentary canal
• Mechanical digestion in the mouth


  1. Food is broken down into smaller pieces by chewing

  2. Pieces are mixed with saliva to form a ball of food called a bolus

  3. Bolus is swallowed and carried down the oesophagus by peristalsis
• Peristalsis


  Wave-like muscular contractions in the smooth wall of the gut which move food through the alimentary canal
• Hydrochloric acid in the stomach


  Kills many harmful micro-organisms that might have been swallowed along with the food
• Acidic conditions in the stomach


  Enzymes work best
• Protein digestion


  Starts in the stomach
• Bile


  Neutralises the acid and emulsifies fat in the small intestine
• Alkaline conditions in the small intestine


  Enzymes work best
• Absorption in the small intestine


  1. Digested food molecules move through the wall of the intestine into the blood

  2. Small intestine has a large internal surface area due to villi

  3. Good blood supply around villi maintains a steep concentration gradient for diffusion
• Villi


  Finger-like projections in the small intestine that provide a large surface area for the absorption of food
• Egestion


  1. Remaining material in the gut (water, bacteria, cells, indigestible substances) is absorbed in the large intestine

  2. Semi-solid waste material called faeces is passed out of the body through the anus
• Overeating can lead to obesity