#DigestiveSystem

#Biology

#HumanNutrition

Introduction

• The digestive system is an example of an organ system

• Some of the digestive system organs make up the alimentary canal; food passes directly through these organs as it moves through the body:

  • mouth

  • oesphagus

  • stomach

  • small intestine, including the duodenum and the ileum

  • large intestine, including the colon, rectum and anus

• Some of the organs of the digestive system do not form part of the route travelled by food, but are still involved with digestion; these are the associated organs, or accessory organs, and include the:

  • salivary glands

  • pancreas

  • liver

  • gall bladder

Function

• The function of the digestive system is to digest food and absorb nutrients

• The digestive system carries out its function in several stages:

  • ingestion: food and drink are taken into the body through the mouth

  • mechanical digestion: food is broken down into smaller pieces without chemical change to the food molecules

  • chemical digestion: large, insoluble molecules are broken down into small, soluble molecules

  • absorption: small food molecules and ions move through the wall of the intestine into the blood

  • egestion: food that has not been digested or absorbed passes out of the body as faeces

• Once nutrients have been absorbed into the blood by the digestive system they can be assimilated into the body; this occurs when they are taken up by the cells of the body

Organs

• Mouth - Food is ingested here and the teeth break it down into smaller pieces during mechanical digestion

  
  

• Salivary glands- Saliva is secreted into the mouth.

  The enzyme amylase in saliva begins to digest starch into maltose.

  Saliva lubricates the food for easy swallowing

• Oesophagus - This tube connects the mouth to the stomach.

  Contractions of the walls of the oesophagus force the food downwards; this is peristalsis

• Stomach - Churning of the muscular stomach walls continues the process of mechanical digestion.

  Protease enzymes begin protein digestion.

  Hydrochloric acid provides a suitable pH for the enzymes and also destroys any pathogens in food

• Liver - Bile is produced here.

  Bile aids the digestion of fats, as well as neutralising stomach acid as it exits the stomach

• Gall bladder - Bile is stored here before being released into the duodenum via the bile duct

• Pancreas - Amylase, protease and lipase enzymes are produced here before being released into the duodenum

• Small intestine: duodenum - Food enters the small intestine from the stomach here.

  The acidic stomach contents are neutralised by bile and become slightly alkaline.

  Enzymes complete chemical digestion here

• Small intestine: ileum - Food and water are absorbed into the blood via villi in the lining of the ileum

• Large intestine: colon- Remaining water is absorbed from food into the blood, and the solid waste left behind in the colon forms faeces

• Large intestine: rectum - Faeces are stored here prior to egestion

• Large intestine: anus - Faeces leave the body via the anus; this is egestion