B6.2: The Alimentary Canal

Digestion is defined as the breakdown of large, insoluble food molecules into small, water soluble molecules using mechanical and chemical processes

1. Ingestion: the taking of substances (e.g. food and drink) into the body through the mouth
2. Mechanical Digestion: the breakdown of food into smaller pieces without chemical change to the food molecules
3. Chemical Digestion: the breakdown of large, insoluble molecules into small, soluble molecules (which can be absorbed)
4. Absorption: the movement of small food molecules and ions through the wall of the intestine into the blood
5. Assimilation: the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
6. Egestion: the passing out of food that has not been digested or absorbed, as faeces, through the anus

1. Mouth + Salivary glands: the opening where food is taken in and the glands that secrete saliva
• The food is ingested through the mouth
• Your mouth chews and mechanically digests the food, breaking it down to smaller pieces which have higher surface to volume ratios. This allows it to digest faster
• Your salivary glands secrete saliva which contains amylase (enzyme for starch) and breaks some starch into amylases
• The food is turned into a lump called the bolus and is covered with saliva, allowing it to be swallowed easily
2. Oesophagus: Tube connecting mouth to the stomach
• Walls of the oesophagus contract to push the food down to the stomach
3. Stomach: Muscular organ that pounds the bolus and secrets acids and enzymes for digestion
• Muscular walls of the stomach squeeze the food and combine it with digestive juices
• The digestive juices break down the food with strong acids and enzymes, including protease which breaks down protein
• The hydrochloric acid kills harmful bacteria in the food and provides the optimum pH level for the protease enzymes to function
• Food goes to the small intestine
4. Small intestine: Narrow part of the intestine (made up of the duodenum and ileum)
• In the duodenum, the food from the stomach is digested by enzymes produced in the duodenum and the pancreas
• In the ileum, The nutrients from the food is absorbed
• The ileum is lined with villi (fingerlike projections) on its surface, which increase its surface area to absorb the nutrients
5. Large intestine: the long tube that connects the small intestine and anus
• Water is absorbed and faeces remain
6. Anus: The opening of the gastrointestinal tract that expels waste products
• Faeces (waste material) are stored in the rectum
• When the rectum is full, the faeces (waste material) are released from the anus

In addition to the organs in the alimentary canal, there are also other organs associated with it and help it digest food

• Pancreas
• Produces enzymes amlyase, protease, and lipase
• Secrets enzymes in a fluid with an alkaline pH to the duodenum to raise the pH of the food remains from the stomach
• Liver
• Produces bile (an alkaline fluid which helps digest) to break down fats into smaller droplets
• Gall Bladder
• Stores bile produced in the liver to release in the duodenum