• Digestion is the breaking down of food substances into simple and soluble form. Therefore, the digestive system is the part of the human body which works together for digestion of food.
• The human digestive system is made up of five major parts; mouth, gullet/oesophagus, stomach, small intestine, and large intestine.

The Human Digestive System (Photo Credit: LumenLearning.com)

Functions of Parts of the Digestive System

• Mouth
  • It holds the teeth that chew the food.
  • It takes in the food.
  • It chews the food into small pieces and rolls it up for swallowing.
  • It produces the saliva that mixes with food to soften it before it is swallowed.
  • It produces enzymes that initiate the digestion of certain classes of foods.
• Gullet – is a long tube or passage way that:
  • connects the mouth to the stomach.
  • provides passage for food into the stomach.
• Stomach – is a sac-like organ that:
  • provides temporary storage for the swallowed food.
  • gradually releases the food into the small intestine for further digestion.
  • churns the swallowed food into a semi-liquid that can pass into the small intestine.
  • produces gastric juices and enzymes which act on the food.
  • has muscles that relax and contract to enable the movement of food to the small intestine.
  • acidifies food.
• Small Intestine – is a long narrow tube of about 2.5 cm in human adults.
  • It has three components called the duodenum, jejunum, and ileum.
  • Bile produced in the liver neutralizes the acidic food from the stomach.
  • Enzymes in digestive juices finalize the breakdown of food into absorbable form.
  • There is absorption of food into the blood stream through villi.
  • There is final digestion of all foods.
• Large Intestine: is a tube five times shorter than the small intestine and twice in diameter.
  • It has three main components; ascending colon, transverse colon, and descending colon.
  • It absorbs water from undigested food materials.
  • It leaves waste substances called feces which is passed from time to time through the anus.
  • It does not carry out any digestion.

Digestion of Food

• Digestion starts from the mouth for certain classes of food while for others, digestion starts much later as the food goes down the digestive system.
• In each part of the digestive system, certain chemicals called enzymes act on the food to bring about the breaking down of the food.
• Digestion does not take place in the gullet and large intestine.
• Vitamins, mineral salts and water do not undergo digestion. They exist in soluble states and are absorbed directly into the body.
• Digestion of proteins only starts in the stomach.
• An enzyme called pepsin digests proteins into peptones.
• An enzyme called ptyalin digests carbohydrates into simple sugars.
• An enzyme called renin changes fat in milk into semi solid, with the final digestion product as fatty acid and glycerol.
• The end-products of digestion are:
  • Simple sugar (glucose) from carbohydrates.
  • Amino acids from proteins.
  • Fatty acids and glycerol from fats and oil.
    

    Food	End Products
    Carbohydrates	Simple sugar
    Proteins	Amino acids
    Fats and Oils	Fatty acid and glycerol
    Vitamins	Soluble
    Mineral	Soluble
    Water	Soluble

• At the end of digestion, all the products, vitamins, salts, and water are absorbed in the small intestine through special structures called villi.
• The undigested food materials are passed into the large intestine.

Food Test