Human Digestive System


The human digestive system functions in nutrition. It is a long tube stretching the entire length of the body. This tube, as a whole, is called the alimentary canal.


  • Nutrition: the way in which organisms obtain and use food
  • Herbivore: an animal that eats only plant material; e.g. deer.
  • Carnivore: an animal that eats only animals material; e.g. lion.
  • Omnivore: an animal that eats both animal and plant material; e.g. pig.

Nutrition in the human:

  • Nutrition: the way in which organisms obtain and use food.

There are four stages of nutrition:
  1.  Ingestion: the taking in of food into the mouth.
  2.  Digestion: the breakdown of food.

  • There are two types: mechanical and chemical digestion.
    • Mechanical digestion: the physical breakdown of large food particles into smaller ones.
    • Chemical digestion: the breakdown of food using enzymes.

  3.  Absorption: the passage of single biomolecules from the gut into the cells lining the gut.
  4.  Egestion: the getting rid of undigested material.

Structure and function of the human digestive system:

The human digestive system

Mouth: also known as the buccal or oral cavity and has a number of functions. It contains teeth and tongue that both function in mechanical digestion. Saliva also acts in the mouth.

Teeth: the teeth are located in the mouth. There are 32 teeth is a fully formed healthy adult mouth: 8 incisors; 4 canines; 8 premolars; and 12 molars.

The human dental formula is: 2(I 2/2; C 1/1; PM 2/2; M 3/3)

  • I stands for incisor and this formula shows that there are four on the top (2 on each side) and four on the bottom (2 on each side). Incisors function in cutting through food.
  • C stands for canine and this formula shows that there are a total of four canines (2 on the top, one on each side and 2 on the bottom, one on each side). Canines function in tearing and ripping food.
  • PM stands for pre-molars and this formula shows that there are a total of eight (4 on the top, two on each side and four on the bottom, two on each side). Premolars have cusps that function in grinding and crushing food.
  • M stands for molar and this formula shows that there are a total of 12 (6 on the top, three on each side and 6 on the bottom, three on each side. However, a person rarely has all 12 molars present. This is because four of these are wisdom teeth and very often wisdom teeth stay within the gum and never break through. Molars also have cusps that function in grinding and crushing food.

Salivary glands: produce saliva which is a mixture of water, mucous, and the enzyme amylase which digests starch into maltose.

Pharynx: the pharynx (or throat) is involved in swallowing.

Oesophagus: tube that is approximately 30 cm long and carries food from the pharynx to the stomach by contraction of smooth muscle in its wall. This is called peristalsis.

  • Peristalsis: rhythmical waves of contraction of smooth muscle pushing food along the alimentary canal in one direction only.

Stomach: muscular bag, located in the upper left abdomen, that receives food from the oesophagus. It stores food for approximately 2 hours and mixes it with gastric juice. This mixture is called chyme (see below).

  • Gastric juice: gastric juice is produced by the gastric glands of the stomach wall (mucosa). It is composed of:
  1. Water: moistens the food and acts as the medium in which chemical digestion occurs. It also takes part in the hydrolysis reactions that are important in chemical digestion.
  2. Mucous: protects the internal wall of the stomach from digesting itself.
  3. Hydrochloric acid: creates an acidic environment within the stomach (with a pH as low as 1). The acid helps to digest proteins by denaturing them and kills pathogens.
  4. Pepsin: released from the cells of the gastric glands in an inactive form called pepsinogen. This is so that the enzyme does not digest the cell before being released. Once pepsinogen comes into contact with the acidic conditions of the stomach it is converted to the active pepsin which digests proteins into peptides.

Duodenum: 30 cm long tube that receives chyme from the stomach. It is also the section of the alimentary canal where most chemical digestion occurs.

Pancreas: is an organ located underneath the liver and stomach. It secretes pancreatic juice for digestion of food. Pancreatic juice contains:

  1. Water: acts as the medium in which enzyme reactions occur and takes part in hydrolysis reactions.
  2. Mucous: protects the internal surface (mucosa) of the duodenum and ileum.
  3. Sodium bicarbonate: neutralises acidic chyme entering from the stomach.
  4. Lipase: digests lipids into fatty acids and glycerol.
  5. Amylase: digests starch into maltose.

Liver: largest internal organ of the human body. It is located in the upper right abdomen. The liver has many functions:

  • Breaks down old, worn out red blood cells and the haemoglobin.
  • Breaks down excess amino acids, in a process called deamination, into urea (which is then excreted by the kidneys).
  • Produces bile: bile contains water, mucous, salts (for emulsifying lipids), cholesterol and bile pigments (bilirubin and biliverdin).
  • Stores fat-soluble vitamins A, D, E and K.
  • Stores minerals, such as iron.
  • Produces plasma proteins (such as complement – see Chapter 34)
  • Stores glycogen – which is the storage form of carbohydrates in animals.
  • Detoxifies alcohol and other toxins present in the body.

Gall bladder: small bag-like organ located underneath the liver. It stores bile produced by the liver before releasing it into the duodenum.

Bile duct: carries bile and pancreatic juice to the duodenum.


Ileum: 6m long tube functioning in absorption of the products of digestion. The mucosa of the ileum has special adaptations that enable it to carry out absorption very efficiently. The ileum has:

  • Villi and microvilli – increase the surface area of the ileum available for absorption.
  • A good blood capillary network – enables the products of digestion to be transported away quickly to where they are needed.
  • A lymph supply (each villus has a lacteal) – responsible for the absorption of lipids.

Structure of a villus:


Appendix: small blind-ending tube attached to the caecum that is thought to function as part of the defence system.

Caecum: first part of the large intestine. Functions in absorption of water. It contains a large number of bacteria that function in producing vitamins and preventing the growth of pathogenic bacteria in the gut.

Colon: part of the large intestine. Functions in absorption of water, vitamins and minerals. There are bacteria present in the colon that produce vitamins that can then be absorbed and prevent growth of pathogenic bacteria.

Rectum: any undigested and unabsorbed material left at the end of the process is stored in the rectum as faeces  before being released.

Anus: opening of the rectum. It is a sphincter that is consciously controlled.

Hepatic portal system: network of blood vessels in the intestines connected to another network of blood vessels in the liver by the hepatic portal vein. Nutrients are absorbed by the small intestine and transported to the liver before being released to the rest of the body through the hepatic vein.

Balanced Diet:

  • A balanced diet is one that contains all seven nutrients in the correct proportions.

The seven nutrients are:

  1. Carbohydrates
  2. Lipids:
  3. Proteins
  4. Fibre
  5. Vitamins
  6. Minerals
  7. Water

Food pyramid

There are recommended daily amounts of different food types to have in the diet. An imbalance in any one of the nutrients or food types can lead to malnutrition.