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Subjects /Biology / Digestive System of Human Body

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22 Apr 2021

Animals get their food from plants either directly by eating plants or indirectly by eating animals that eat plants.

Some animals eat both plants and animals.

Humans require food for growth, repair and functioning of the body.

Animal nutrition includes nutrient requirement, mode of intake of food and its utilisation in the body.

What is digestion?

  • The component of food such as carbohydrates are complex substances. These complex substances cannot be utilised as such. So, they are broken down into simpler substances. The breakdown of complex components of food into simpler substances is called digestion.
  • In single-celled organisms, the food may be taken by the entire surface.
  • In multi-celled organisms, different parts are specialised to perform the function of digestion.

Digestion in Humans

The alimentary canal is a long tube extending from mouth to the anus. The food passes through this continuous canal which begins at the buccal cavity and ends at the anus.

The canal is divided into various compartments:

  • Buccal cavity
  • Food pipe or oesophagus
  • Stomach
  • Small intestine
  • Large intestine ending in rectum
  • Anus

The food component gradually get digested as food travels through the various compartments.

The inner walls of the stomach and small intestine and various glands associated with the canal such as salivary gland, liver, and the pancreas secrete digestive juices.

The digestive juices convert the complex substances of food into simpler ones.

The digestive tract and the associated glands together constitute the digestive system.

The complete process of digestion can be divided into five parts namely:

  • Ingestion
  • Digestion
  • Absorption
  • Assimilation
  • Defecation


This part includes mouth and buccal cavity.

The process of taking the food into the mouth is called ingestion.


  • Our teeth process the food into smaller and of same texture material.
  • Each tooth is rooted in a separate socket in the gums.
  • Our teeth vary in appearance and perform different functions.
  • Molar teeth are used for grinding of food.
  • Canine are used for chewing and grinding of food.
  • Incisor are used for piercing and tearing of food.

Salivary Glands

  • These glands are present in the mouth.
  • These secrete saliva
  • Saliva contains enzyme called amylase or ptyalin.
  • These enzymes in saliva breaks down the starch into sugars.
  • The nature of saliva is acidic i.e. the ph value is lower than 7.


  • It is a fleshy muscular organ attached at back to the floor of the buccal cavity.
  • It is free at the front and can be moved into all the directions.
  • It mixes the saliva with food during chewing and helps in swallowing food.
  • We taste food with the tongue.
  • It has taste buds that detect different tastes of food.

Food pipe or Oesophagus

  • The chewed food passes into the food pipe after swallowing.
  • The muscles in the alimentary canal contract and expand regularly to push the food in forward direction. This movement of food is called peristaltic movement.
  • From the mouth, the food is taken to the stomach, where digestion of the food takes place.


  • The stomach is the thick walled bag.
  • It is the widest part of the alimentary canal.
  • The stomach is a large organ which expands when food enters it.
  • It receives food from the food pipe from one end and opens into the small intestine at the other end.
  • The food that we eat is made of complex substances, so to get the nutrients out of it, it needs to be broken down into simpler substances. This process takes place in stomach.
  • The muscular walls of the stomach of the stomach help in the mixing the food thoroughly with more digestive juices.

Gastric glands

  • These glands secrete the gastric juice which is acidic and light yellow in colour.
  • The digestive juices breakdown the protein into simpler substances. It releases:
  • Hydrochloric Acid
    • It creates an acidic medium which helps the enzyme pepsin to act.
    • It also kills the bacteria that may enter with the food.
  • Pepsin
    • It breaks down the protein into peptones.
  • Mucus
    • It protects the inner lining of the stomach from the action of the acid under normal conditions.
    • If mucus is damaged, it causes the problem of acidity.

Digestion and Absorption in Small Intestine

The exit of food from stomach to the small intestine is regulated by the sphincter muscle.

There are two parts of intestine:

  • Small intestine
  • Large intestine

Small Intestine

  • Small intestine is the longest part of the alimentary canal which is fitted into a compact space because of extensive coiling.
  • The length of the small intestine differs in various animals depending on the food they eat. For example, herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Whereas, meat is easier to digest, so carnivores like tiger have a short small intestine.
  • The small intestine is the site of complete digestion of carbohydrates, proteins and fats.
  • The food coming from the stomach is acidic and has to be made alkaline for the pancreatic enzymes to act.
  • Liver secretes bile juice which creates alkaline medium in the small intestine.
    • Bile juice is stored in a sac into Gall bladder.
    • It also helps in the digestion of fats. Fats are present in the intestine in the form of large globules which makes it difficult for enzymes to act on them. Bile salts break them down into smaller globules increasing the efficiency of enzyme action.
  • Pancreas secrets pancreatic juice.
    • It is a light cream coloured gland located just below the stomach.
    • Pancreatic juice contains:
      • Trypsin for digestion of protein in the small intestine.
      • Amylase to convert the protein and peptone into polypeptides and amino acids.
      • Lipase for breakdown of emulsified fats.
  • Intestinal Juice is secreted by walls of the small intestine which contains glands.
    • It is alkaline in nature.
    • Intestinal juice contains the following enzymes:
      • Lactase: To convert lactose into glucose.
      • Lipase: To convert emulsified fats into glycerol and fatty acids.
      • Sucrase: To convert sucrose into glucose and fructose.
      • Maltase: To convert maltose into glucose.
  • The enzymes present in the juice help in to convert:
    • Protein into amino acids
    • Carbohydrates into glucose
    • Fats into fatty acids and glycerol
  • Small intestine is divided into three parts:
    • Duodenum: It is the starting part of the small intestine. Here, the action of bile juice and pancreatic juice takes place.
    • Jejunum: Here the digestion is completed and the absorption of the digested food takes place. Here, intestinal juice is present.
    • Ileum: The absorption and assimilation of the digested food takes place.

Absorption in Small Intestine

  • Absorption refers to the process of passing the completely digested food into the blood vessels present in the walls of the small intestine.

Villi are the finger like outgrowths present into the inner walls of the small intestine.

  • The villi increase the surface area for the absorption of the digested food.
  • Each villus has a network of thin and small blood vessels close to the surface.
  • The surface of the villi absorbs the digested food material.


  • The process of transporting the absorbed substances via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by the body.
  • In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released.

Did You Know

  • Enzymes in the digestive system separate the food into different nutrients.
  • The small intestine is 22-23 feet long.
  • Stomach growling is called borborygmic. It is just louder when your stomach is empty.
  • You burp to release the extra air you swallow if you eat fast, drink carbonated drinks or smoke.