The various parts of human digestive system are:



The Digestion in Humans starts with the Ingestion of food. Ingestion means ‘taking in’ - food is taken in by the person, and it reaches the mouth.

(1) Mouth:

The mouth or buccal cavity contains teeth, tongue and salivary glands.


The teeth are used to chew the food and break it into smaller pieces so that it is digested easily.


The tongue then moves the food around the mouth properly so as to mix food with the saliva.  

Salivary Glands:

The salivary glands secrete saliva

  • It contains salivary amylase , an enzyme which helps in the chemical digestion of starch. 
  • Enzymes are biological substances that help in the progress of the reaction.


(2) Oesophagus:

It is also known as the food pipe .

  • From the mouth, the food is pushed into the food pipe (oesophagus) with the action of the tongue. 
  • The walls of the oesophagus contract and expand to help the food move down the pipe into the stomach. This expansion and contraction movement is called Peristalsis. 


(3) Stomach:

Due to the peristaltic movements of oesophagus, the food enters the stomach where a variety of juices act on it. 

  • The food is churned in the stomach for about 3-4 hours where a semi-solid paste is formed. 
  • The stomach wall contains three tubular glands in it. The glands present in this wall of stomach secrete gastric juices
  • The gastric juice contains hydrochloric acid , enzyme pepsin and mucus .


Hydrochloric acid: 

  • It kills any bacteria which may enter the stomach along with food.
  • Turns the gastric juice acidic in nature.



  • The enzyme pepsin works in this acidic medium to digest the protein present in the food to convert it into smaller molecules. 
  • This protein digestion begins in the stomach.



  • Mucus helps protect the lining of the stomach from the acidic secretion of HCl. 
  • If mucus is not secreted the lining of the stomach will be degraded leading to formation of ulcers.


(4) Small Intestine:

This partially digested food in the stomach now moves forward into the small intestine .

  • The exit of food from the stomach is regulated by sphincter muscle which releases it in small amounts into the small intestine.
  • The small intestine is the largest part of the alimentary canal. It is called the small intestine because it is very narrow
  • The small intestine receives secretions of two glands : liver and pancreas.


Liver : 

  • The liver secretes bile juice which is normally stored in the gallbladder and is a greenish yellow liquid .
  • Bile performs two functions :
  • It provides an alkaline medium for the acidic food coming from the stomach for the pancreatic enzymes to act on it
  • Bile salts break down the fats present in food into smaller globules making it easy for the pancreatic enzymes to act and digest them.


Pancreas : 

  • Pancreas secrete pancreatic juices which contain digestive enzymes like pancreatic amylase, trypsin and lipase.
  • The enzyme amylase breaks down starch
  • The enzyme trypsin digests proteins
  • The enzyme lipase breaks down emulsified fats .


  • The enzymes present in the intestinal juice complete the digestion of complex carbohydrates into glucose , proteins into amino acids and fats into fatty acids and glycerol
  • After digestion the molecules of food become very small and can then be absorbed by the walls of the small intestine and move further into our blood.
  • The small intestine is the region for absorption of digested food because of the presence of villi
  • Villi are small fingerlike projections which increase the surface area of the small intestine thereby increasing the rate of absorption.


  • From the bloodstream,  the digested food reaches all parts of the body where it becomes assimilated as part of cells.
  • The digested food which is not used by our body immediately is stored in the form of a carbohydrate called glycogen in our liver.


(5) Large Intestine: 

The undigested food passes from the small intestine into a wider tube called the large intestine.


  • Here, most of the water gets absorbed from the undigested food and thus the leftover part becomes almost solid. The solid waste is excreted through the anus as faeces or stool.  
  • The exit of the waste is called egestion or defecation. It is controlled by the anal sphincter.



Dental Caries: 

  • Dental Caries or tooth decay is the damage that occurs to teeth which can potentially result in cavities, dental abscesses or even tooth loss .
  • This begins as a result of plaque deposition on teeth. Plaque is a sticky deposit on teeth in which bacterial growth happens fast. 
  • This bacterial multiplication can convert the sugars in our food to acid. 
  • Enamel is the outermost, hard, white layer of a tooth
  • This acid may cause enamel deterioration.

STAGES OF TOOTH DECAY - Teachoo.jpg                                

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CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo