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Human Digestive System

Human beings depend on other organism for food, therefore are called heterotrophic organisms, they need nutrients for various day to day activities. The complete process of nutrition is divided into five stages:
  • Ingestion
  • Digestion
  • Absorption
  • Assimilation
  • Defecation


The process of taking food into the mouth is known as ingestion.


  • There is some food which is not directly absorbable, so the process of conversion of non-absorbable food into the absorbable form is known as digestion.
  • The digestion of food is started from the mouth.
  • In mouth there are Salivary gland which secrets the Saliva in mouth in which two types of enzymes are found, ptyalin and maltase.
  • These enzymes convert the simple sugar and make it digestible.
  • Around 1.5 litre of saliva is secreted in human on an average day, it is acidic in nature (pH 6.8)
  • Through food pipe food reach into the stomach.

Digestion in Stomach:

  • As the food reaches in the stomach gastric glands secretes the gastric juice, this is a light yellow acidic acid.
  • Hydrochloric acid secreted from the Oxyntic cells of the stomach kills all the bacteria coming with food the; and accelerates the reaction of enzymes.
  • Hydrochloric acid makes the food acidic by which ptyalin reaction of the saliva end.
  • Pepsin and Renin are the enzymes in the gastric juice.
  • Pepsin breaks down the protein into peptones.
  • Renin breaks down the Caseinogen into Casein.

Digestion in Duodenum:

  • As the food reaches the duodenum bile juice form the liver combines with it.
  • Main function of the bile juice is to convert the acidic food into alkaline, as it is alkaline in nature.
  • Pancreatic juice form pancreas combines with food and it contains the following enzymes:
  1. Trypsin: It converts the protein and peptone into polypeptides and amino acid.
  2. Amylase: It converts the starch into soluble sugar.
  3. Lipase: It converts the emulsified fats into glycerol and fatty acids.

Small Intestine:

  • Here the process of digestion completed and absorption of digested foods start.
  • In small intestine, intestinal juices secrete and it is alkaline in nature and around 2 liters of intestinal juice secretes per day.
  • Intestinal juice contains the following enzymes:
  1. Erepsin: It converts the remaining protein and peptone into amino acids.
  2. Maltase: It converts the maltose into glucose.
  3. Sucrase: It converts the sucrose into glucose and fructose.
  4. Lactase: It converts the lactose into glucose and galactose.
  5. Lipase: It converts the emulsified fats into glycerol ad fatty acids.


  • The process of reaching the digested food into the blood is called absorption.
  • The absorption of digested foods takes place through small intestine villi.


  • Use of absorbed food in the body is called assimilation.


Undigested food reaches into large intestine where bacterias converts it into faeces which is excreted through anus.

Disorders of digestive system:

Here are some important digestive disorder in human beings.


Expulsion of food from mouth due to irritation in stomach.


Infectious disease resulting in loose frequent bowel.


Yellow colouration of skin and mucous membrane.

Gall stone:

Cholesterol crystallises to from gall stone.


difficulty of defecation due to decreased mobility in large intestine.