digestion system and function

What is digestion? The process in which the complex food,  insoluble molecules, and the food that we can not be absorbed directly by the blood that we eat, is broken down into simple, soluble substances, which can be absorbed by the body is called digestion. 

Humans use physical and chemical methods for the digestion of food. Physical methods such as chewing and grinding the food in the mouth and chemical methods like the addition of digestive juices (enzyme) to food by the body itself.

The Human Digestive System 

The human digestive system consists of the alimentary canal and its associated glands. liver and pancreas. The human alimentary canal which runs from mouth to anus is about 9 meters long tub. Humans have various organs of the digestive system:

Mouth :                                            

  • Teeth chew and grind the food,
  • saliva changes starch into sugar.

Oesophagus (food pipe)

  • Carries food from mouth to stomach.


  • Food is churned and mixed digestive juice.


  • Secretes digestive juice.


  • Secretes digestive juice.

Small intestine  

  • Digestion is complete and food is absorbed.

Large intestine  

  • The absorption of water takes place.

  • It stores the undigested waste mater called feces.

  • Undigested solid waste is passed out as stool.

Glands associated with digestive system:

The glands associated with the human digestive system are Salivary glands, the liver, the pancreas, gall bladder. The ducts of various glands open into the alimentary canal.

Human digestive process

The various phases of digestion in the human being are as follows: the whole process of humans conducted in 5 steps.
  1. Ingestion.
  2. Digestion,
  3. Absorption,
  4. Assimilation, and
  5. Ejection.


Digestion of food begins in the mouth itself. The mouth cavity (buccal cavity) contain teeth, tongue and salivary glands. The teeth cut the food into small pieces, chews, and grinds. the teeth help in physical digestion.

The function of salivary glands

The salivary glands in our mouth produce saliva (water fluid). The saliva is secreted by three pairs of salivary glands (one below the tongue, second at the base of the ear, and third on the inner side of the angles of the lower jaws. The tongue helps in mixing this saliva with food.

Saliva is a fluid containing water, salts and a slimy mucous.it also contains an enzyme called amylase which converts starch into maltose. (Chemical digestion)

Starch --salivary/amylase → sugar (maltose)

Digestion of food remains incomplete in the mouth.

Oesophagus (food pipe) :

The slightly digested food in the mouth is swallowed by the tongue and goes down the food pipe called the esophagus. The esophagus carries food to the stomach. The food moves down the esophagus by peristalsis.no digestion occur in the esophagus.

Peristalsis movement: 

peristalsis is the slow wave-like movement seen in the walls of the esophagus due to the contraction and relaxation of its muscles. Peristalsis occurs throughout the alimentary canal. this movement enables the food to be pushed downward.

The digestion process in the stomach:

The stomach is a J-shaped organ present on the left side of the stomach.it contains glands called gastric glands which secrete gastric juice. Gastric juice contains 3 substances: hydrochloric acid, enzyme pepsin, and mucus.

  • Hydrochloric acid kills the germ and makes an acidic medium, which activates the enzyme pepsin.
  • Pepsin converts proteins into a simpler compound called peptones.
  • Proteins - pepsin → Peptones.
  • The protein casein present in the milk is converted into curd by the help of another enzyme, the rennin.
  • Casein → Rennin ⇢ Curd

The food remains in the stomach for about 3-4 hrs or more. During this time stomach also churns the food thoroughly along with the digestive juices and changes it into a pulp-like thick paste called chyme. The chyme moves into the first part of the small intestine.

The digestion process in the Small intestine:

The small intestine is the largest part of the alimentary canal.  it is about 6.5 m Long in an adult man. The food remains in the small intestine for about 3-5 hrs for digestion and absorption.the short upper 'U' shaped part of the small intestine, containing from the stomach, is called the duodenum. Duodenum receives a common duct that brings secretions from both the liver (bile juice) and the pancreas (pancreatic juice).

The villi:

The small intestine is the main area for the absorption of digested food.the inner lining of the small intestine contains numerous tiny finger-like structures called villi (singular villus). Each villus has a network of thin blood vessels close to its surface. The surface of the villi absorbs amino acid and glucose to pass them into the blood system whereas fatty acid passes into lymph vessels and vitamins and minerals are directly absorbed through the wall of the intestine.

The liver:  


The liver is reddish-brown and is the largest gland in the body. It is situated on the right side of the body, below the chest region.

The liver secretes bile. Bile is a greenish-yellow liquid made in the liver and stored in the gall bladder.it is alkaline and contains salt. Bile performs the following function.

👉 Makes the acidic food coming from the stomach alkaline.

👉 Bile salts break the fat present in the food into smaller globules, which makes enzymes easy to digest. This process is called emulsification.

The pancreas

The pancreas secretes pancreatic juice. the pancreatic juice contains enzymes namely amylasetrypsin, and lipase which help in the digestion of carbohydrates, protein, and fats respectively.

👉 Amylase: starch converting into maltose.

👉 Trypsin: converts protein and peptones into peptides.

👉 Lipase: converts emulsified fats into fatty acid and glycerol.

The second part of the small intestine is called the jejunum, no digestion takes place jejunum.

The semi-digested food enters the last part of the small intestine called the ileum. The inner lining of the ileum contains glands called intestine glands which produced intestinal juice erepsin, maltase, sucrase, and lactate.

👉 Erepsin : converts peptides into amino acids.

👉 Maltase : converts maltose into glucose.

👉 Sucrase : converts sucrose into glucose and fructose.

👉 Lactase: converts lactose into glucose and galactose.

The small intestine is the place of the complete digestion of food (such as carbohydrate, protein, and fat)

Digestion process in the large intestine:   
Large intestine (ejection): the large intestine is about 1.5 m long and consists of 3 regions Caecum, colon, and rectum. the large intestine, not secrets any enzyme, it mainly absorbs water from the undigested food.after absorption of water the undigested food reaches the rectum is semi-solid.
The rectum is 15 cm long and last part of the large intestine.it stores the undigested waste matter called faces. The rectum opens to the outside at the Anus. The process of eliminating undigested food through the anus is called ejection.

Conclusion of the digestive system:

Assimilation: It is the utilization of digested food by body cells. The nutrients absorbed into the blood and transported to the rest of the body, and used to obtain energy, build new cells, and repair the old cells.

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