Human Digestive System

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.

Teeth:

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

Tongue:

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.

Pepsin:

• 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:

• 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.