How the digestive system works

1st stop: Mouth
Both physical and chemical digestions take place in the mouth. The teeth break down food mechanically into small pieces to increase the surface area of the food exposed to the enzymes. The food is mixed with saliva produced by the saliva glands. Saliva contains the digestive enzyme, amylase, which breaks down starch in the food into maltose, a type of sugar. The other types of food molecules are not broken into smaller molecules in the mouth.

2nd stop: Oesophagus (gullet)
The gullet is a long and narrow tube joining the mouth and the stomach. When the food is swallowed, it does not "drop" or "fall" into the stomach through the oesophagus by the action of gravity. Instead, the oesophagus has strong muscles in its walls. These muscles contract and relaz to produce a wave-like movement that pushes the food into the stomach. This involuntary wave-like pattern of contractions is called peristalsis.

3rd stop: stomach
The stomach is a muscular bag which is able to expand to take in different amounts of food. The muscles in the wall of the stomach contract and churn the food to mix with a digestive juice called gastric juice.

Protease
Gastric juice is produced by the lining of the stomach and contains hydrochloric acid and proteases. The proteases in the stomach digest the proteins in the food into shorter chains of amino acids

Hydrochloric acid
Proteases in the stomach work well in acidic conditions. The hydrochloric acid provides the acidic condition in the stomach for the proteases to work . It also kills bacteria which may have been swallowed with the food.

The food stays in the stomach for a few hours and is slowly moved by peristalsis to the small intestine. By the time the food leaves the stomach, it would have turned into a thick mass of semi-liquid called chyme.

4th stop: Small intestine
the small intestine is a long and narrow tube joining the stomach and the large intestine. The small intestine is where most of the chemical digestion and absorption of food take place.
As the chyme enters the small intestine from the stomach, it is mixed with secretions from the liver, pancreas and the small intestine itself.

Bile
The liver produces a liquid called bile. It is stored in the gall bladder released into the small intestine through the bile duct to break down fats into tiny fat droplets.

The pancreas produces pancreatic juice and the small intestine produces intestinal juice. Glucose, amino acids, fatty acids and glycerol are the main end products of digestion. These molecules are now small enough to pas through the wall of the small intestine and into the bloodstream.

The digested food is absorbed by the small intestine. Some water, vitamins and mineral salts are absorbed too. These nutrients then diffuse out of the intestinal cells and and most of them enter the bloodstream through the capillaries. The end product of digestion are used for cellular procesesses like respiration, grouwth, and tissue repair.

5th Stop: large intestine
The undigested food moves from the small intestine into the large intestine by peristalsis.
The undigested food is made up largely of fibres which come from the cellulose that makes up the cell walls of vegetables and fruits. This provides roughage which helps in bowel movement. Some water is present too.

There are many different types of bacteria living in the large intestine. Most of these bacteria are harmless. Some are beneficial as they are able to make vitamins which are needed by the body. The cells lining the large intestine absorb these vitamins as well as mineral salt and excess water.

The mixture of undigested food and dead bacteria is called faeces and is stored temporarily in the rectum. Most of the time, the muscles in the rectum stay contracted. When the ring-like muscles between the rectum and anus relax, faeces is passed out of the body through the anus, This is known as defaecation.