Human Digestive System


1. Digestion is the process of breaking down large food molecules into smaller soluble molecules that can be readily absorbed by the body.


2. Digestion occurs in an eight-metre long tube called the gut or alimentary canal.

The human digestive system includes the alimentary canal and organs such as the salivary glands, liver, gall bladder and pancreas


3. The gut has an opening (mouth) into which food is taken and another (anus) from which undigested material is ejected.


4. Below is the flow of food particles in the alimentary canal.



5. There are two types of digestion - physical digestion and chemical digestion.


6. Physical digestion is the mechanical breakdown of food by the chewing action of the teeth and the churning action of the stomach.


7. Chemical digestion involves the use of enzymes to break down food substances.


8. Enzymes are protein substances which act as catalysts and generally speed up the chemical reactions in our body. Enzymes that break down food substances in the gut are called digestive enzymes.


9. There are three main types of digestive enzymes.

(a) Carbohydrases break down carbohydrates.

(b) Proteases break down proteins.

(c) Lipases break down fats and oils.


Process of digestion

1. Digestion in the mouth

(a) Food is broken down into smaller pieces by the teeth.

(b) At the same time, food is mixed with saliva from the salivary glands.

(c) Saliva contains salivary amylase (a kind of carbohydrase) which catalyses the breaking down of starch into maltose (a kind of sugar).

(d) Mucus in the saliva lubricates the food.


2. Swallowing and peristalsis

(a) Our tongue shapes the food into a round lump called bolus before it is swallowed.

(b) The bolus is swallowed down the oesophagus.

(c) The bolus is pushed along the oesophagus to the stomach by a wave-like action called peristalsis.

(d) Persitalsis is caused by the contraction and relaxation of muscles in the oesophagus.


3. Digestion in the stomach

(a) The stomach is a muscular sac with a volume of 2 to 4 litres.


(b) Functions of the stomach:

(i) Stores food for 2 to 6 hours

(ii) Assists in the mechanical breakdown of food (peristaltic and churning actions break apart the food substances)

(iii) Secretes gastric juices

(c) Gastric juices contain proteases and hydrochloric acid.

(d) Proteases catalyse the breakdown of proteins into polypeptides or peptones.

(e) Functions of hydrochloric acid:

(i) Stops the action of salivary amylase

(ii) Provides an acidic medium suitable for the action of proteases

(iii) Changes the inactive forms of proteases to their active forms

(iv) Kills most micro-organisms in food

(f) Food is gradually converted to a semi-fluid substance called chyme.

(g) Mucus in the stomach protects the stomach wall from being destroyed by the acid and proteases.

(h) Chyme is pushed into the duodenum. 


4. The liver and the pancreas

(a) The main function of the liver in digestion is to produce bile.

(b) Bile is a brownish-green liquid which contains bile salts and bile pigments.

(c) Bile is stored in the gall bladder and released into the duodenum through a tube called the bile duct.

(d) The pancreas produces pancreatic juice which is released into the duodenum through the pancreatic duct.


5. Digestion in the duodenum

(a) The duodenum is the first part of the small intestine.

(b) The duodenum receives bile from the liver and pancreatic juice from the pancreas.

(c) Functions of bile:

(i) Neutralises acidic chyme from the stomach

(ii) Emulsifies fats into oil droplets for lipase to act upon

(d) Functions of pancreatic juice:

(i) Neutralises acidic chyme

(ii) Digests carbohydrates, lipids and proteins

- Pancreatic amylase breaks down starch into maltose.

- Protease changes proteis to polypeptides.

- Lipase turns fats and oil into fatty acids and glycerol.


6. Digestion in the lower part of the small intestine

(a) Tiny glands in the small intestine produce intestinal juice which contains several types of digestive enzymes.

(b) Carbohydrates such as maltose, sucrose and lactose are converted to simple sugars, polypeptides to amino acids and fats to fatty acids and glycerol.


           The end products of digestion

Class of food

End product




Amino acids


Fatty acids and glycerol


 Summary of digestion

Place of digestion




Digestive action


 Salivary glands


 Salivary amylase

 Starch to maltose










 Gastric glands





 Gastric juice



 Proteins to   polypeptides or   peptones


 Hydrochloric   acid

 - Kills micro-   organisms

 - Provides acidic   medium for   action   of   proteases

 - Stops action of   salivary amylase

 - Converts   inactive forms of   proteases to   active   forms







 Emulsifies fats




 Pancreatic juice

 Pancreatic   amylase

 Starch to maltose


 Proteins to   polypeptides


 Fats to fatty   acids   and glycerol


 Lower part of   small intestine


 Intestinal   glands


 Intestinal juice


 Maltose to   glucose


 Polypeptides to   amino acids


 Fats to fatty   acids   and   glycerol