The human digestive system breaks down food through physical and chemical digestion. Physical digestion involves chewing and stomach churning while chemical digestion uses enzymes to break foods into smaller molecules. Digestion begins in the mouth and travels through the esophagus, stomach, and small and large intestines. In each section, glands secrete enzymes to break down carbohydrates, proteins, and fats into smaller molecules like glucose, amino acids, and fatty acids that the body can absorb.

2. Human Digestive System
•Digestion is the process of breaking down large
food molecules into smaller soluble molecules that
can be readily absorbed by the body.
•Digestion occurs in an eight – metre long tube
called the gut or alimentary canal.

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

4. •The gut has an opening (mouth) into which food is
taken and another (anus) from which undigested
material is ejected.
•Below is the flow of food particles in the alimentary
canal.
Mouth Oesophagus Stomach
Anus
Large
intestines
Small
intestines
Duodenum

5. •There are two types of digestion – physical digestion and
chemical digestion.
•Physical digestion is the mechanical breakdown of
food by the chewing action of the teeth and the
churning action of the stomach.
•Chemical digestion involves the use of enzymes to break
down food substances.
•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.

6. There are three main types of digestive enzymes.
a.Carbohydrates – break down carbohydrates
b.Proteases – break down proteins
c.Lipases – break down fats and oils.

8. Process of Digestion
•Digestion in the mouth
a.Food is broken down into smaller pieces by the teeth.
b.At the same time, food is mixes with saliva from the
salivary glands.
c.Saliva contains salivary amylase (a kind of
carbohydrates) which catalyses the breaking down of
starch into maltose ( a kind of sugar)

10. •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.Peristalsis is caused by the contraction and
relaxation of muscles in the oesophagus.

12. •Digestion in the stomach
a.The stomach is a muscular sac with a volume of 2 to
4 litres.
b.Functions of the stomach:
Stores food for 2 to 6 hours.
Assist in the mechanical breakdown of food
(peristaltic and churning actions break apart the food
substances)
Secretes gastric juices.
c. Gastric juices contain proteases and hydrochloric
acid.
d. Proteases catalyse the breakdown of proteins into
polypeptides or peptones.

14. e. Functions of hydrochloric acids:
Stops the action of salivary amylase
Provides an acidic medium suitable for the action of
proteases.
Changes the inactive forms of proteases to their
active forms
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.

15. •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.

17. •Digestion in the duodenum
a.The duodenum is the first part of the small
intestines.
b.The duodenum receives bile from the liver and
pancreatic juice from the pancreas.
c.Functions of bile:
Neutralizes acidic chyme
Digests carbohydrate, lipids and proteins
- pancreatic amylase breaks down starch into
maltose.
- protease changes proteins to poliypeptides.
- lipase turns fats and oil into fatty acids and
glycerol.

19. •Digestion in the lower part of the small intestine
a.Tiny glands in the small intestine produce intestinal
juice which contains several type of digestive
enzymes.
b.Carbohydrate turns maltose into amino acids, and
lipases convert fats to fatty acids and glycerol
Class of food End product
Carbohydrates Glucose
Proteins Amino acids
Fats Fatty acids and glycerol
The end products of digestion

21. Place of
digestion
Glands Secretion Enzyme Digestive action
Stomach Gastric
glands
Gastric
juice
Proteases Starch to maltose
Hydrochloric
acid
•Kills
microorganisms
•Provides acidic
medium for action
of proteases
•Stops action of
salivary amylase
•Converts inactive
forms of
proteases to
active forms
Summary of digestion

22. Duodenum Liver Bile Emulsifies fats
Pancrea
s
Pancreati
c juice
•pancreatic
amylase
•Proteases
•lipase
•starch to
maltose
•Protein to
polypeptides
•Fats to fatty
acids and
glycerol
Lower
parts of
small
intestines
Intestin
al
glands
Intestinal
juice
•Carbohydr
ate
•Protease
•lipase
•Maltose to
glucose
•Polypeptides to
amino acids
•Fats to fatty
acids and
glycerol