THE DIGESTIVE SYSTEM
© PDST Home Economics
 Without food, water and oxygen, human beings could
not survive.
 The digestive system is a set of organs which change
what we eat into substances that can be used in the
body.
 These substances can be used for energy, growth and
repair.
THE DIGESTIVE SYSTEM
 The alimentary canal is a tube that runs from the
mouth to the anus
 It is composed of the mouth, oesophagus, stomach,
small intestine and the large intestine
 As food passes through the alimentary canal it is
changed and the nourishment is taken into the blood
 Waste passes out the end of the canal
 Certain organs and glands add juices to the canal at
various points
Mouth
1. Chemical digestion (amylase
converts starch to maltose)
Oesophagus
2. Physical digestion (teeth break
food down into smaller
pieces)
Connects mouth
to stomach
Stomach
Liver
Produces
1.
Holds the food for a while
bile for the
2.
Physical digestion (food is
churned and mixed)
digestion of
fats
3.
Gall
Stores bile
bladder
Chemical digestion (assisted
by HCl)
Small intestine
1.
Chemical digestion
2.
Pancreas
Large intestine
(colon)
Produces digestive
juices
Appendix
Anus
Absorption of
nutrients into blood
1.
Elimination of waste
2.
Absorption of water
Rectum
Stores faeces
Food can be broken down (digested) in one of two
ways:
1. Physical Digestion
This is where large pieces of food are broken down
into smaller pieces of the same food
2. Chemical Digestion
This is where food is broken down into a different
substance that can easily pass into the blood
Mouth
 The food is broken down by the teeth and mixed with
saliva.
 Saliva is excreted by three pairs of glands:
• The parotid gland (below the ear)
• The submandibular (under the tongue)
• The sublingual (under the tongue)
 Saliva contains water, mucus and the enzyme salivary
amylase.
Functions of Saliva
 It lubricates food with mucus, making it easier to
swallow.
 It contains the enzyme salivary amylase, which acts on
cooked starch turning some of it into maltose.
 It keeps the mouth and teeth clean.
 The ball of food that leaves the mouth is known as a
bolus.
Functions of the Tongue
 Taste: it is covered with thousands of taste buds.
These are sensitive to salt, sweet, sour and bitter
chemicals in food and drink.
They help us enjoy food and drink and warn us when
food, drink are off or inedible.
 Chewing: the tongue aids chewing by moving the food
around the mouth, pushing it between the teeth and
covering it with saliva, which contains enzymes that
start the digestive process.
The food is turned into a partially digested mass known
as a bolus.
 Swallowing: when the food is ready to travel to the
stomach, the tongue pushes it to the back of the mouth.
Tongue Taste Centres
Epiglottis
Bitter
Papillae
Taste Buds
Sour
Salt
Sweet
Oesophagus
 The food passes into the pharynx (a muscular tube
behind the mouth) and down the oesophagus.
 The epiglottis a small flap of cartilage blocks the
entrance to the larynx, this stops the food going down
the wrong way and prevents choking.
Structure of the Oesophagus
& Functions
 It is a muscular tube.
 It leads from the pharynx to the stomach.
Function
 To carry chewed food from the pharynx to the
stomach.
 Food moves along it by a muscular contraction known as
peristalsis.
 The muscle fibres contract and relax which acts like a
wave on the tube, pushing the bolus forward.
 It’s lining secretes mucus to lubricate the passage of
food.
The Stomach
Oesophagus
Cardiac Sphincter
Pyloric Sphincter
Duodenum
Body of Stomach
Stomach Structure
 It is a J-shaped, elastic organ.
 Food enters it from the oesophagus through the cardiac
sphincter.
 The cardiac sphincter, is a valve that stops back flow of
the stomach`s contents.
 Food leaves the stomach through the pyloric sphincter
into the duodenum (first part of the small intestine).
 The walls of the stomach is made up of layers of
muscle.
 It has an inner mucous membrane.
 This membrane has lots of folds.
 When the stomach is full these folds stretch out,
enabling it to expand, then they contract when the
stomach empties.
Functions of The Stomach
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It digests protein through the action of enzymes.
It churns food with the gastric juices.
It helps lubricate the food by producing mucus.
It absorbs alcohol.
It kills bacteria by producing hydrochloric acid.
Gastric Juices
 Hydrochloric acid neutralises bacteria and activates
pepsin.
 Rennin is an enzyme that curdles milk protein in infants.
 Pepsin is an enzyme that breaks down proteins into
peptones.
Structure of the Small Intestine
 It is seven metres long.
 It is divided into three parts:
 The duodenum
 The jejunum
 The ileum
 The walls has four layers:
 A muscular layer
 A layer containing blood vessels, lymph vessels, and
nerves.
 A submucous layer,
 A mucous layer.
 The inner wall is covered in villi, tiny hair like
projections which increase the surface area for
absorption.
 Each villi contain blood vessels and lymph vessels.
Functions of The Small Intestine
Digestion
 Pancreatic juice is secreted into the duodenum and
contains the following enzymes:
 Trypsin: converts proteins into shorter chains.
 Lipase: converts fats into fatty acids and glycerol.
 Amylase: converts starch into disaccharides.
 Bile: emulsifies fats (breaks them into smaller
droplets).
 Intestinal Juices have the following enzymes:
 Maltase, sucrase, lactase: change disaccharides
into monosaccharides.
 Peptidase: changes polypeptides into amino acids.
Absorption
 Digested food is absorbed through the villi walls.
 Fats, fatty acids and glycerol are passed into the
lymph system.
 Amino acids and sugars pass along the portal vein to
the liver.
Ascending
Transverse
Colon
Colon
Descending
Colon
Caecum
Apendix
Anus
Rectum
What is the Large Intestine
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





It deals with waste.
It is about 1.5m long.
It consists of the following:
The caecum: a small pouch; the ileum empties its
contents into the caecum through the ileo-caecal valve.
The colon: ascending, transverse, descending colon.
The appendix: narrow tube attached to the caecum.
The rectum.
The anus
Functions
 Whatever remains of the food, is passed into the large
intestine
 To reabsorb water and vitamins left in digestive waste.
 It secretes mucus to help the movement of faeces.
 Short term storage of faeces in the rectum.
 Many bacteria live in the large intestine, they are
harmless in the colon and may be useful e.g. produce
Vitamin K.
 Defecation: peristalsis pushes waste along the colon and
then it is passed out of the body.
ENZYMES
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An enzyme is a biological catalyst
A catalyst speeds up chemical reactions
Enzymes speed up biological reactions
All chemical reactions that take place in living systems
require the action of an enzyme
ENZYMES
 Digestive enzymes break food down into smaller, more
soluble substances
 This allows the food to be absorbed into the blood
ENZYMES
 An example of a digestive enzyme is
amylase
 Amylase is present in saliva
 Amylase chemically breaks down starch
ENZYMES
Amylase converts starch into a sugar called maltose
STARCH  MALTOSE
ENZYMES
AMYLASE
STARCH
MALTOSE
ENZYMES
 The substance that an enzyme works on is known as its
SUBSTRATE
 The substance formed by the enzyme is known as its
PRODUCT
 Therefore starch is the substrate for amylase and
maltose is its product
STAGES OF NUTRITION
There are four stages in human nutrition:
1. Eating (also called “ingestion”)
2. Digestion
3. Absorption of digested food into the blood
4. Elimination of undigested food (also called “egestion”)
STAGES OF NUTRITION
 Digestion changes food into a form that can enter the
blood
 Physical (mechanical) digestion breaks food down into
smaller pieces
 Chemical digestion breaks food down into different,
more soluble substances
 The contents of the alimentary canal are pushed
along by a rhythmic pulsing of the muscles of the
intestines
 This is known as peristalsis
TEETH
 An adult human has 32 teeth, 16 in either jaw
 The shape and size of the tooth varies with the job it
has to do
 There are four different types of tooth
TEETH
Incisors
These cut and bite food
Canines
These grasp and tear food
Pre-molars
These grind and chew food
Molars
These also grind and chew food
TEETH
Incisor
Pre-molar
Canine
Molar
TEETH
 The type of teeth that an animal has depends on what it
eats
 A herbivore eats plant material and has very large
incisors and molars
 A carnivore eats other animals and needs very large
canines
 An omnivore (e.g. humans) eats all kinds of food and
needs and use all of the types of tooth equally