Without food, water and oxygen, human beings could
The digestive system is a set of organs which change
what we eat into substances that can be used in the
These substances can be used for energy, growth and
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
1. Chemical digestion (amylase
converts starch to maltose)
2. Physical digestion (teeth break
food down into smaller
bile for the
1. Holds the food for a while
2. Physical digestion (food is
churned and mixed)
3. Chemical digestion (assisted
1. Chemical digestion
2. Absorption of
nutrients into blood
1. Elimination of waste
2. Absorption of water
Food can be broken down (digested) in one of two
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
The food is broken down by the teeth and mixed with
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
Functions of Saliva
It lubricates food with mucus, making it easier to
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
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.
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
It is a muscular tube.
It leads from the pharynx to the stomach.
To carry chewed food from the pharynx to the
Food moves along it by a muscular contraction known as
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
Body of Stomach
It is a J-shaped, elastic organ.
Food enters it from the oesophagus through the cardiac
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
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
Functions of The Stomach
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.
Hydrochloric acid neutralises bacteria and activates
Rennin is an enzyme that curdles milk protein in infants.
Pepsin is an enzyme that breaks down proteins into
Structure of the Small Intestine
It is seven metres long.
It is divided into three parts:
The walls has four layers:
A muscular layer
A layer containing blood vessels, lymph vessels, and
A submucous layer,
A mucous layer.
The inner wall is covered in villi, tiny hair like
projections which increase the surface area for
Each villi contain blood vessels and lymph vessels.
Functions of The Small Intestine
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
Intestinal Juices have the following enzymes:
Maltase, sucrase, lactase: change disaccharides
Peptidase: changes polypeptides into amino acids.
Digested food is absorbed through the villi walls.
Fats, fatty acids and glycerol are passed into the
Amino acids and sugars pass along the portal vein to
What is the Large Intestine
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.
Whatever remains of the food, is passed into the large
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
Defecation: peristalsis pushes waste along the colon and
then it is passed out of the body.
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
Digestive enzymes break food down into smaller, more
This allows the food to be absorbed into the blood
An example of a digestive enzyme is
Amylase is present in saliva
Amylase chemically breaks down starch
Amylase converts starch into a sugar called maltose
The substance that an enzyme works on is known as its
The substance formed by the enzyme is known as its
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”)
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
Physical (mechanical) digestion breaks food down into
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
This is known as peristalsis
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
These cut and bite food
These grasp and tear food
These grind and chew food
These also grind and chew food
The type of teeth that an animal has depends on what it
A herbivore eats plant material and has very large
incisors and molars
A carnivore eats other animals and needs very large
An omnivore (e.g. humans) eats all kinds of food and
needs and use all of the types of tooth equally
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