The circulatory system uses the heart to pump blood throughout the body via blood vessels. The heart has four chambers - two upper atria and two lower ventricles - which work together to circulate blood in two loops. Valves between the chambers prevent backflow of blood. During each heartbeat, the atria contract together then the ventricles contract to pump blood out of the heart.

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KS4 Biology
The Heart and
Circulatory System

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How the heart pumps blood
The Heart and Circulatory System
The circulatory system
Structure of the heart
Valves in the heart
Summary quiz
Contents

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Which organs are involved in
this system?
How do substances move around the body?
The body has its own transport
system that carries substances
around the body.
The body’s transport system is
called the circulatory system.
Why is it given this name?
heart
blood vessels
blood

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What is carried by the circulatory system?
carbon
dioxide
oxygen
Which gases are transported to and from the body’s cells
by the blood flowing in the circulatory system?
Oxygen is the gas needed for respiration and is transported
to the body’s cells.
Carbon dioxide is the waste gas produced by respiration
that must be carried away from the body’s cells.

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The arrangement of the circulatory system means that these
two types of blood do not mix. Why is this important?
Two types of blood
The circulatory system carries two types of blood:
oxygen-rich
blood
c
oxygen-poor
blood
blood travelling
to the body cells
high oxygen content
low carbon dioxide
content
blood travelling
away from the body cells
low oxygen content
high carbon dioxide
content

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How are the two types of blood (oxygen-rich and
oxygen-poor) kept apart inside the heart?
At the heart of the circulatory system
The heart is the organ at the centre of the circulatory system.
It pumps blood around the body.

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Inside the heart
The inside of the heart is divided into two sections so that
the two types of blood (oxygen-rich and oxygen-poor)
are kept apart.
oxygen-poor
blood
right side
of the heart
oxygen-rich
blood
left side
of the heart
Remember that the heart is always labelled as if it is in a
body facing you, so the right side of the heart is on the
left of the diagram.

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The two sides of the heart

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How does blood circulate around the body?
Blood is pumped
around the body by
the heart.
It takes about 30
seconds for blood to
go once around the
body.
Starting with the left
side of the heart,
what route does the
blood follow to
complete one circuit
of the body?

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How does blood circulate around the body?
The left side of
the heart pumps
oxygen-rich blood
to the rest of the body.
This blood supplies
the body’s cells with
oxygen.
What gas does the
blood then pick up
from the body’s cells
and where does the
blood go next? body’s
cells
body’s
cells

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The oxygen-poor
blood needs to lose
the carbon dioxide and
pick up more oxygen.
How does it do this?
How does blood circulate around the body?
Blood picks up
carbon dioxide from
the body’s cells.
This oxygen-poor
blood then travels
back to the right side
of the heart.
body’s
cells
body’s
cells

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lungslungs
Where does this
oxygen-rich blood
then travel to?
How does blood circulate around the body?
Next, the right side
of the heart pumps
oxygen-poor blood
to the lungs.
In the lungs the blood
gets rid of the waste
carbon dioxide and
collects more oxygen.
body’s
cells
body’s
cells

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The oxygen-rich
blood then returns
to the left side of the
heart.
This completes the
blood’s journey
around the body.
Why is the journey
of blood through the
circulatory system
called a double
circulation?
How does blood circulate around the body?
body’s
cells
body’s
cells
lungslungs

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During one complete
circuit of the body,
blood passes through
the heart twice.
The heart has two
jobs to do and so the
circulatory system
involves a double
circulation.
What are the two jobs
that the heart carries
out during this double
circulation?
A double circulatory system
body’s
cells
body’s
cells
lungslungs

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Which way does blood flow?

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The Heart and Circulatory System
The circulatory system
Structure of the heart
How the heart pumps blood
Valves in the heart
Summary quiz
Contents

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The structure of the heart – exterior
The heart pumps blood around the circulatory system.
What is the heart made of?
The heart is made of muscle and keeps pumping blood
around your body, even when you are asleep!
muscle
tissue
What do the blood vessels on the outside of the heart do?

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The heart needs blood too!
The heart is full of blood but also needs its own blood
supply so that the muscle can keep pumping.
blood vessels
supply blood
to muscle tissue
The blood vessels on the outside of the heart carry
oxygen-rich blood to the heart muscle cells.
Oxygen-poor blood is then carried away from these
cells by outer blood vessels and back into the heart.
muscle
tissue

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The structure of the heart – interior
The inside of the heart is divided into two sections to keep
oxygen-rich and oxygen-poor blood separate.
Each side of the heart is also divided into two sections.
Each section of the heart is called a chamber.
How many chambers are there?
right side
of the heart
left side
of the heart
4

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The chambers of the heart
The four chambers of the heart have special names:
A lower chamber is called a ventricle.
An upper chamber is called an atrium (plural atria).
right
ventricle
right
atrium
left
ventricle
left
atrium

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What do atria and ventricles do?
The chambers of the heart have different functions.
The atria collect blood that enters the heart.
The ventricles pump blood out of the heart.
blood to
the body
blood from
the body
blood to
the lungs
blood from
the lungs

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Inside the heart – labels

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Inside the heart – labelling activity

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Chambers of the heart – activity

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The Heart and Circulatory System
The circulatory system
Structure of the heart
How the heart pumps blood
Valves in the heart
Summary quiz
Contents

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Preventing backflow
Blood always flows in the same direction as it moves
through the heart during each circulation of the body.
Why is it important that blood does not flow backwards?

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The chambers of the heart are separated by valves
which prevent blood from flowing in the wrong direction.
Heart valves
There are valves between the atria and the ventricles…
…and there are valves leading out of the ventricles.
valve between
right atrium and
right ventricle
valve between
left atrium and
left ventricle
valve leading
out of
right ventricle
valve leading
out of
left ventricle

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Naming the heart valves

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How are valves held in place?
The valves between the atria and ventricles are connected
to the inner walls of the heart by tough tendons.
valve open

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How are valves held in place?
The tendons allow the valves to close and hold the valve
flaps in place. They prevent the valves from flipping up
and turning inside out. Why is this important?
valve open valve closed

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How do valves work?
A valve acts like a door that only opens in one direction.
If the door is held by someone at a fixed point, only the arm
moves as the door opens and closes.
When the door is closed the arm is fully extended, so the
door can only be opened in one direction.

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How do valves work?
A valve acts like a door that only opens in one direction.
In the heart, the tendons holding the valve are like the
arm holding the door.
One end of each tendon is fixed to the wall of the heart
and so the valve can only open in one direction.

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The Heart and Circulatory System
The circulatory system
Structure of the heart
How the heart pumps blood
Valves in the heart
Summary quiz
Contents

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How does the heart pump blood?
Imagine the force needed
to squeeze a tennis ball.
That’s how much force
the heart uses to pump
blood around the body!
How does the heart produce
enough force to keep doing
this 24 hours a day?
The heart can pump blood
because it is made of muscle.
Muscle tissue works by
contracting (squeezing)
and relaxing.

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How does the heart pump blood?
All the parts of the heart on
either side, work together
in a repeated sequence.
The two atria contract and
relax; then the two ventricles
contract and relax.
This is how blood moves
through the heart and is
pumped to the lungs and
the body.
One complete sequence of
contraction and relaxation is
called a heartbeat.

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Heartbeat animation

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Stages of a heartbeat
Stage 1:
A heartbeat begins
with the heart muscle
relaxed and valves
closed.
Blood flows into the
two atria and both
sides fill up with blood.
This blood has to be
pushed through the
valves to get into the
ventricles. How does
this happen?

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Stages of a heartbeat
Stage 2:
The atria contract and
the blood is squeezed
which causes the
valves leading to the
ventricles to open.
Blood then flows from
the atria into the
ventricles.
What happens to the
open valves when the
atria are empty?

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Stages of a heartbeat
Stage 2 (continued):
The valves between
the atria and the
ventricles close.
This prevents any
backflow.
What happens next
to the blood in the
ventricles?

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Stages of a heartbeat
Stage 3:
Almost immediately,
the ventricles contract
and the blood is
squeezed again.
The pressure of the
blood forces open the
valves leading out of
the heart.
Blood is pumped out
of the heart.
What happens to the
open valves when the
ventricles are empty?

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Stages of a heartbeat
Stage 3 (continued):
When the ventricles
are empty, the valves
leading out of the
heart close and the
heart muscle relaxes.
This completes the
sequence of
contraction and
relaxation in one
heartbeat.
What will happen
next?

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Stages of a heartbeat
Stage 1 (again):
The atria fill up with
blood as the heartbeat
sequence begins
again.
Why are the walls
of the atria thinner
than the walls of
the ventricles?
Why is the wall of the
left ventricle thicker
than the right
ventricle?

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What does a doctor hear when they listen to a patients’ heart?
Listening to a beating heart: lub-dub
The “lub” is caused by
the closing of the valves
leading to the ventricles.
The “dub” is caused by
the closing of the valves
leading out of the heart.
lub-dub,
The sound of a heartbeat is the sound of the heart valves.
lub-dub, lub-dub, lub-dub, lub-dub, lub-dub…

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Measuring a beating heart
You can measure how
fast your heart is beating
by taking your pulse.
Each pulse that you feel
is due to the pressure of
blood leaving the heart as
the left ventricle contracts.
Place the fingertips of one
hand on the opposite wrist,
where an artery passes
near the surface of the skin.
What is your heart rate,
in beats per minute,
right now?

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…in one hour?
…in one day?
…in one year?
…in 70 years?
How many heartbeats?
If your heart beats at an average rate of 70 times
per minute, how many heartbeats are there…
70 x 60 =
4,200 x 24 =
100,800 x 365 =
36,792,000 x 70 =
100,800
36,792,000
2,575,440,000
4,200

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The sequence of a heartbeat

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Journey of blood around the body

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The Heart and Circulatory System
The circulatory system
Structure of the heart
How the heart pumps blood
Valves in the heart
Summary quiz
Contents

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Multiple-choice quiz