KS4 Biology
The Heart and
Circulatory System
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© Boardworks Ltd 2004
Heartbeat animation
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?
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?
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?
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?
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?
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?
Listening to a beating heart: lub-dub
What does a doctor hear when they listen to a patients’ heart?
lub-dub,
lub-dub,
lub-dub,
lub-dub,
lub-dub,
lub-dub…
The sound of a heartbeat is the sound of the heart valves.
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.