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There are two stages in a heart beat. They are
called systole and diastole. For your GCSE biology exam you should be able to label a
diagram of the heart, put arrows on the diagram to show how blood flows through the heart,
and explain how the muscles and valves force blood through the heart. You should also know
that the left hand side of the heart pumps blood from the lungs to the rest of the body
and that the right hand side pumps blood from the body back to the lungs.
Here
are two labelled diagrams to show what happens during systole and diastole:
How the Heart Pumps Blood
Systole This should really be called "ventricular systole", but
don't worry about this for GCSE.
During systole the thick muscular walls of the ventricles
contract. This happens to both sides of the heart at about the same time. The diagram
shows systole in the left side of the heart. The contraction of the ventricular muscle
raises the pressure in the ventricle. The high pressure in the ventricle forces the
bicuspid valve to close and forces blood up the aorta. The diagram shows that the
semi-lunar valves are pushed open.
When your doctor measures your blood pressure and says that it is
120 over 90 he or she means that the blood pressure in your arteries is 120 when the left
ventricle is contracting.
The diagram also shows that the atrium is relaxing. Blood rushes
into the left atrium from the pulmonary vein: the pressure of the blood in the pulmonary
vein is just enough to stretch the walls of the atrium which are muscular but are much
thinner than the walls of the ventricle.
The bicuspid valve prevents blood from
going back from the ventricle into the atrium. Some people have a "heart
murmur", this is when the bicuspid valve does not close properly and some blood does
go backwards. The bicuspid valve is supported by tendons which look rather like the
strings of a parachute. These tendons prevent the bicuspid valve from being blown inside
out.
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Diastole This should really be called "ventricular diastole",
but don't worry about this for GCSE.
During diastole the thick muscular walls of the ventricles relax.
Again, this happens to both sides of the heart. The pressure of the blood in the
ventricles falls low enough for the bicuspid valve to open. At the same time, the atrium
contracts. Blood is forced from the atrium into the ventricle. This blows the ventricle up
like a balloon. The semi-lunar valves close because of the pressure in the aorta. If you
have been told that your blood pressure is 120 over 90, the second figure is the pressure
of the blood in your atreries when the ventricle is relaxing.
The diagram also shows you that the entrance to the left atrium
from the pulmonary vein is closed: this prevents blood from being forced back the way it
came. The muscle of the atrium is just strong enough to force blood into the ventricle and
stretch its walls.
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I hope that you will be now able to:
- label a diagram of the heart;
- put arrows on the diagram to show where blood
enters and leaves the heart;
- put arrows to show how blood goes through the
heart;
- recognise diagrams of systole and diastole;
- explain what the valves do;
- explain what happens in systole and diastole.
However, you will have to check with your text
book for the labels to the right side.
O You could buy a sheep's heart from your butcher. It will cost you
less than £1 and you can have great fun cutting it open and finding out how it works. You
don't need any special equipment; just use a sharp kitchen knife and a clean chopping
board but be careful === get your mum or dad to help you.
Here are the two diagrams side by side to help you see
the difference between the two stages:
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