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KS4 Biology
Blood Vessels
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Contents
Blood Vessels
What are blood vessels?
Arteries
Veins
Capillaries
Summary activities
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Blood vessels and the circulatory system
The circulatory system is
made up of the heart, blood
and blood vessels.
What are blood vessels and
what do they do?
Blood vessels are the
network of tubes that carry
the blood, pumped by the
heart, around the body.
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What is the total length of blood vessels?
Make a guess at the total length,
in kilometres, of all the blood
vessels in an adult human?
100, 000 kilometres
That’s more than twice the
distance around the Earth
at the equator!
There are a lot of blood vessels
so they must be important.
Are all blood vessels the same?
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Different types of blood vessels
There are three types of blood vessels, as shown in this
magnified part of the circulatory system.
blood to
blood from
the heart
the heart
artery
vein
Why are there different types of blood vessels?
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Different types of blood vessels
The different blood vessels have different jobs to do
in carrying blood around the body.
blood to
blood from
the heart
the heart
artery
vein
carries blood
away from
the heart
carries blood
back into
the heart
carries blood
to and from the
body’s cells
Do all blood vessels carry the same type of blood?
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Contents
Blood Vessels
What are blood vessels?
Arteries
Veins
Capillaries
Summary activities
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What is an artery?
Arteries are the blood vessels that carry blood away from
the heart.
blood to
blood from
the heart
the heart
artery
carries blood
away from
the heart
Blood that is pumped from the heart to the body’s cells
along the arteries is oxygen-rich.
Is this oxygen-rich blood under high or low pressure when
it leaves the heart?
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Cross-section of an artery
The arteries carry blood at high
pressure away from the heart.
Looking at the cross-section of
an artery, why is it suitable for
carrying blood at high pressure?
thick outer wall
thick inner layer
of muscle and
elastic fibres
narrow
central tube
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Blood flow in arteries – high pressure
Imagine using a hosepipe and
covering half of the open end
with your thumb. What happens
to the pressure of the water?
The water is released under
higher pressure and flows faster.
In the same way, arteries have
a narrow central core and thick
muscular walls.
This means that blood from the
heart is kept at high pressure and
flows quickly to reach every part
of the body, even the little toes!
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Blood flow in arteries – stretching
With each heartbeat, a surge of blood enters the arteries
and they expand to cope with the increased blood flow.
Which part of an artery allows it to expand?
thick inner layer
of muscle and
elastic fibres
increased
blood flow
The elastic fibres allow the artery to stretch under pressure.
The muscle fibres contract to push the blood along and
keep it flowing.
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Blood flow in arteries – detecting
There is a simple way of
detecting how an artery
stretches under the
pressure of each heartbeat.
What is it?
You can feel your pulse
because of the stretching
of an artery that passes
between bone and the
surface of the skin.
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Contents
Blood Vessels
What are blood vessels?
Arteries
Veins
Capillaries
Summary activities
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What is a vein?
Veins are the blood vessels that carry blood back into
the heart.
blood to
blood from
the heart
the heart
vein
carries blood
back into
the heart
Blood that travels from the body’s cells to the heart along
the veins is oxygen-poor.
Is this oxygen-poor blood under high or low pressure as
it returns to the heart?
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Cross-section of a vein
The veins carry blood at low
pressure back into the heart.
Looking at the cross-section of
a vein, why is it suitable for
carrying blood at low pressure?
thin outer wall
thin inner layer
of muscle and
elastic fibres
wide
central tube
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Comparing cross-sections
Compare the cross-section of a vein and an artery.
Why are they different?
Veins do not need to keep blood flowing quickly at high
pressure and so they have much thinner walls than arteries.
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Blood flow in veins – preventing backflow
Blood travels back to the heart in veins at low pressure.
If the blood pressure is too low, what, for example,
might happen to the blood in leg veins?
blood
to the
heart
possible
backflow
of blood
gravity
If low-pressure blood has to move against gravity, it might
slow down further and even flow in the wrong direction!
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Blood flow in veins – valves
Veins have valves to prevent backflow.
vein valve
open
blood
to the
heart
When blood flows along
veins it pushes past
the valves, which can
only open in one direction.
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backflow
prevented
vein valve
closed
If blood in a vein does
flow backwards,
it is trapped by
closed valves.
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Blood flow in veins – muscle contraction
Veins have valves to prevent backflow. They also have thin
walls and so need help from nearby muscles to push blood
towards the heart.
How does this work? (Think of a tube of toothpaste!)
blood
to the
heart
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Blood flow in veins – muscle contraction
Many veins are surrounded by muscles. When you move,
these muscles contract and squeeze the veins.
This pushes blood along the veins back towards the heart.
(Just like squeezing a tube of toothpaste!)
blood
keeps
flowing
to the
heart
blood
to the
heart
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Contents
Blood Vessels
What are blood vessels?
Arteries
Veins
Capillaries
Summary activities
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What is a capillary?
Capillaries are the tiny blood vessels that carry a blood
supply to and from the body’s cells.
What blood vessels are linked by capillaries?
artery
vein
Capillaries link arteries to veins.
Capillaries are the only blood vessels where substances
can be exchanged between the blood and body cells.
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Cross-section of a capillary
The capillaries carry blood to and
from the body’s cells.
Looking at the cross-section of
a capillary, why is it suitable for
the exchange of substances
between the blood and body cells?
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What happens in a capillary?
Capillaries have very thin walls
for the exchange of substances
between the blood and
surrounding body cells.
waste
products
How does this happen?
Substances are exchanged
by diffusion.
useful
substances
Useful substances in the
blood diffuse across the
capillary wall into body cells.
Waste products from the
body cells diffuse across the
capillary wall into the blood.
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Gas exchange in a capillary bed
A network of capillaries is called a capillary bed.
What gases are exchanged by diffusion in a capillary bed?
carbon
dioxide
oxygen
Oxygen in the blood diffuses across the capillary wall
into body cells for respiration.
Carbon dioxide from the cells diffuses across the
capillary wall into the blood.
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Contents
Blood Vessels
What are blood vessels?
Arteries
Veins
Capillaries
Summary activities
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Different blood vessels – activity
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What do blood vessels do?
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Which type of blood vessel?
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Multiple-choice quiz
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