There are three main types of blood vessels - arteries, veins, and capillaries. Arteries carry oxygen-rich blood away from the heart at high pressure, and have thick muscular walls to maintain pressure. Veins carry oxygen-poor blood back to the heart at low pressure, and have thinner walls. Capillaries link arteries and veins and allow for the exchange of substances between blood and body cells through their very thin walls.

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KS4 Biology
Blood Vessels

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Contents
Blood Vessels
What are blood vessels?
Arteries
Capillaries
Summary activities
Veins

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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?
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?
100,000 kilometres

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There are three types of blood vessels, as shown in this
magnified part of the circulatory system.
Different types of blood vessels
Why are there different types of blood vessels?
blood from
the heart
blood to
the heart
artery vein

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Different types of blood vessels
The different blood vessels have different jobs to do
in carrying blood around the body.
vein
carries blood
back into
the heart
artery
carries blood
away from
the heart
Do all blood vessels carry the same type of blood?
blood to
the heart
blood from
the heart
carries blood
to and from the
body’s cells

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Contents
Blood Vessels
What are blood vessels?
Arteries
Capillaries
Summary activities
Veins

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What is an artery?
Arteries are the blood vessels that carry blood away from
the heart.
Is this oxygen-rich blood under high or low pressure when
Blood that is pumped from the heart to the body’s cells
along the arteries is oxygen-rich.
blood to
the heart
blood from
the heart
carries blood
away from
the heart
artery
it leaves the heart?

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Cross-section of an artery
thick outer wall
thick inner layer
of muscle and
elastic fibres
narrow
central tube
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?

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Blood flow in arteries – high pressure
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!
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.

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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
Capillaries
Summary activities
Veins

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What is a vein?
Veins are the blood vessels that carry blood back into
the heart.
Is this oxygen-poor blood under high or low pressure as
it returns to the heart?
Blood that travels from the body’s cells to the heart along
the veins is oxygen-poor.
blood to
the heart
blood from
the heart
vein
carries blood
back into
the heart

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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?
Cross-section of a vein
thin outer wall
thin inner layer
of muscle and
elastic fibres
wide
central tube

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Compare the cross-section of a vein
Comparing cross-sections
Veins do not need to keep blood flowing quickly at high
pressure and so they have much thinner walls than arteries.
and an artery.
Why are they different?

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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?
gravity
blood
to the
heart possible
backflow
of blood
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.
blood
to the
heart
backflow
preventedvein valve
open
vein valve
closed
When blood flows along
veins it pushes past
the valves, which can
only open in one direction.
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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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 flow in veins – muscle contraction
blood
to the
heart
blood
keeps
flowing
to the
heart

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Contents
Blood Vessels
What are blood vessels?
Arteries
Capillaries
Summary activities
Veins

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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?
What is a capillary?
Capillaries link arteries to veins.
Capillaries are the only blood vessels where substances
can be exchanged between the blood and body cells.
artery vein

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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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Capillaries have very thin walls
for the exchange of substances
between the blood and
surrounding body cells.
How does this happen?
What happens in a capillary?
Substances are exchanged
by diffusion.
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.
useful
substances
waste
products

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Gas exchange in a capillary bed
oxygen
carbon
dioxide
A network of capillaries is called a capillary bed.
What gases are exchanged by diffusion in a capillary bed?
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
Capillaries
Summary activities
Veins

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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