Had a look
Nearly there
AS & A le
vel
Nailed it!
3.1.1a, b, h
Gas exchange surfaces
Living organisms need to be able take in the oxygen they need for aerobic respiration. They also need to
remove carbon dioxide waste.
surface
volume = 2
red blood
cells
ll a r y
carbon
dioxide
red blood
cells
ow
in
capi
oxygen
blo o d
fl
The features that make the lungs an efficient exchange
surface are:
•Increased surface area due to many alveoli.
•The walls of the alveoli and the capillaries are thin
(only one cell thick), decreasing the distance needed
for the diffusion of gases.
•A good blood supply. There is a capillary close to
each alveolus, rapidly taking the oxygenated blood
away and bringing in deoxygenated blood. This
ensures that a steep concentration gradient for
oxygen and carbon dioxide is maintained.
To work out the SA : V ratio of
a cube, you need to know the
surface area and the volume.
You then divide the surface area by the volume.
For example, if the cube is 2 cm by 2 cm and
has 6 sides, the surface area is:
2 × 2 × 6 = 24 cm2
The volume will be:
2 cm × 2 cm × 2 cm = 8 cm3
That makes the SA : V ratio:
24
=3
8
The SA:V ratio is 3.
Why do multicellular organisms need a
specialised gas exchange surface?
w
flo
t
ou
surface
volume = 6
sides = 3
surface = 32 × 6 = 54
sides = 33 = 27
ai r
sides = 1
surface = 12 × 6 = 6
sides = 13 = 1
In mammals, the gas
exchange surface is in
the lungs. The trachea
branches into two bronchi,
which in turn branch into
many bronchioles. At the
end of the bronchioles
are air sacs called alveoli.
d
an
In single cell organisms, such as bacteria,
this can be done by diffusion, without the
need for a specialised exchange surface.
This is because a bacterium’s surface area
to volume ratio (SA:V) is large. It has a
large surface area compared to its volume.
In multicellular organisms, such as us,
the SA:V ratio is very small. The oxygen
could diffuse into our bodies but it would
take too long to reach the cells. For this
reason, we and other multicellular organisms
need a specialised gas exchange surface
to increase the SA:V ratio and maintain
metabolic activity.
Features of the gas
exchange surface
in
Surface area to volume ratio
(3 marks)
They have a small SA : V ratio.
Gases will not diffuse quickly enough.
Specialised gas exchange surfaces increase the
surface area, and therefore the SA : V ratio.
The histology of exchange surfaces
You can see that the walls of the alveoli are
very thin. These are made from squamous
epithelium (see page 45 for a reminder about this
specialised tissue). The alveolar walls also contain
some elastic fibres
so that the alveoli
can recoil back to
their original size
after expiration.
The alveoli are
associated with
many capillaries.
The gas exchange system in bony fish is an
example (see page 53).
In plants, the root hair cells are an efficient
exchange surface. They have a large surface
area to increase the rate of diffusion.
Why is it important to maintain a steep
concentration gradient for the gases being
exchanged?
(2 marks)
49