Diffusion gas exchange in
the human lungs
- plus comments on COPD and ventilators
Doc Brown's Biology exam study revision notes
There are various sections to work through,
after 1 they can be read and studied in any order.
of biology notes on exchange surfaces
3. Diffusion gas exchange in
the human lungs
- plus comments on COPD and ventilators
The lungs are the means of
transferring oxygen from air to the blood stream (blood plasma) and to
remove the waste gas carbon dioxide.
Know and understand that in humans:
surface area of the lungs is greatly increased by
the alveoli - millions of tiny air sacs of the end of the tiny bronchiole tubes in the lungs
where the gas exchange by diffusion takes place, but some basic stuff
before we reach the alveoli.
your head, the thorax is the upper part of you body (between the neck and
abdomen) and separated from the lower part of your body by the diaphragm muscle.
The lungs are rather like a large pink 'sponge' protected by the rib cage
and surrounded by pleural membranes which aid the movement of the chest
When you breathe in, the air passes down the trachea which splits into
two tubes called bronchi, two bronchus, each of which goes to one of your
The bronchi split into lots of smaller tubes called
finally end in small sacs called alveoli surrounded by tiny blood vessels
to effect the gas exchange (O2 in <=> CO2 out)..
The diagram shows the connection between
the mouth, windpipe (trachea), bronchus, bronchiole, lungs with their
alveolus and alveoli sub-structures.
Know and understand that the lungs are in the upper
part of the body (thorax), protected by the ribcage and separated from the
lower part of the body (abdomen) by the diaphragm.
should be able to recognise these structures of the lungs in the diagram
where the lungs are in the thorax.
The ribcage physically protects
the lungs and heart from being easily crushed and damaged.
In between the ribs are the
intercostal muscles which help to move air in an out of the lungs
To increase the efficiency of
gas exchange in the lungs the bronchus divides in two (the bronchi), so each
lung gets a good supply of air.
Each bronchus divides and divides into many
bronchioles with a tiny sac at the end of each one - the alveoli -
tiny sacs that
considerably increases the area for oxygen and carbon dioxide gas exchange.
The 'ventilation' pathway for air
(including oxygen) from breathing in through your mouth or nose is:
inhaled air ==> trachea ==>
bronchus ==> bronchiole ==> alveoli ==> individual alveolus (air
sac) in the lungs
The mouth and nasal passages
filter, warm and moisten the inhaled air, before finally reaching
Know and understand that the breathing system
takes air into and out of the body so that oxygen from the air can diffuse
into the bloodstream for respiration, and waste carbon dioxide from
respiration, can diffuse out of the bloodstream
into the air.
This gas exchange happens in the
lungs which has millions of tiny air sacs called alveoli at the ends of the
finest bronchiole tubes - a large surface area for gas exchange.
Surrounding the alveoli are many small arteries (fine
capillaries) bringing a good supply of 'dark red' deoxygenated blood to the lungs
- the thin walls of the fine capillaries of the small arteries
mean a short distance to enable faster diffusion rates for the
gases and they form a large surface area for gas exchange.
exchange occurs on the specialised moist thin membrane surfaces of the alveoli and the fine blood
vessels - the moisture in the membranes is good for dissolving gases and
increases the rate of gaseous diffusion.
When the blood from the rest of
the body arrives at the alveoli in the lungs it contains a
relatively high concentration of carbon dioxide and low
concentration of oxygen.
This maximises the diffusion
concentration gradients for the gas exchange i.e. the blood to
absorb fresh oxygen from the alveoli and the expulsion of carbon
dioxide from the blood in breathing out.
Direction of diffusion
gradients - from high to low concentration:
When air enters the alveoli
it has a greater concentration than the deoxygenated blood.
The steep concentration
gradient produces very efficient diffusion of oxygen into the
air in lungs ==> alveoli ==> blood, favours
oxygen transfer by diffusion through the alveoli membranes
Deoxygenated blood has a
greater concentration of carbon dioxide than the external air,
so it will diffuse out of the blood.
CO2 blood ==> alveoli
==> air in lungs, favours
carbon dioxide transfer by diffusion through alveoli membranes
Therefore the oxygen diffuses out
of the air into the blood capillaries of the alveoli (from high to
low concentration) and carbon dioxide diffuses out in the opposite
direction from the blood to the air in the lungs (again, from high
to low concentration).
So, oxygen, from breathing in, is transferred from the air in the
alveoli into the fine veins which carry the 'bright red' oxygenated blood away to
where it is needed in the rest of the body. Simultaneously carbon dioxide
diffuses in the opposite direction, from the deoxygenated blood into the alveoli
and breathed out.
The alveoli are well designed by
evolution to perform this gas exchange efficiently - refer to repeated
(from left to right with increasing detail)
Alveoli are very efficient
exchange surfaces and the
adaptations to increase the rate of transfer
of gas molecules are:
(i) The alveoli have a huge surface area because
of their tiny spherical sac like structure,
have a larger surface area : volume ratio than larger
spheres. For a given radius:
surface area /
volume = 3 / r. For more on this see
(ii) The sac walls are
very thin, only one cell thick, to reduce diffusion
distance and hence reduce diffusion time - giving a faster rate
of gas exchange,
(iii) The cell membrane lining is moist to
dissolve gases which can diffuse down their concentration gradients across
the exchange surface.
(iv) The alveoli have an
excellent blood supply from numerous tiny blood vessels - vein and artery
capillaries. Each alveolus is surrounded by blood capillaries that ensure
efficient transfer and the gas exchange can function down the
steepest concentration gradients.
BREATHING: Know and understand that to make air move into the
lungs the ribcage moves out and up and the diaphragm becomes flatter.
changes are reversed to make air move out of the lungs.
Know the movement of air
into and out of the lungs is known as ventilation - the mechanism of
should be able to describe the mechanism by which ventilation takes place,
including the relaxation and contraction of muscles leading to changes in
pressure in the thorax.
The lungs consist of soft sponge-like tissue protected
by the rib cage.
The diaphragm is a muscle located
underneath the ribcage. It moves up when it relaxes and down when it
In the trachea, or windpipe, there are tracheal rings,
also known as tracheal cartilages which are strong and flexible
tissue that help support the trachea while still allowing it to move and
flex during breathing.
Breathing involves changes in the thorax that
produce ventilation of the lung.
breathe in, the
intercostal muscles contract, expanding the rib cage, and the diaphragm also
contracts making it flatter, both of which increase the volume of the
This has the effect of
decreasing the pressure in the lungs and allowing fresh air to be easily drawn in,
the air will flow in to the lungs naturally, due to the pressure difference between the
air in the lungs (lower pressure) and the 'outside' air (higher pressure).
Overall the air is drawn in down through the trachea
which splits in two tubes (bronchi, 2 bronchus) and further splitting of
the airways into the smaller tubes of the bronchioles with the tiny air
sacs called alveoli at their ends, where the gas exchange takes place.
breathing out, the
intercostal muscles relax (ribcage contracts), the diaphragm relaxes and
moves up, so the combined effect is to decrease the volume of the lungs and increase the air pressure and
waste air is expelled from the lungs.
of lung volume
Lung volume is the quantity of air you can breathe
in for a single breath and varies from person to person e.g.
children will have a smaller lung volume than
adults, taller people tend to have larger lung volumes and rib
Lung volume can be measured with a spirometer
machine, which, after you breathe in to full capacity, you
breathe out through a tube connected to the machine which measures
the volume of expelled air.
Your effective lung volume and surface area for gas
exchange can be reduced by disease e.g. emphysema or cancer, both
can be caused by inhalation of dust (e.g. miners) and smokers (tar
Most people, most of the time when at rest, breathe
in and out about 12 to 16 times a minute.
Your breathing rate will increase if you are engaged
in more intense aerobic activity like running it may increase to 40
to 60 times a minute.
You can do a simple time experiment:
e.g. lets say you breathe in and out 75 times in
5 minutes when just sitting down.
breathing rate = 75 / 5 = 15 times/minute.
Repeat the experiment when briskly walking and
you will find your rate of breathing increased because of
increasing muscle demands for more oxygen and expulsion of waste
obstructive pulmonary disease (COPD)
Chronic obstructive pulmonary disease
(COPD) is the name for a group of lung conditions that cause breathing
They include emphysema – damage to
the air sacs in the lungs and chronic bronchitis – long-term inflammation of the airways
COPD is a common condition that mainly affects middle-aged or older
adults who smoke.
The problems are often caused by long-term exposure to irritants (particles in tobacco smoke, any
kind of fine dust e.g. mineral or coal, atmospheric particles from
vehicle exhaust) which damage and destroy the walls of the alveoli.
This means the gas exchange in
the lungs (O2 <=> CO2) is not as efficient as
it needs to be.
For COPD sufferers, the breathing problems tend to get gradually worse
over time and can limit your normal activities (readily become 'out of
breath'), although medication treatment can help keep the condition
Pandemic footnote in June 2020
As I'm adding this section on
COPD, we are in the middle of the Covid-19 coronavirus pandemic.
In the more serious cases, people
are suffering from breathing problems due to the virus causing
inflammation in the lungs - and this is where artificial ventilation
systems using oxygen are used to save lives.
to aid breathing
Ventilators move air
into and out of a persons lungs, where they cannot work unaided. This may be because some injury or medical
condition or undergoing an operation, which prevents them from breathing
This used to be done by a large
'capsule' called an 'iron lung' which encased the whole body of the patient
except for the head.
The pressure in the capsule is
mechanically lowered to allow the lungs to expand and take in air and then
raised to make the lungs contract and expel air.
However the blood flow in the
lower part of the body can be poor and giving rise to poor circulation side
Modern ventilators work by
pumping air in a go/stop cycle, using a mouth piece connection, directly
into the lungs to expand them and push out the ribcage.
When the pump temporarily stops,
the ribcage relaxes, contracting the lungs and expelling the air.
This is a much more convenient
method with a wide range of applications, and, it doesn't interfere with the
body's blood supply, but there can be problems if the alveoli (may burst)
can't cope with the artificially increased air supply.
Human circulatory system - heart, lungs, blood,
blood vessels, causes/treatment of cardiovascular disease
Possible practical work
You can use
sensors, eg spirometers, to measure air flow and lung volume
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