COSTA
College of St. Thomas Aquinas
(Kindergarten Primary and Secondary School)
Rua Das Rosas No 293, Maputo, Mozambique
GRADE 11 - BIOLOGY
Gaseous exchange in the lungs
To supply the cells of our body with a continuous supply of oxygen for respiration and to remove
the carbon dioxide generated by respiration, we have evolved a specialised exchange surface for
gas exchange within the breathing system. The efficiency of this system is further improved by
ventilation of this exchange surface and by having an efficient blood supply - both of which
maintain a suitable concentration gradient.
The lungs
The lungs are part of the breathing system which is adapted for two functions:


ventilation – the movement of air into and out of the lungs
gas exchange – the 'swapping’ of gases between the alveolar air and the blood
The lungs are located within the upper part of your body called the thorax. They are surrounded
by the ribcage (which protects them) and in between the ribs are intercostal muscles which play
a role in ventilating the lungs.
Beneath the lungs is a muscular sheet called the diaphragm. This separates the lungs from the
abdomen of the body and also plays a role in ventilating the lungs.
Diagram of the lungs
Within the lungs is a network of tubes through which air is able to pass. Air is firstly warmed,
moistened and filtered as it travels through the mouth and nasal passages. It then passes through
the trachea and down one of the two bronchi and into one of the lungs.
After travelling into the many bronchioles, it finally passes into some of the millions of tiny sacs
called alveoli, which have the specialised surfaces for gas exchange.
Ventilation
When you inhale:
1. The intercostal muscles contract, expanding the ribcage outwards and upwards.
2. The diaphragm contracts, pulling downwards to increase the volume of the chest.
3. Pressure inside the chest is lowered and air is sucked into the lungs.
When you exhale:
1. The intercostal muscles relax, the ribcage drops inwards and downwards.
2. The diaphragm relaxes, moving back upwards, decreasing the volume of the chest.
3. Pressure inside the chest increases and air is forced out.
Mechanical ventilation
When a person stops breathing on their own, mechanical ventilation can be used until the patient
is able to recover and again breathe independently. This is done by machines called ventilators which fall into two main types:
1. Negative pressure ventilators - the patient is placed in an airtight machine from the
neck down, and a vacuum is created around the thorax. This creates a negative pressure,
which leads to the expansion of the thorax and a decrease in pressure. As a result, air is
drawn into the lungs. As the vacuum is released, the elasticity of the lungs, diaphragm
and chest wall cause exhalation.
2. Positive pressure ventilators - air is forced into the lungs through a tube which is
inserted into the trachea. As the ventilator pumps air in, the lungs inflate. When the
ventilator stops, the elasticity of the lungs, diaphragm and chest wall cause exhalation.
The table below lists some of the pros and cons of using artificial ventilators.
Artificial
ventilator
Uses
Advantages
Disadvantages

Negative
pressure
Positive
pressure
Developed and
used from the
1920s to treat polio
sufferers
Used extensively
since the 1950s

Effective at treating
many polio patients
over the years

Useful during
operations, where
surgeons need
access to the body


Patient is confined to the
machine
The vacuum on full-body
machines can affect the
abdomen, leading to the
pooling of blood in lower
parts of the body
Long-term ventilation
requires the tube to be
surgically inserted into the
trachea through the neck
Artificial
ventilator
Uses
Advantages

Disadvantages
Effective at
ventilating the lungs
Gas exchange
Within the alveoli, an exchange of gases takes place between the gases inside the alveoli and the
blood.
Blood arriving in the alveoli has a higher carbon dioxide concentration which is produced during
respiration by the body’s cells. However, the air in the alveoli has a much lower concentration of
carbon dioxide, meaning there is a concentration gradient which allows carbon dioxide to diffuse
out of the blood and into the alveolar air.
Similarly, blood arriving in the alveoli has a lower oxygen concentration (as it has been used for
respiration by the body’s cells), while the air in the alveoli has a higher oxygen concentration.
Therefore, oxygen moves into the blood by diffusion and combines with the haemoglobin in red
blood cells to form oxyhaemoglobin.
This table shows the differences (approximate figures) in the composition of inhaled and exhaled
air.
Gas
% of inhaled air% of exhaled air
Oxygen
21
16
Carbon dioxide0.04
4
Nitrogen
79
79
Adaptations of the alveoli
To maximise the efficiency of gas exchange, the alveoli have several adaptations:




They are folded, providing a much greater surface area for gas exchange to occur.
The walls of the alveoli are only one cell thick. This makes the exchange surface very
thin - shortening the diffusion distance across which gases have to move.
Each alveolus is surrounded by blood capillaries which ensure a good blood supply. This
is important as the blood is constantly taking oxygen away and bringing in more carbon
dioxide - which helps to maintain the maximum concentration gradient between the blood
and the air in the alveoli.
Each alveolus is ventilated, removing waste carbon dioxide and replenishing oxygen
levels in the alveolar air. This also helps to maintain the maximum concentration gradient
between the blood and the air in the alveoli.
QUESTIONS TO ANSWER
1. Explain what is wrong with each of these statements.
a) Inspired air is mostly carbon dioxide.
____________________________________________________________
____________________________________________________________
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b) Expired air contains more moisture than inspired air.
____________________________________________________________
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c) The nitrogen content of expired air is the same as inspired because
nitrogen cannot get into the body.
____________________________________________________________
____________________________________________________________
____________________________________________________________
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2. Fill in the crossword below
1
2
3
5
6
7
8
10
9
4
Across:
The MAXIMUM amount of oxygen
the body is able to use in one minute.
The type of "exchange" which takes
5. place when blood moves between the
lungs and the blood vessels.
6. Technical word for "Breathing In"
The total amount of air the lungs can
7.
hold
8. Technical word for "Breathing Out"
Haemaglobin carries oxygen - these
10.
contain haemaglobin
3.
3.
Down:
1. Breathed out in quantities of approximately 4%
The volume of OXYGEN the body actually
2. uses in one minute (not just how much air is
breathed in)
Breathed in in quantities of about 21% and
4. breathed out in quantities of 16% - more of this
is needed by the muscles when exercising.
The VOLUME of air breathed in and out in one
9.
breathing cycle