The document summarizes the key components and functions of the respiratory system involved in gas exchange. It describes how the lungs maximize surface area for diffusion through alveoli with thin walls. It also explains how ventilation creates concentration gradients that drive diffusion of oxygen into the blood and carbon dioxide out of the blood in the lungs.

1. GAS EXCHANGE
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2. The gaseous exchange system
 Cleans and warms air that enters during
breathing
 Maximises the surface area for diffusion of O2
and CO2 between blood and atmosphere
 Minimises the distance for this diffusion
 Maintains adequate gradients for this diffusion
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3. The Respiratory System Nasal cavity: air
passing over the
A flap of elastic
mucous membrane of
tissue that
the nasal cavity is
forms a lid
moistened, warmed
over the
and filtered
opening of the
trachea
Passage where the nose and
mouth come together
Contains 2 ligaments (vocal
cords) that produce sound
when air move through them
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4. Lungs
 Site of gaseous exchange between air and
blood
 Presents a huge surface area to the air
that flows in and out
 In the thoracic (chest) cavity
surrounded by airtight space containing
fluid to allow friction-free movement
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5. Alveoli
 Have very thin epithelial lining
 Surrounded by many blood capillaries
 Alveolar walls contain elastic fibres
which stretch during breathing and recoil
during expiration to help force out air –
allows alveoli to expand
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6. Alveoli (cont)
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7. Gas exchange in alveoli
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8. BRAIN TEASER
How many times must a molecule of oxygen
diffuse across a cell surface membrane in
passing from the inside of an alveolus to
haemoglobin?

9. How does the composition of
inhaled air compare to that of
exhaled air?
% Gas Atmospheric air Alveolar air
(inhaled air) (exhaled air)
Nitrogen 79.0% 79.0%
Oxygen 20.9% 14.0%
Carbon dioxide 0.04% 5.6%
Water vapour 1.0% 6.0%
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10. ALBIO9700/2006JK

11. The ciliated
epithelium

14. TRACHEA

16. The Lung Tissues

18. Bronchiole and Trachea in transverse
section

19. A transverse
cross section
view of the
cartilaginous
trachea
cartilage
Smooth muscle
And elastic fibres
Ciliated epithelium
and goblet cells

21. Cartilage Ciliated Smooth Connective
epithelium muscle tissue
with goblet
cells
Trachea √ √ √ √
Two bronchi √ √ √ √
Bronchiole Cartillage No goblet Gradually √
gradually lost
lost
Alveolar No cartilage No cilia No No
duct No goblet
Alveolar sac No cartilage No cilia No No
No goblet
Alveoli No cartilage No cilia No No
No goblet
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22.  Cartilage: prevents collapse of the tube during
inspiration.
 Cilia: beats rhythmically, gradually sweeping the
mucus up towards the back of the throat, where
it is swallowed.
 Goblet cells: secretes a protective coating
called mucus. Much of the dust and
microorganisms in the air which is breathed in is
trapped in this mucus.
 Smooth muscle: involuntary muscles. When
they contract, they narrow the airways.
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23. Lung volumes and capasities
 Tidal volume: the volume of gas
exchange during one breath in and out.
~450 cm3 during quiet breathing. After
exercise it rises to ~3 dm3.
 Vital capacity: maximum volume of air
that a person can exhale after filling the
lungs to their maximum extent.
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24. Task
1. State 3 ways in which the structure of the
lungs allows efficient gas exchange (4).
2. Explain why the barrier to diffusion must be
as thin as possible (1).
3. Describe how a steep diffusion gradient is
achieved in the lungs (4).

25. Answers
1. State 3 ways in which the structure of the
lungs allows efficient gas exchange (4).
Large surface area (1) provides more space
for molecules to pass through (1), plasma
membranes surrounding the cytoplasm create
a permeable barrier allowing diffusion of
oxygen and carbon dioxide (1), alveolus wall is
one cell thick creating a thin barrier for
diffusion (1)
2. Explain why the barrier to diffusion must be
as thin as possible (1).
It reduces the distance gases have to diffuse
(1)

26. Answers
1. Describe how a steep diffusion gradient is achieved in the
lungs (4).
Blood brings carbon dioxide from the tissues to
the lungs ensuring the carbon dioxide
concentration in the blood is higher than that of
the air in the alveoli (1), it also carries oxygen
away from the lungs ensuring that the
concentration of oxygen in the blood is kept lower
than air inside the alveoli (1), the movement of the
lungs (ventilation) ensures there is a fresh supply
of oxygen entering the lungs increasing the
concentration of oxygen in the alveoli (1), carbon
dioxide is also removed by ventilation ensuring
that the concentration in the alveoli is lower than
that of the blood (1)

27. Mechanism of ventilation
External intercostal muscles
contract and the internal External intercostal muscles
intercostal muscle relax relax and the internal
intercostal muscle contract
Pulls rib cage up and out
The rib cage drops, mainly
Diaphragm muscles contract due to its own weight
This flattened the diaphragm Force the diaphragm into
domed shape
Both actions increase the
volume of the thorax Diaphragm muscles relax
Pressure in the thorax, hence Pressure in the thorax,
the lung, is reduced to less hence the lung, is increase,
than atmospheric pressure more than atmospheric
pressure
Air enters the lungs, inflating
the alveoli, until the air Air is forced out of the lungs
pressure in the lungs is equal
to that of the atmosphere
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