Hind Leys Biology
F211
Exchange surfaces 4.2
Gas exchange in the lungs
Objectives
 Describe the features of an efficient exchange surface with reference to
diffusion of oxygen and carbon dioxide across an alveolus.
 Describe the features of the mammalian lung that adapt it to efficient
exchange.
 Outline the mechanism of breathing (inspiration and expiration) in mammals,
with reference to the function of the rib cage, intercostal muscles and
diaphragm.
The lungs
Air can pass into the lungs through the nose
and along the trachea, bronchi and
bronchioles. Each part of the airway is
adapted to its function of allowing the
passage of air. Finally, air reaches tiny sacs
called alveoli. The walls of the alveoli are the
surface where the exchange of gases occurs.
(See F4 Foundation Biology disc).
Figure 1 The
lungs and
associated
structures
Figure 2 Transverse
section of lung tissue
http://www.kscience.co.uk/animations/lungs.swf
Gas exchange in the lungs
Gas exchange is the movement of gases by diffusion between and organism and its
environment across a barrier such as the alveolus wall
We shall now consider how the ideal features of an efficient exchange surface apply to
the lungs.
 Large surface area so more space for molecules to pass through. Larger
area than that of skin- size of ½ a tennis court!
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Hind Leys Biology
F211
Exchange surfaces 4.2
 Barrier permeable to oxygen and carbon dioxide- plasma membrane of cells
readily allow diffusion of gases.
 Thin barrier to reduce diffusion distance.
Alveolus and capillary wall both one cell thick
Both walls consist of squamous cells/flattened
Capillaries in close contact
Capillaries narrow so RBCs squeezed against
Examiner tip
Alveoli and capillaries
both have thin walls,
which are one cell thick.
Don’t say ‘alveoli have
thin cell walls’, which is
nonsense!
wall of capillary, closer contact to air in alveolis, and reduce rate at
which they flow past
Total barrier to diffusion less than 1 μm thick

Moist membrane allows gases to dissolve and diffuse through membrane.
Lungs produce surfactant which reduces cohesive forces between water
molecule and prevents the lungs collapsing

Diffusion gradient maintained- close contact with blood supply ensures that
oxygenated blood is rapidly removed from vicinity of lungs and replaced with
blood of a high CO2 concentration so that it can be breathed out.
Maintaining the diffusion gradient
For diffusion to be rapid, a steep diffusion gradient is
needed. This means that there is a high concentration of
oxygen in the alveoli, and a low concentration in the blood
of the capillaries surrounding the alveoli. This gradient
must be maintained, and this is achieved by the action of
the blood transport system and the ventilation
movements.
Figure 3 Capillary network over the surface of alveoli
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F211
Exchange surfaces 4.2
Carbon dioxide is delivered to the lungs from the
tissues by the blood. The concentration of
carbon dioxide is higher in the blood than in the
alveoli so it passes down its concentration
gradient into the alveoli and is breathed out.
Oxygen is carried away from the lungs. There is
a higher concentration of oxygen in the alveoli
than in the blood, so oxygen diffuses down its
concentration gradient from the alveoli into the
blood, and is transported to the tissues.
Figure 4 Gaseous exchange
The heart pumps the blood along the pulmonary
artery to the lungs. Here the artery divides to
form finer and finer vessels that are only just
wide enough to allow red blood cells to squeeze
through. These capillaries cover the surface of
the alveoli.
Ventilation of the lungs brings fresh supplies of air to replace the used oxygen, and
removes air containing carbon dioxide from the alveoli.
Inspiration
Contracts and flattens
Expiration
Relax and rises
External inteostal
muscles
Ribs
Contract
Relax
Up and out
Down and in
Volume of chest
cavity
Pressure in chest
Increases
Decreases
Decreases to become less than
atmospheric pressure
Air moves into lungs
Increases to become greater
than atomospheric pressure
Air moves out of lungs
Diaphragm
Air movement
Table 1 Breathing movements to ventilate the lungs in humans
1. State three ways in which the structure of the lungs allow efficient gas exchange.
2. Explain why the barrier to diffusion must be as thin as possible.
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Hind Leys Biology
F211
Exchange surfaces 4.2
3. Describe how a steep diffusion gradient is achieved in the lungs.
4. Emphysema is a disease that is frequently caused by smoking. In emphysema, the
walls of the alveoli break down, creating larger and fewer spaces in the lungs.
In an investigation of the effects of smoking on lung surface area:volume ratios,
samples were taken from 10 normal lungs and 10 diseased lungs. The mean surface
area of the samples from healthy lungs was 0.275 μm, and that of the samples
from emphysema patients was 0.170 μm.
a) Explain how the breakdown of alveolar walls leads to a decrease in surface
area;volume ratio of the gas exchange surface.
b) Use the results of the study to explain why a person with emphysema has
difficulty getting enough oxygen into the blood.
Refer to pages 52-53 in Biology 1 for further reading and examples.
http://learningat.ke7.org.uk/scienceweb/alevel/biology/AS%20Interactive/e-alevel/as_revision_web_site.htm
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