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Summary CIE A level Biology notes on unit o9 - Gas Exchange
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Everything you need to know about unit 09 of the CIE A level Biology course, including diagrams and notes.
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9.1 - The Gas Exchange system:The gas exchange system is adapted to...
• Clean and warm the air that enters during breathing
• Maximise the surface area for di usion of oxygen and carbon dioxide between the blood and the atmosphere.
• Minimise the distance for di usion.
• Maintain adequate gradients for di usion.
In humans, the gas exchange surface is the alveoli in the lungs - although each individual alveoli is is tiny, the alveoli
collectively have a huge surface area, roughly 70m2 in an adult. This means that a high amount of oxygen and carbon dioxide
can di use in and out at any given point.
The di erent structures in the respiratory system:
• The lungs - the lungs are in the chest cavity, surrounded by the pleural membranes
which enclose an airtight space. This space contains a small amount of a uid to allow
friction-free movement as the lungs are ventilated by the movement of the ribs and the
diaphragm.
• Trachea, bronchi and bronchioles - leading from the throat to the lungs is the trachea
and at the base of the trachea are 2 bronchi which subdivide and branch extensively into
a bronchiole tree in each lung. Each of these divides many times into smaller bronchioles
(terminal bronchioles divide to from even narrower respiratory bronchioles that supply
the alveoli with air).
◦Cartilage is in the trachea and the bronchi, which keeps the airways open and air
resistance low (prevents them from collapsing or busting due to air pressure
changes during breathing).
◦In the trachea, there is a regular arrangement of c-shaped rings of cartilage, but in
the bronchi there are irregular blocks of cartilage instead.
◦The bronchi has a very similar structure to the trachea, the only exception being that the cartilage is more irregular
shaped and doesn’t have the same C-shaped structure.
• Alveoli - At the end of the pathway between the atmosphere and the blood stream are the alveoli. The walls contain elastic
bres which stretch during inspiration and recoil during expiration to help force out air - this elasticity allows the alveoli to
expand according to the volume breathed in. As they are fully expanded during exercise, the surface area available for
di usion increases. A network of capillaries are found next to the alveoli, which allow
easy access for di usion.
The route of an oxygen molecule and the roles of the alveoli...
• Inhalation by the mouth or the nose warms and moistens the air and does initial
ltering.
• Then it enters the trachea where the goblet cells and the cilia move the mucus to the
larynx and up into the mouth.
• Air then enters the left or right bronchus, then through into the terminal bronchiole
which divides to form the respiratory bronchiole.
• Air then enters the alveoli where di usion of oxygen occurs across the squamous
epithelium and capillary endothelial wall and into the red blood cells.
• Carbon dioxide also di uses in the opposite direction and is moved out of the body during exhalation.
Adaptations of the alveoli...
• Thin squamous epithelium wall
◦The squamous epithelium is 0.5um thick and therefore gives a very short di usion distance so that gases can move
over it more easily.
• Close to the capillaries
◦The capillaries lie very close to the alveoli and also have squamous epithelial cells called endothelial which again,
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