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Exchange surfaces & breathing
All living cells need oxygen and nutrients to survive. They also need to remove
waste CO2.
3 main factors that affect need for an exchange system:
Size
SA:V ratio
Level of activity
Size
Small single-celled organisms can rely on diffusion alone on exchange. Larger
organisms have a longer diffusion pathway, so diffusion is too slow to be
efficient.
SA:V Ratio
smallorganisms have a large SA:V ratio, meaning their SA is large enough for
exchange.
Large organisms have a small SA:V ratio, meaning their SA is not sufficient
for exchange.
Organisms can increase their SA by having special structural adaptations.
Level of activity
More active organisms have a higher metabolic rate, so require more energy
for respiration – good exchange surface is required.
Need for energy is increased for those animals that maintain body
temperature.
Features of a good exchange surface:
Large SA to provide more space for molecules to pass through. Often
done through folding walls and membranes, e.g. villi and microvilli.
Thin barrier to reduce diffusion distance – barrier must be permeable,
e.g. alveoli
Good blood supply to maintain the concentration gradient, e.g. remove
O2
Mammalian gaseous exchange system
Air passes through nose, along trachea, bronchi and bronchioles until it reaches
alveoli.
, Lungs are protected by the ribcage. Ribs are held together by intercostal
muscles. The action of these muscles and the diaphragm produce breathing
movements (ventilation)
Gaseous exchange in the lungs
Gases diffuse through thin walls of alveoli. Oxygen passes from air in alveoli to
blood in capillaries. Carbon dioxide passes from blood to air.
Lungs must maintain a steep concentration gradient to ensure diffusion can
continue.
Large surface area
Alveoli are small individually, but there are so many of them that there is a
huge surface area – 70m2
Alveoli lined by surfactant which evaporates as we breathe out. Reduces
surface tension of water molecules which would cause collapse. Maintains air
sac shape.
Thin barrier to reduce diffusion distance
Number of adaptations:
Alveolus and capillary wall are 1 cell thick
Both walls consist of squamous cells – flattened or very thin – short
pathway.
Capillaries are in close contact with alveolus wall – short diffusion
pathway.
Capillaries are so narrow that RBC are squeezed against wall – closer to
air in alveoli and reducing their rate of flow.
Elastic so recoil, helping ventilation – creates concentration gradient.
Surfactant maintains surface area and prevent collapse.
Good blood supply
The blood supply maintains a steep concentration gradient, so gases
continue to diffuse.
Blood transports CO2 from tissues to lungs. Ensures concentration is higher in
blood than in alveoli. Blood transports O 2 away so concentration is lower than in
alveoli.
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