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Gas exchange system
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AS Bio notes Raida 12DGas exchange
9.1 Gas exchange
The human gas exchange system links the circulatory system with the atmosphere and is
adapted to:
Clean and warm the air that enters during breathing
Maximise the surface area for diffusion of oxygen and carbon dioxide between the blood
and atmosphere
Minimise the distance for this diffusion
Maintain adequate gradients for this diffusion
Most organisms need a supply of oxygen for respiration
In single-celled organisms, the oxygen simply diffuses from the fluid outside the cell,
through the cell surface membrane and into the cytoplasm
Multicellular organisms usually have a specialised gas exchange surface where oxygen
from the external environment can diffuse into the body and carbon dioxide can diffuse
out
In humans, the gas exchange surface is the alveoli (alveolus) in the lungs
The alveoli collectively have a huge surface area
This means that a large number of oxygen and carbon dioxide molecules can diffuse
through the surface at any one moment giving a high rate of gas exchange
The large surface area is also required because oxygen is not very soluble in water
9.2 Lungs
The lungs are in the thoracic (chest) cavity surrounded by the pleural membranes which
enclose an airtight space
This space contains a small quantity of fluid to allow friction-free movement as the lungs
are ventilated by the movement of the ribs and diaphragm
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