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The Respiratory System

Explain, in terms of surface area: volume ratio, why multicellular organisms need
specialised exchange surfaces and single-celled organisms do not.
Multicellular organisms are larger and so have a smaller surface area to volume
ratio than single-celled organisms.
Multicellular organisms have higher metabolic activity and so a greater…

Page 2

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o The pressure in the thorax decreases to below atmospheric pressure.
o Air moves into the lung down the pressure gradient.
o The diaphragm relaxes.
o The external intercostal muscles relax and the rib cage moves down and in.
o The volume in the thorax decreases.
o The pressure…

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Explain the meanings of the terms tidal volume and vital capacity.
Tidal volume: The volume of air moved in and out of the lungs with each breath when
you at rest. It is approximately 0.5dm3 and provides the body with enough oxygen
for its resting needs while removing enough carbon…

Page 4

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o Count the number of breaths per minute. E.g. in the trace above, there
are 10 peaks (or inhalations and exhalations) in 60 seconds. Therefore
the breathing rate is 10 breaths per minute.
To measure oxygen uptake:
o Because the volume of carbon dioxide breathed out is the same as…


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