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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 demand for
oxygen and glucose.
The outer surface is not large enough to enable gases and nutrients to enter its
body fast enough to keep all the cells alive.
The diffusion distance would be too great for nutrients and gases to travel from
the surface to the centre of the organism.
Larger organisms therefore combine specialised exchange surfaces e.g. alveoli in
lungs with a transport system (heart and blood vessels) to move substances around
Describe the features of an efficient exchange surface, with reference to diffusion
of oxygen and carbon dioxide across an alveolus.
In the alveoli, oxygen dissolves in the water film and then diffuses down a
concentration gradient into the blood. Carbon dioxide diffuses down a
concentration gradient from the blood into the alveoli.
Exchanges surfaces have 3 key features:
1. Large surface area (provided by alveoli)
2. Steep concentration gradient (due to regular ventilation caused by diaphragm
and intercostal muscles, stretch and recoil of elastic tissue and an extensive
3. Short diffusion distance (the epithelium of the alveolus is only one cell thick
and the wall of the capillary is only one cell thick).
Efficient exchange surfaces are also permeable and moist (to allow substances to
Describe the features of the mammalian lung that adapt it to efficient gas exchange.
Many alveoli Large surface area to volume ratio for
Thin barrier (only two cells thick) Short diffusion distance
Good blood supply To carry dissolved gases to and from the
alveoli, maintaining the diffusion gradient
Regular ventilation To refresh the air in the alveoli and
maintain the diffusion gradient
Elastic tissue Stretches and recoils to help expel air
Outline the mechanism of breathing (inspiration and expiration) in mammals, with
reference to the function of the rib cage, intercostal muscles and diaphragm.
o The diaphragm contracts and flattens.
o The external intercostal muscles contract, moving the rib cage up and out.
o The volume in the thorax increases.
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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 in the thorax increases to above atmospheric pressure.
o Air moves out of the lung down the pressure gradient.…read more
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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 dioxide to maintain a safe level.
Vital capacity: The largest volume of air that can be moved in and out of the lungs
in any one breath.…read more
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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 the
volume of oxygen breathed in, the reduction in volume of air in the
spirometer (due to the soda lime absorbing the CO2) is equal to the
volume of oxygen used by the subject.…read more