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Question A: Describe and contrast the gas exchange system of fish and
mammals
Over time, organisms have evolved to occupy different environments, resulting
in the need to physically adapt in order to survive in the often hostile conditions. Fish
and mammals are greatly contrasting in their physical forms due to their (usually)
opposing environments of water and air, respectively, with one notable difference
being the structure of their gas exchange systems.
Water as a respiratory medium requires that fish have a much more efficient
gas exchange system than that of mammals, as water contains less oxygen than air
and is far more dense and viscous (Campbell et al., 2008: 916), making gill ventilation
energetically costly. Oxygen demand is also the drive for control of respiration when in
water, so an effective oxygen extraction system is crucial. The gas exchange system
of fish consists of evaginations (outfoldings) of the epithelium, known as gills, which
are suspended in the water, and which are usually covered by the operculum and thus
considered internal. The gills are composed of cartilaginous gill arches containing the
blood vessels, from which branch two rows of filaments, which themselves are then
covered in small plates known as lamellae (the respiratory units). This structure
creates an extremely large surface area for gas exchange.
In fish there is a unidirectional flow of water through the gills, with the
movement of water being created in one of two ways. The first, used by most fish, is
the double-pump system. To start, the operculum is closed and the mouth open, and
the floor of the mouth is pulled down to increase the volume of the buccal cavity,
causing water to rush into the mouth. The mouth is then closed and the operculum
opened, with the mouth base pushed upwards, compressing the buccal cavity and
forcing the water out over the gills. The second mechanism, known as ram ventilation,
is highly efficient and employed by fast-swimming fish such as tuna and mackerel. The
fish swim along with their mouths open, using the ‘free’ energy already spent on
locomotion to drive the water over the gills, rather than using further energy to move
the muscles of the mouth and operculum.
In combination with a unidirectional flow of water through the gills, high
efficiency is achieved through a counter-current system i.e. the blood in the capillaries
flows in the opposite direction to the water flow. This means that at any one point, the
percentage of oxygen saturation in the blood is lower than that of the water flowing
alongside it, thus maintaining a gradient of oxygen exchange from the water into the
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blood. This system is much more efficient at extracting oxygen than a concurrent
system, extracting around 80% of the available oxygen (Campbell et al., 2008: 918),
and allows for the highest percentage blood oxygen saturation possible.
In contrast to fish, the gas exchange system of mammals is much less efficient,
with humans only extracting about 25% of the oxygen inhaled (Campbell et al., 2008:
916). Gills collapse in air, so as organisms moved from the water to land, new
respiratory organs were required. In mammals these are the lungs, and exist as
invaginations (infoldings) of the epithelium, contained within the thoracic cavity. The
lungs are a repeatedly branching system of tubes, starting with the trachea, then the
bronchi and bronchioles, and then finishing in the sac-like alveoli (the respiratory
units). Unlike fish, there is a bi-directional flow of air into and out of the lungs, following
the change of drive controlling respiration from oxygen demand to the need to excrete
carbon dioxide – this inhalation and exhalation is known as breathing, and is the
process that provides ventilation of the lung surface. Mammals employ negative
pressure breathing which pulls air into the lungs. During inhalation, the diaphragm
contracts and moves downwards, and the external intercostal muscles (between the
ribs) contract, pulling the ribs up and out, which increases the volume and lowers the
pressure of the thoracic cavity, causing air to rush in. During exhalation, the
diaphragm and external intercostal muscles relax, reducing the thoracic volume, and,
along with elastic recoil, push the air out of the lungs. The internal intercostal muscles
are used during forced exhalation.
There are also key features present in a mammalian lung that are not present
in fish gills. Firstly, any debris in the air inhaled cannot simply flow out at the other end
of the lung, so mucus is ‘wafted’ upwards towards the throat by cilia that line the
bronchi, trapping any particles and removing them from the lung (the ‘mucus
escalator’). The second feature is surfactant, which lines the alveolar fluid layer and
terminal airways, reducing surface tension to stabilise them and prevent collapse
(Orgeig et al., 2007).
Overall, fish and mammal gas exchange systems share the same purpose, but
the differing drives for respiration and extremely contrasting environments have forced
these organisms to evolve highly complex systems to extract the oxygen needed for
survival.
Word Count: 828
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References
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Campbell, N.A, Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V. and Jackson, R.B.
2008. Biology. 8th ed. San Francisco: Pearson Benjamin Cummings
Orgeig, S. et al., 2007. The anatomy, physics, and physiology of gas exchange surfaces: is there a
universal function for pulmonary surfactant in animal respiratory structures? Integrative and
Comparative Biology [e-journal] 47 (4), 610. Available through: EBSCO host Electronic Journal Service
[Accessed 20th February 2011]