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Essay A, Student 3, Marker 3
Describe and contrast the gas exchange
system of fish and mammals
Fish and mammal gas exchange systems differ greatly physiologically however; both
systems take oxygen from the environment and release carbon dioxide back through
a respiratory surface.
The respiratory organs of fish are the gills which are used for gas exchange under
water. These are highly complex structures. Gills are evaginations which are found
outside the fish’s body protected by the operculum. They are found between the
opercular and buccal cavities. Gills are designed to minimise diffusion distances for
gas exchange in water. (Randall, Burggren, French 2002) Water has diffusion
coefficient that is approximately 104 times smaller than that of air and, therefore, per
volume. Water will always have lower oxygen content than air (Liss 1973) as, in
order to maximise oxygen taken from water, the system must be very efficient.
The highly specialised structure of the gills facilitates diffusion. Although gills are in
water and are therefore constantly wet, a mucous is secreted to aid quick diffusion
across the membrane and create a boundary layer between the epithelium and
water. The layer of epithelial cells lining the lamellae is only 3-5µm thick which
enables fast exchange of gas by minimising the diffusion distance. Gills require a
large surface area to volume ratio in order to make gas as exchange efficient as
possible. (Campbell et al 2008) Gills are made up of 4 branchial arches on either
side of the head. The arches have 2 rows of filaments, each of which are flattened
dorsoventrally. Lamellae are found on both sides of the filaments, greatly increasing
the surface area of the gills. These structures form a sieve that water filters through.
(Randall, Burggren, French 2002)
Essay A, Student 3, Marker 3
Respiratory surfaces require a constant supply of oxygen. Fish use either the active
double pump system or RAM ventilation (which uses free energy from locomotion) to
create a unidirectional flow of water through the gills. Water is taken in through the
mouth over the gills and then out through the operculum. This continually refreshes
the oxygen supply (Roberts 1975)
Current flow is important in gas exchange as it creates a Countercurrent
constant
Concurrent
gradient for diffusion across the respiratory surface.
Fish use a counter current flow system for gas
exchange; this is achieved by a unidirectional blood
flow across the respiratory surface with water flowing in
the opposite direction. This system allows a greater
difference in partial pressure of oxygen between the
Fig 1
Fig 2
environment and that of the blood; which ensures quick gas exchange
across the surface (Rahn 1966) (Fig 1), more than 80% of oxygen can be
extracted from water in this way. (Campbell et al 2008)
The efficiency of a gas exchange system is important. The more efficient a gas
exchange system is the less energy it uses and the more it produces. The gas
exchange system in fish is more efficient than that of mammals. This could be due to
the properties of water: high viscosity, high-density, low oxygen content and a small
diffusion coefficient for oxygen and carbon dioxide. (Piper, Scheid 1972)
Mammalian respiratory organs are invaginations unlike that of the fishes and are
found inside the body, and are used for gas exchange in air. The lungs are made up
of an intricate network of bronchioles and alveoli.
Lungs protect the respiratory
surfaces and help control water loss and, like gills, provide a large surface area for
gas exchange to take place. (Randall, Burggren, French 2002)
Essay A, Student 3, Marker 3
Like the gills the lungs are also highly specialised to support efficient gas exchange.
Lungs are used for gas exchange in air. A surfactant is secreted which moistens the
surface of the alveoli, helping diffusion and also preventing them from collapsing.
The epithelial cells lining the alveoli are thicker than the lamellae of the fish at 50300µm thick; there is not so much need for it to be as thin as the fish’s as there is a
higher oxygen concentration in air than water. Lungs also require a large surface
area to volume ratio for making gas as exchange efficient as possible. The air
travels down the trachea, which is supported by cartilaginous rings, in to the lungs.
The air then travels down the two bronchi and then into the smaller branches of the
bronchioles and into the alveoli. (Campbell et al 2008)
Mammals unlike fish use bidirectional flow, where air is taken into and out of the
lungs the same way. A large amount of energy is used when reversing the flow of
air, where fish use little or no energy, creating a flow of water over the gills. Due to
the large amounts of oxygen in the air, this energy consumption does not affect gas
exchange. (Randall, Burggren, French year?) The percentage of oxygen in the air
counteracts the fact that oxygen is not always fully replenished as the lungs carry out
tidal ventilation. This could not occur in fish as the percentage of oxygen in water is
so much lower, it would therefore be inefficient.
Mammals use a concurrent gas exchange system, where both air and blood flow in
the same direction (Fig 2). This system is less efficient than the counter current of
fish and only 50% of oxygen can be extracted in this way. (Campbell et al 2008)
Although the physiological structure of the gills and lungs look different they have
much the same features: large surface area to volume ratio and moist and thin
surfaces. The gas exchange systems used by the fish and mammals are however
somewhat different.
Over all fish are more efficient at gas exchange in than
mammals, which is mainly due to the differences in respiratory media.
Word count: 944
Essay A, Student 3, Marker 3
References:
Campbell.N, Reece.J, Urry.L, Cain.M, Wasserman.S, Minosky.P, Jackson.R, (2008)
biology. 8th Ed. P 915–920
Liss.P, (1973) Process of Gas Exchange Across an Air-water Interface, Deep Sea
Research and Oceanographic Abstracts. Volume 20, Issue 3 P 221-238
Piper.J, Scheid.P. (1972) maximum gas transfer efficiency of models for fish gills,
avian lungs and mammalian lungs, respiration physiology. Volume 14, issues 1-2,
P115–124
Rahn.H.(1966) aquatic gas exchange: theory 1, respiration physiology. volume 1
issue 1, P1–12
Randall.D, Burggren. W, French.K. (2002) animal physiology mechanisms and
adaptions. 5th Ed. P 545. W.H.Freeman and company, New York.
Roberts. J, (1975) Active Branchial and Ram Gill Ventilation in Fishes, Biological
Bulletin. Vol. 148, Issue 1 P85-105
Figures 1 and 2 taken from Randall.D, Burggren. W, French.K. (2002) animal
physiology mechanisms and adaptions. 5th Ed. P 561 W.H.Freeman and company,
New York.