Essay A, Student 13, Expert Marker
Describe and contrast the gas exchange system of fish and mammals
The process of respiration is vital to the survival off both mammals and fish. The demand for
oxygen and the need to expel carbon dioxide is overcome by very intricate organs and systems. I
will explain the main principles and how these organisms meet the requirements for gas exchange.
For gas exchange to take place across a membrane, it needs to be permeable, moist and as thin
as possible. Both lungs and gills tackle these problems with differing levels of efficiency.
I will firstly start with the mammalian invaginations that are the lungs. These are a complex
network of vessels that decrease in size from the trachea down to the 150 million alveoli in an
average human pair of lungs.
The air supply is created by changes in volume of the thoracic cavity. This cavity consists of a
hemispherical base called a diaphragm and sides that are made up of ribs and both internal and
external intercostal muscles. Inhalation occurs when the external intercostal muscles and the
diaphragm contract. This forces the ribs out and the pressure within the cavity drops. Air is
subsequently drawn in. For exhalation the internal intercostal muscles contract and the diaphragm
relaxes. This reduces the cavity volume and the rise in pressure pushes air out.
The first problem overcome is the lack of moisture in the air. Goblet cells in the trachea produce
a constant supply of mucus and this ensures the pulmonary system never desiccates.
The oxygen rich air works its way to the alveoli and then diffuses across the thin membrane to
the capillary network that surrounds each one, this distance is between 0.5-2.5µm. As the diffusion
gradient changes, carbon dioxide diffuses into the lungs and is then expelled via exhalation.
This is tidal ventilation as the flow of air is bi-directional through the lungs. This system has a low
efficiency and most mammals manage 25% oxygen extraction from the air they breathe. There is
a category of animals that have developed a more efficient system. These are birds. If they had a
system as inefficient as ours they simply would not be able to fly as it is the most costly form of
locomotion, especially at the altitudes that many migratory birds reach. The key feature are air
sacs, these are found either side of the lungs. The air travels through the trachea to a posterior
sac, then through the lungs in one direction and then into an anterior air sac. From here it the re
enters the trachea without having passed through the lungs. This means that the air in the lungs
will have a higher O2 concentration overall and therefore more can be extracted. This overcomes
the problem of low oxygen levels.
Fish have developed a highly efficient process and this is mainly due to the unidirectional flow of
water over their gills. This is vital as water can hold between 20-40 times less oxygen than air,
combine this with a diffusion speed up to 8000x slower then it becomes pretty obvious why fish
have got a far more effective system.
Fish use one of two methods to create a flow of water over their gills. This first is a double pump
system.
To draw water in the fish will lower the base of its mouth, shut its operculum and open its mouth.
This is functionally analogous to how our thoracic cavity works. Water is drawn in and over the
gills. The mouth is then shut, buccal and opercular cavity decrease by raising the mouth and
water is forced out via the open operculum.
To reduce energy costs many active predatory fish such as Tuna Thunnus and Mackerel S.
scrombus use a system called ram ventilation. This is very simple, the mouth and operculum are
permanently open, water passes through the gills as they swim. The energy is free as it is already
spent on locomotion.
The gills themselves are very intricate structure with a large surface area and very thin filaments.
The branchial arches give them support and contain the major blood vessels. From these there
are v-shaped filaments, the last stage are the lamellae. These coat both sides of the filaments
and are thin plates no more than 50µm apart. The lamellae are incredibly thin and allow O 2 and
CO2 to diffuse across it’s membrane, the high surface area increases the rate at which this can
happen.
Essay A, Student 13, Expert Marker
There is a further stage that can affect the efficiency of O 2 extraction. This is the diffusion of
oxygen into the blood. Fish mainly have a counter current system, where blood flows in an
opposite direction to the blood. This means that blood with 75% oxygen levels will pass water with
90% levels and will still extract O2. The potential efficiency is 90% but this is rarely met due to
water flows rates. Most fish have this method yet some do use what is called a con-current
system. Water flows in the same direction as the blood. This means they meet oxygen
concentration equilibrium at roughly 50% this means there is no diffusion gradient and therefore
50% is the maximum efficiency.
One mechanism used in gaseous exchange that is common to both water and air breathers are
the presence of pigments in the blood. These consist of proteins and a metal prosthetic group.
Haemoglobin and Haemocyanin. These pigments greatly increase the O2 capacity of blood. The
average value of O2 dissolved in body fluid is 3ml O2/l but in blood with haemoglobin it rises to
250 ml O2/l in terrestrial vertebrates and 100 ml O2/l in fish.
The main difference in the evolution of respiration mechanisms between land and water breathers
is driven by the different limiting factors. In water this is the supply of O 2, as it is dramatically
less than in air. The extraction of CO2 is not a problem and CO2 is very soluble in H2O. For
terrestrial air breathers the problem in not the supply of oxygen but the removal of CO 2.