Insects
Gas exchanges in insects are unusual as they do not use blood transport system to carry
oxygen and carbon dioxide between air and the body tissues. Instead, insects have tracheal
system where the air is transported throughout the body by the branching of tracheal tubes.
The air enters through the spiracles which are openings (with valves) along each side of the
abdomen. These openings are kept open by the trachea bound by the chitin. However, the
spiracles can be shut by the valves to reduce water loss. The air diffuses through the tracheal
tubes to the tracheoles. As insects have open transport system, the transport of gas is slow.
Therefore muscular pumping of the abdomen (breathing) assists the movement of the gases
into/out of the body and along the tranchea. More large, active insects such as the locusts
have air sacs that compress and expand to assist in moving air through the tracheal tubes. The
branching of the tracheal tubes increases the surface area so more exchange of gases can take
place. Tracheoles are finer branches and lack chitin so they pass between individual cells.
Oxygen and carbon dioxide dissolve in the moist environment at the end of the trancheoles
and diffuse in/out of the cell.
The body water can be lost easily as insects are exposed in the air. Therefore the hard
exoskeleton with waxy outer layer and the valves on the spiracles reduce water loss.
For efficiency of gas exchange, the insects have large surface area to volume ratio as this
reduces the time diffusion takes place and the large surface area increases the rate of gas
exchange. Therefore this limits the size of insect to be small so that it maintains its large
surface area to volume ratio.
Mammals
Mammals require high amount of oxygen as they are active homeotherms. High demand in
oxygen is met by the complex, internal lung which is a closed, double transport system. The
oxygen passes through the nasal cavity. The air is kept moist so that when it passes through
the rest of the system, it prevents the surfaces becoming too dry. The air goes down the
trachea which is covered in mucus so that it traps any small particles. Then the mucus is
brought up to the nasal cavity by the beating of cilia. Trachea is strengthened and supported
by cartilage which also allows flexibility of the neck. It stops the trachea from collapsing
during ventilation (breathing). Trachea then divides into two bronchi. The air diffuses down
the bronchi into the bronchioles and to the alveoli where the gas exchange occurs between the
thin membranes of the alveoli and the blood capillaries. The alveoli provide large, moist
surface area for diffusion of oxygen and carbon dioxide to take place. Efficiency of gas
exchange is also increased by the thin membranes of the alveoli as this reduces the distance in
which diffusion has to occur. The alveolus has round shape and is surrounded by capillaries.
This is to increase surface area of where diffusion takes place so that diffusion rate is
increased
Fish
Fish have closed, single circuit system where the gas exchange occurs at the gills. The gills
are very efficient and its extraction rates are three times the rate of human lungs. The fish
gulps in the water and shuts the oral valve, forcing the water to exit through the gills. The
gills are made of many filaments and the filaments are divided further into thin lamellae. The
thin foldings of lamellae increases the surface area where gas exchange can occur. In bony
fish, gills are protected by the operculum which is a gill cover. The filaments are supported
by bony structures. Cartilaginous fish don’t have gill covers but they have gill slits where the
water exits. Efficiency of gas exchange is increased by the gills as they are kept apart from
each other by the water. This increases surface area for more gas exchange. The gills are
dependent on the buoyancy of water for their support. When in air the gill collapses and the
fish suffocates. The gills are also rich in capillaries which increase the rate of diffusion. The
blood flow in the lamellae is in the opposite direction to the flow of water. This is called
counter-current effect. Parallel current flow would not achieve the same oxygen extraction
rates because the concentrations across the gill would quickly equalize.
Comparison
Both insects and mammals live in terrestrial environment where as the fish lives in the
aquatic environment. Insects and mammals live in the atmosphere where the oxygen is
abundant. 20% of the air is oxygen where as only 1% of the volume is oxygen in water.
Breathing in mammals is not as efficient as the fish because 30% is in ‘dead space’ in the
bronchi and the bronchioles. However, this isn’t a huge problem for mammals as they are
terrestrial animals. Fish have gills that are very efficient and can achieve an 80% extraction
rate of oxygen from water. The numerous thin foldings of the lamellae, (rich in capillaries)
increases surface area so increases surface area for diffusion
Although the oxygen is abundant in the air, water loss is a disadvantage for the insects. The
oxygen and the carbon dioxide have to dissolve in water and then be diffused in/out. Fish are
surrounded by water; therefore water loss and dissolving of oxygen and carbon dioxide is not
a problem for them. To prevent water loss, the spiracles in insects have valves which can be
shut. Also, the hard exoskeleton with waxy outer layer prevents water from evaporating. This
keeps the moisture level up so that more oxygen can be dissolved and be diffused into the cell
and carbon dioxide out of the cell.
The size of fish and mammals are larger compared to insects. This is due to the blood
circulatory system that is used for transport of oxygen and carbon dioxide in mammals and
fish. Insects do not use the blood transport system for gas exchange but the oxygen diffuses
directly in and carbon dioxide directly out of the cells. This limits the size of insects to be
small so that the distance in which the diffusion has to occur is short. Diffusion only occurs in
the alveoli of the mammals and the lamellae of the fish and then the oxygen is carried by the
haemoglobin. This allows them to grow in size as the blood circulatory system can transport
oxygen throughout the body.
All the diversity in structure of mammals, insects and fish increases the rate of gas exchange.
This helps them to survive and be successful in their habitats.