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Gas exchange is a biological process that is essential for the survival of an organism. The purpose of this process is to obtain oxygen from the
environment (which is necessary for cellular respiration, and to dispose of CO2, a bi product of respiration. An efficient gas exchange system has 3
requirements. It must have a large surface area over which the gases can diffuse. A large surface area maximises the amount of area available for the
gases to exchange. It must also be moist. Oxygen must first dissolve into a fluid before it can diffuse into a cell. Having a moist gas exchange surface
ensures the gases can be dissolved. Thirdly, the membrane through which gases diffuse must be thin. Gas exchange is driven by concentration gradients.
Diffusion is the process which moves the gases. The rate of diffusion is determined by the concentration gradient and the distance so having a smaller
distance for the gases to travel increases the rate at which gas exchange can occur. A thick membrane would mean a slower rate if diffusion because the
gases have to travel further. In this essay I will discuss the diversity in structure and function of gas exchange systems of three animal groups, Insects,
fish and mammals. I will also discuss how these gas exchange systems have allowed each animal group to be successful in their environment.
Unlike fish and mammals, insects do not have a circulatory system with blood that transports oxygen. Insects get the oxygen they need directly from the
air. Insects have openings on the side of their bodies called spiracles. These spiracles have valves that the insect can actively open and close to allow
more air to enter the body. Requirements for oxygen change depending on activity so when active, the valves are open, when not active, the valves close
Tracheal tubes are tubes that connect the spiracle to the internal part of the body. As the tubes move into the body they branch continuously into smaller
and smaller trachioles. Almost every cell in an insects body is in close contact with a trachiole. The trachioles are the site of gas exchange in insects. Air
moves into an insects body by diffusion from the atmosphere. As it enters the tracheal tubes which lead to the trachioles, the oxygen in the air becomes
dissolved in a fluid in the tips of the trachioles. Once dissolved, oxygen the diffuses through the trachioles wall (which is one cell thick) and into the cell.
Because respiration creates CO2 there is a higher concentration of CO2 in the cell than in the air, so the CO2 diffuses out of the cell, intot he trachioles and
into the air. Insects do not really actively ventilate their body unlike mammals and fish. They rely on diffusion alone to move air from the atmosphere
into their body. Because of this, insects are limited by the size to which they can grow. If an insect were to large then the rate of diffusion would be
lower, meaning they may not be able to get enough oxygen to all body parts to meet oxygen requirements. Also, although the tracheal tubes are lined
with chitin to hold their structure, to much mass of an insect body could weigh down on the tubes and they may collapse. It is important to note that some
insects do ventilate their bodies by rhythmic pulsating of the body. Despite the limitations on size, insects are still highly successful. The continual
branching of tracheal tubes provides a large surface area for gas exchange and with almost every cell in the body being in contact with a trachiole, the
cells are feed oxygen directly form the environment without the need to develop an internal circulatory system.
Fish, like mammals, have an internal, closed circulatory system which is responsible for transporting oxygen to cells and removing CO2 from cells. The
gas exchange structure in fish is called the gills. The main support structure of the gills is called the gill arch and is made of cartilage. This means it can
support itself, but the other parts of the gills are soft tissue and are supported by water. Running of the gill arch are many gill filaments, and on the
surface of the gill filaments are flattened disc like structures called lamellae. The gill filaments and lamellae need to be supported by water, and , in its
absence, will collapse. The lamellae are the site of gas exchange in fish. They have a blood supply running through them and oxygen that is dissolved in
the water diffuses through the tissue of the lamellae (one cell thick) and into the blood running through the lamellae. Because the lamellae are so small,
and there are thousands of them, fish have an extremely large surface area for gas exchange to occur over. Also, because fish live in water, their gas
exchange system is always moist. Compared to air, water is extremely dense and has much less oxygen in it, fish have adapted a gas exchange system
which ensures they maximise the amount of oxygen they take from the water. This system is called a counter current system. This means that the
movement of blood through the lamellae runs in the opposite direction to the water flowing over the lamellae. This means that as oxygen poor blood
enters the lamellae it encounters water with a higher oxygen concentration so oxygen diffuses into the blood. As the blood picks up oxygen it encounters
fresh water with an ever higher oxygen content than itself, so diffusion of O2 in and CO2 out occurs over the entire length of the lamellae. This system is
so efficient that fish can take up to 80% of the dissolved oxygen out of the water. If oxygen poor blood ran in the same direction as the flow over the
water, then diffusion would occur at the start of the lamellae but the rate of diffusion would lower the further along the lamellae you went because the
water would have lost oxygen(be oxygen poor) and the blood would have picked it up.
The structure of the gas exchange system in fish has limited them to living in an aquatic environment (few exceptions to some fish that have a lung) If a
fish is taken out of water, the gill filaments and lamellae collapse, decreasing the surface area of the gas exchange system, also the gills would dry out and
the fish would suffocate as no oxygen could diffuse into the lamellae. Dessication of the gas exchange system would cause an organism to die. Fish can
ventilate their gills by ram ventilation. This is simply swimming with the mouth open so that water moves in through the mouth and out over the gills.
They can actively ventilate their gills by the movement of the mouth and operculum, like a pump. When a fish opens its mouth, it can close it’s
operculum. Water enters the mouth. Then when the fish closes its mouth and opens its operculum the water is forced out over the gills. This is analogous
to clenching your fist under water.
Unlike fish (except a few), mammals have an internal gas exchange organ. This is called the lung. Mammals solved the problem of dessication by placing
their gas exchange system inside the body. The mouth and nose are attached to the trachea, a pipe that has cartilage rings in it to hold it open. The trachea
branches into 2 bronchi, one leading to each lung. The bronchi branch continuously into bronchioles which end in small hollow sacs called alveoli. The
continuous branching and many alveoli provide a massive surface area for gas exchange. The alveoli are the site of gas exchange and, if you were to lay
out a human gas exchange surface it would be the size of a tennis court. Breathing in mammals is controlled mainly by the muscles of the ribcage and
diaphragm. When the diaphragm contracts, the volume of the thoracic cavity decreases, creating a n environment with a higher pressure system than that
of the atmosphere, so air is sucked out. When the diaphragm relaxes, the volume of the chest cavity increases, meaning the pressure in the atmosphere is
greater than in the lung, so air is drawn into the lung. AS air moves down the bronchioles and to the alveoli, the oxygen in the air becomes dissolved in a
fluid in the alveoli. This oxygen then diffuses through the alveoli into the blood supply, and then around the body. Alvioli are very thin and have a high
supply of blood vessels over them, so the gas does not have far to travel to get into the blood.
Mammals have therefore developed a gas exchange system which has allowed them to move terrestrial environments, despite the threat of dessication.
Although our gas exchange system works well on land, it could not function properly in water. Firstly, because it does not have a large enough surface
area to take the oxygen out of the water, which in terms of concentration is low compared to air. Secondly, water is very dense and mammals lack a
sytem to move water in and out of the body.
In this essay I have discussed the structure and function of gas exchange systems in three groups of animals that show a range of diversity in their forms.
I have discussed the implications of each gas exchange system for each animal and how it has allowed them to be successful in their given environment.