Essay A, Student 1, Expert Marker
Describe and contrast the gas exchange system of fish and mammals
[e1]Nearly all living cells respire to get the energy they need for metabolic processes. Respiration requires glucose and oxygen and produces carbon dioxide and water along with the release of ATP for energy. If the concentration of carbon dioxide gets too high itbecomes toxic so organisms need a system that can constantly provide oxygen and remove carbon dioxide. Organisms use many different gas exchange systems to achieve this and significant differences can be noted between air and [e2]water breathers which shall be covered later[e3].
For a gas exchange system to be effective it needs to be permeable to gases, thin, moist and have a large surface area to volume ratio[e4]. The gas exchange occurs by simple diffusion,a thin membrane provides a shortdiffusion distance making it easier for gases to be exchanged. A moist membrane also benefits diffusion, water breathers have a naturally moist gas exchange system but terrestrial animals have to provide their own moisture. A large surface area to volume ratio provides more sites for gas exchange; smaller animals already have this but more complex, multicellular animals require evaginations or invaginations. Evaginations are outfoldings of the epidermis such as gills and are usually found on water breathers, terrestrials usually have invaginations (infoldings) such as lungs which are contained in their own coelomic cavity; both increase the surface area to volume ratio. Convection in fluid pumps also increases the efficiency of gas exchange in more complex organisms by maintaining gas diffusion gradients. Convection can be just of the internal or external medium but fish and mammals use convection of both media – water/air and blood. In these cases the blood contains respiratory pigments which are composed of a protein and a prosthetic metal group; these increase the capacity for oxygen in blood[e5].[e6]
Fish use gills as their gas exchangeorgans; these are located on each side of the head and are covered by a protective bony plate called the operculum. Each side has four cartilaginous arches whichhave perpendicularly two columns of filaments which form a V-shape. [e7]The filaments have two rows of thin, semi-circular plates on the top and bottom of each filament. These discs are the lamellae; they contain a matrix of tiny blood vessels and are the respiratory units (site of gas exchange), analogous to the alveoli of lungs. Carried within this blood is the pigment haemoglobin which is found in nearly all vertebrates and gives the red blood cells their colour[e8]. Haemoglobin contains Fe2+ but haemocyanin which is found in arthropods and molluscs contains Cu2+ so their blood is blue.[e9]
Mammalian lungs have a similar structure of components of ever decreasing size, both fish and mammals use this [e10]to increase the surface area of their gas exchange sites, improving efficiency. In mammals the trachea, a pipe which leads down from the nose and mouth, splits into two bronchi which lead into each lung, these bronchi then branch into many bronchioles which in turn go to terminal bronchioles, respiratory bronchioles then finally alveoli. [e11]The larger three structures are supported by cartilage, similar to the arches and filaments of fish. The alveoli are highly folded and contain many small sacs as sites of gas exchange; they are covered in capillaries which provide a constant supply of blood. As referred to previously air breathers have to provide their own moisture for gas exchange; mucus (a sticky, slimy secretion) is continually produced by specialised goblet cells which line the epithelium. [e12]It also provides protection of the lungs, particles and pathogens adhere to the secretion which is then wafted up to the throat by cilia (hair like outfoldings) and swallowed.[e13]
Although parallels can be drawn between the structures of fish and mammalian gas exchange systems the ventilation of each is very different. Lung ventilation is operated by the thoracic cavity – the diaphragm, ribs and intercostal muscles. The diaphragm is a hemispherical sheet of muscle at the base of the ribs and the internal and external intercostal muscles are found between the ribs. During inhalation the diaphragm and external ICM contract moving the ribs up and out, increasing the volume of the thoracic cavity. This increase produces a lower pressure within the cavity so air is drawn into the lungs by diffusion. During exhalation the muscles relax and the volume decreases due to elastic recoil pushing air back out of the lungs. The intercostal muscles are only used during forced exhalation e.g. sneezing.[e14]
The movement of water through the gills is unidirectional[e15], unlike that of air in lungs but a similar system of increasing volume is used. The mouth opens and the base of the buccal cavity is pulled down, increasing the volume and drawing water in and over the gills. The mouth is then closed, the operculum opened and the base of the cavity moves up pushing the water out across the gills. Some fish also use ram ventilation where they open both the mouth and operculum and use the whole organism locomotion to drive water over the gills. Mammals cannot use ram ventilation however synchronisation of movement and breathing has been seen to be functionally analogous.
In mammals gases will simply diffuse across the alveoli and capillary walls and then removed from the gas exchange site but fish have a highly efficient counter-current exchange system. Gases still diffuse across the membranes but the blood in the lamellae flows in the opposite direction to the water passing over them so the oxygen is gradually transferred from the water to the blood achieving maximum oxygen extraction. The oxygen content of water is much lower than that in air, diffusion is also much slower and water is much more dense and viscous than air so fish require this system to prevent ventilation being energetically inefficient.
Ultimately for efficient respiration both fish and mammals require an effective gas exchange system. Therefore they will have similar features to achieve the aforementioned requirements of an effective respiratory system but we will always see many contrasting features due to the different eternal convection media they use.[e16]
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[e1]Make heading font bold!
[e2]Good!
[e3]Reference?
[e4]Good
[e5]Reference?
[e6]Good
[e7]Reference?
[e8]Interesting!
[e9]Reference?
[e10]Unclear
[e11]This takes a lot for air breathers to build, so wqhy have it all?
[e12]Yes, to maintain moisture/reduce water loss
[e13]Reference?
[e14]Reference
[e15]This important! Make more of this!
[e16]Add a reference list.