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Question A: Describe and contrast the gas exchange system of fish and mammals
Over time, organisms have evolved to occupy different environments, resulting in the need to physically adapt in order to survive in the often hostile conditions. Fish and mammals are greatly contrasting in their physical forms due to their (usually) opposing environments of water and air, respectively, with one notable difference being the structure of their gas exchange systems.
Water as a respiratory medium requires that fish have a much more efficient gas exchange system than that of mammals, as water contains less oxygen than air and is far more dense and viscous (Campbell et al., 2008: 916), making gill ventilation energetically costly. Oxygen demand is also the drive for control of respiration when in water, so an effective oxygen extraction system is crucial. The gas exchange system of fish consists of evaginations (outfoldings) of the epithelium, known as gills, which are suspended in the water, and which are usually covered by the operculum and thus considered internal. The gills are composed of cartilaginous gill arches containing the blood vessels, from which branch two rows of filaments, which themselves are then covered in small plates known as lamellae (the respiratory units). This structure creates an extremely large surface area for gas exchange.
In fish there is a unidirectional flow of water through the gills, with the movement of water being created in one of two ways. The first, used by most fish, is the double-pump system. To start, the operculum is closed and the mouth open, and the floor of the mouth is pulled down to increase the volume of the buccal cavity, causing water to rush into the mouth. The mouth is then closed and the operculum opened, with the mouth base pushed upwards, compressing the buccal cavity and forcing the water out over the gills. The second mechanism, known as ram ventilation, is highly efficient and employed by fast-swimming fish such as tuna and mackerel. The fish swim along with their mouths open, using the ‘free’ energy already spent on locomotion to drive the water over the gills, rather than using further energy to move the muscles of the mouth and operculum.
In combination with a unidirectional flow of water through the gills, high efficiency is achieved through a counter-current system i.e. the blood in the capillaries flows in the opposite direction to the water flow. This means that at any one point, the percentage of oxygen saturation in the blood is lower than that of the water flowing alongside it, thus maintaining a gradient of oxygen exchange from the water into the blood. This system is much more efficient at extracting oxygen than a concurrent system, extracting around 80% of the available oxygen (Campbell et al., 2008: 918), and allows for the highest percentage blood oxygen saturation possible.
In contrast to fish, the gas exchange system of mammals is much less efficient, with humans only extracting about 25% of the oxygen inhaled (Campbell et al., 2008: 916). Gills collapse in air, so as organisms moved from the water to land, new respiratory organs were required. In mammals these are the lungs, and exist as invaginations (infoldings) of the epithelium, contained within the thoracic cavity. The lungs are a repeatedly branching system of tubes, starting with the trachea, then the bronchi and bronchioles, and then finishing in the sac-like alveoli (the respiratory units). Unlike fish, there is a bi-directional flow of air into and out of the lungs, following the change of drive controlling respiration from oxygen demand to the need to excrete carbon dioxide – this inhalation and exhalation is known as breathing, and is the process that provides ventilation of the lung surface. Mammals employ negative pressure breathing which pulls air into the lungs. During inhalation, the diaphragm contracts and moves downwards, and the external intercostal muscles (between the ribs) contract, pulling the ribs up and out, which increases the volume and lowers the pressure of the thoracic cavity, causing air to rush in. During exhalation, the diaphragm and external intercostal muscles relax, reducing the thoracic volume, and, along with elastic recoil, push the air out of the lungs. The internal intercostal muscles are used during forced exhalation.
There are also key features present in a mammalian lung that are not present in fish gills. Firstly, any debris in the air inhaled cannot simply flow out at the other end of the lung, so mucus is ‘wafted’ upwards towards the throat by cilia that line the bronchi, trapping any particles and removing them from the lung (the ‘mucus escalator’). The second feature is surfactant, which lines the alveolar fluid layer and terminal airways, reducing surface tension to stabilise them and prevent collapse (Orgeig et al., 2007).
Overall, fish and mammal gas exchange systems share the same purpose, but the differing drives for respiration and extremely contrasting environments have forced these organisms to evolve highly complex systems to extract the oxygen needed for survival.
Word Count: 828
References
· Campbell, N.A, Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V. and Jackson, R.B. 2008. Biology. 8th ed. San Francisco: Pearson Benjamin Cummings
· Orgeig, S. et al., 2007. The anatomy, physics, and physiology of gas exchange surfaces: is there a universal function for pulmonary surfactant in animal respiratory structures? Integrative and Comparative Biology [e-journal] 47 (4), 610. Available through: EBSCO host Electronic Journal Service [Accessed 20th February 2011]