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Essay A, Student 3, Marker 3 Describe and contrast the gas exchange system of fish and mammals Fish and mammal gas exchange systems differ greatly physiologically however; both systems take oxygen from the environment and release carbon dioxide back through a respiratory surface. The respiratory organs of fish are the gills which are used for gas exchange under water. These are highly complex structures. Gills are evaginations which are found outside the fish’s body protected by the operculum. They are found between the opercular and buccal cavities. Gills are designed to minimise diffusion distances for gas exchange in water. (Randall, Burggren, French 2002) Water has diffusion coefficient that is approximately 104 times smaller than that of air and, therefore, per volume. Water will always have lower oxygen content than air (Liss 1973) as, in order to maximise oxygen taken from water, the system must be very efficient. The highly specialised structure of the gills facilitates diffusion. Although gills are in water and are therefore constantly wet, a mucous is secreted to aid quick diffusion across the membrane and create a boundary layer between the epithelium and water. The layer of epithelial cells lining the lamellae is only 3-5µm thick which enables fast exchange of gas by minimising the diffusion distance. Gills require a large surface area to volume ratio in order to make gas as exchange efficient as possible. (Campbell et al 2008) Gills are made up of 4 branchial arches on either side of the head. The arches have 2 rows of filaments, each of which are flattened dorsoventrally. Lamellae are found on both sides of the filaments, greatly increasing the surface area of the gills. These structures form a sieve that water filters through. (Randall, Burggren, French 2002) Essay A, Student 3, Marker 3 Respiratory surfaces require a constant supply of oxygen. Fish use either the active double pump system or RAM ventilation (which uses free energy from locomotion) to create a unidirectional flow of water through the gills. Water is taken in through the mouth over the gills and then out through the operculum. This continually refreshes the oxygen supply (Roberts 1975) Current flow is important in gas exchange as it creates a Countercurrent constant Concurrent gradient for diffusion across the respiratory surface. Fish use a counter current flow system for gas exchange; this is achieved by a unidirectional blood flow across the respiratory surface with water flowing in the opposite direction. This system allows a greater difference in partial pressure of oxygen between the Fig 1 Fig 2 environment and that of the blood; which ensures quick gas exchange across the surface (Rahn 1966) (Fig 1), more than 80% of oxygen can be extracted from water in this way. (Campbell et al 2008) The efficiency of a gas exchange system is important. The more efficient a gas exchange system is the less energy it uses and the more it produces. The gas exchange system in fish is more efficient than that of mammals. This could be due to the properties of water: high viscosity, high-density, low oxygen content and a small diffusion coefficient for oxygen and carbon dioxide. (Piper, Scheid 1972) Mammalian respiratory organs are invaginations unlike that of the fishes and are found inside the body, and are used for gas exchange in air. The lungs are made up of an intricate network of bronchioles and alveoli. Lungs protect the respiratory surfaces and help control water loss and, like gills, provide a large surface area for gas exchange to take place. (Randall, Burggren, French 2002) Essay A, Student 3, Marker 3 Like the gills the lungs are also highly specialised to support efficient gas exchange. Lungs are used for gas exchange in air. A surfactant is secreted which moistens the surface of the alveoli, helping diffusion and also preventing them from collapsing. The epithelial cells lining the alveoli are thicker than the lamellae of the fish at 50300µm thick; there is not so much need for it to be as thin as the fish’s as there is a higher oxygen concentration in air than water. Lungs also require a large surface area to volume ratio for making gas as exchange efficient as possible. The air travels down the trachea, which is supported by cartilaginous rings, in to the lungs. The air then travels down the two bronchi and then into the smaller branches of the bronchioles and into the alveoli. (Campbell et al 2008) Mammals unlike fish use bidirectional flow, where air is taken into and out of the lungs the same way. A large amount of energy is used when reversing the flow of air, where fish use little or no energy, creating a flow of water over the gills. Due to the large amounts of oxygen in the air, this energy consumption does not affect gas exchange. (Randall, Burggren, French) The percentage of oxygen in the air counteracts the fact that oxygen is not always fully replenished as the lungs carry out tidal ventilation. This could not occur in fish as the percentage of oxygen in water is so much lower, it would therefore be inefficient. Mammals use a concurrent gas exchange system, where both air and blood flow in the same direction (Fig 2). This system is less efficient than the counter current of fish and only 50% of oxygen can be extracted in this way. (Campbell et al 2008) Although the physiological structure of the gills and lungs look different they have much the same features: large surface area to volume ratio and moist and thin surfaces. The gas exchange systems used by the fish and mammals are however somewhat different. Over all fish are more efficient at gas exchange in than mammals, which is mainly due to the differences in respiratory media. Word count: 944 Essay A, Student 3, Marker 3 References: Campbell.N, Reece.J, Urry.L, Cain.M, Wasserman.S, Minosky.P, Jackson.R, (2008) biology. 8th Ed. P 915–920 Liss.P, (1973) Process of Gas Exchange Across an Air-water Interface, Deep Sea Research and Oceanographic Abstracts. Volume 20, Issue 3 P 221-238 Piper.J, Scheid.P. (1972) maximum gas transfer efficiency of models for fish gills, avian lungs and mammalian lungs, respiration physiology. Volume 14, issues 1-2, P115–124 Rahn.H.(1966) aquatic gas exchange: theory 1, respiration physiology. volume 1 issue 1, P1–12 Randall.D, Burggren. W, French.K. (2002) animal physiology mechanisms and adaptions. 5th Ed. P 545. W.H.Freeman and company, New York. Roberts. J, (1975) Active Branchial and Ram Gill Ventilation in Fishes, Biological Bulletin. Vol. 148, Issue 1 P85-105 Figures 1 and 2 taken from Randall.D, Burggren. W, French.K. (2002) animal physiology mechanisms and adaptions. 5th Ed. P 561 W.H.Freeman and company, New York.