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Gas Exchange Gas exchange supplies oxygen for cellular respiration and disposes of carbon dioxide: an overview � Gas exchange is the uptake of molecular oxygen (O2) from the environment and the discharge of carbon dioxide (CO2) to the environment. � The source of oxygen, the respiratory medium, is air for terrestrial animals and water for aquatic animals. � The atmosphere is about 21% O2 (by volume). � Dissolved oxygen levels in lakes, oceans, and other bodies of water vary considerably, but they are always much less than an equivalent volume of air. � The part of an animal where gases are exchanged with the environment is the respiratory surface. � Movements of CO2 and O2 across the respiratory surface occurs entirely by diffusion. � The rate of diffusion is proportional to the surface area across which diffusion occurs, and inversely proportional to the square of the distance through which molecules must move. � Therefore, respiratory surfaces tend to be thin and have large areas, maximizing the rate of gas exchange. � In addition, the respiratory surface of terrestrial and aquatic animals are moist to maintain the cell membranes and thus gases must first dissolve in water. ...read more.


2. Gills are respiratory adaptation of most aquatic animals � Gills are outfoldings of the body surface that are suspended in water. � The total surface area of gills is often much greater than that of the rest of the body. � Water has both advantages and disadvantages as a respiratory medium. � There is no problem keeping the cell membranes of the respiratory surface moist, since the gills are surrounded by the aqueous environment. � However, O2 concentrations in water are low, especially in warmer and saltier environments. � Thus, gills must be very effective to obtain enough oxygen. � This flow pattern is countercurrent exchange. Tracheal systems and lungs are respiratory adaptations of terrestrial animals � As a respiratory medium, air has many advantages over water. � Air has a much higher concentration of oxygen. � Also, since O2 and CO2 diffuse much faster in air than in water, respiratory surfaces exposed to air do not have to be ventilated as thoroughly as gills. � When a terrestrial animal does ventilate, less energy is needed because air is far lighter and much easier to pump than water and much less volume needs to be breathed to obtain an equal amount of O2. ...read more.


� Since the lungs do not completely empty and refill with each breath cycle, newly inhaled air is mixed with oxygen-depleted residual air. � Therefore, the maximum oxygen concentration in the alveoli is considerably less than in the atmosphere. � This limits the effectiveness of gas exchange. � Ventilation is much more complex in birds than in mammals. � The entire system - lungs and air sacs - is ventilated when the bird breathes. � Air flows through the interconnected system in a circuit that passes through the lungs in one direction only, regardless of whether the bird is inhaling or exhaling. � Instead of alveoli, which are dead ends, the sites of gas exchange in bird lungs are tiny channels called parabronchi, through which air flows in one direction. � This system completely exchanges the air in the lungs with every breath. � Therefore, the maximum lung oxygen concentrations are higher in birds than in mammals. � Partly because of this efficiency advantage, birds perform much better than mammals at high altitude. � For example, while human mountaineers experience tremendous difficulty obtaining oxygen when climbing the Earth's highest peaks, several species of birds easily fly over the same mountains during migration. ...read more.

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Response to the question

The candidate answers the question a very high level covering a vast range of ventilation types, and how each one differs between species and medium.

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Response to the question

The candidate answers the question a very high level covering a vast range of ventilation types, and how each one differs between species and medium.

Level of analysis

An essay which covers gas exchange. The candidate starts with a well explained definition of oxygen in different animals, and provides a simple definition of oxygen in relation to the different mediums which is a great start. The candidate is wrong in thinking that only aquatic animals need a moist exchange surface, as mammals do as well. The candidate goes on to explain different respiratory adaptations well, but does not explain the different types of fish and how they adapted very well. They could also have included adaptations such as a respiratory swimbladder. They explain the adaptations of birds well.

Quality of writing

Punctuation, grammar and spelling done well. The format of the essay is all over the place with different sized bullet points and no subheadings for different sections and different sized font and text which is very untidy.

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Reviewed by skatealexia 06/08/2012

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