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The respiratory gases that animals must exchange are oxygen (O2) and carbon dioxide (CO2). Cells need to obtain O2 from the environment to produce an adequate supply of ATP by cellular respiration (see Chapter 9). CO2 is an end product of cellular respiration, and it must be removed from the body to prevent toxic effects. Gas exchange systems of animals consist of (1) specialized body surface areas where these gases can move between the body and the external environment, (2) mechanisms that ventilate the environmental side of those surfaces with air or water, and (3) mechanisms that circulate extracellular fluids on the internal sides.
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Respiratory gases dioxide diffuse from areas of higher partial pressure to areas of lower partial pressure.
Fick’s law of diffusion describes the factors that determine the rate of diffusion of a gas between two locations.
Rising temperature decreases oxygen solubility in water and increases the metabolism of aquatic ectotherms.
With increasing altitude, oxygen partial pressure decreases, which decreases the rate of oxygen diffusion into the respiratory systems of animals.
Diffusion is the only means by which respiratory gases are exchanged between an animal’s internal body fluids and the outside medium (air or water). There are no active transport mechanisms to move respiratory gases across biological membranes. Because diffusion is a physical process, knowing what physical factors influence rates of diffusion helps us understand the diverse adaptations of gas exchange systems. (You may want to review the coverage of diffusion in Key Concept 6.3.)