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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

CHAPTER 48 GAS EXCHANGE IN ANIMALS 48.1 What Physical Factors Govern Respiratory Gas Exchange? Respiratory Gases the gases that animals must exchange are oxygen and carbon dioxide. Cells need to obtain 2 from the environment to produce an adequate supply of ATP by cellular respiration. CO is an end product of cellular respiration, and it must be removed from the body to 2 prevent toxic effect. Diffusion is the only means by which respiratory gases are exchanged between the internal body fluids of an animal and the outside medium (air or water). There are no active transport mechanisms to move such gases across biological membranes. Diffusion is driven by concentration differences - The concentrations of different gases in a mixture are expressed by the partial pressures of those gases. To do this, the total pressure must be known first (this is measured with a barometer). Barometric Pressure atmospheric pressure. Partial Pressure of Oxygen at sea level is 20.9% of 760 mm Hg. - The concentration of respiratory gases in a liquid such as water is a little more complicated because the solubility of the gas in the liquid is involved. Ficks law applies to all systems of gas exchange Ficks Law of Diffusion an equation that describes the factors that determine the rate of diffusion of a molecule from an area of higher concentration to an area of lower concentration. Q = DA x P 1 P 2 L Q = rate a substance diffuses between 2 locations D = diffusion coefficient A = area over which substance diffuses P1and P 2 concentration of gas at 2 locations L = distance between these locations - Animals can maximize D by using air instead of water - Other adaptations for maximizing respiratory gas exchange maximize surface area for exchange (A) maximize concentration gradient across exchange surface (C >> 1 ) 2 Air is a better respiratory medium than water - Oxygen can be obtained more easily from air that from water for several reasons: The O 2ontent of water is much higher that the O co2tent of an equal volume of water. Oxygen diffuses about 8000 times more rapidly in air than in water. When an animal breathes, it does work to move water or air over its specialized gas exchange surfaces. More energy is required to move water than to move air because water 800 times denser than air and about 50 times more viscous. - The slow diffusion of O m2lecules in water affects air-breathing animals as well as water- breathing ones. It limits the efficiency of 2 distribution from gas exchange surfaces to the sites of cellular respiration in air-breathing animals. Animal cells with low rates of metabolism are forced to be no more than a couple of millimeters away from a good source of environmental O . 2 - Therefore, there are severe size and shape limits on many species of invertebrates that lack internal systems for distributing O2. High temperatures create respiratory problems for aquatic animals - Water breathers Most water breathers are ectotherms. - As the temperature of the water gets warmer, their body temperature and metabolic rate rise. They need more O as2the water gets warmer. - Warm water holds less dissolved gas than cold water does. As water temperature goes up, they must extract more and more O from2an environment that is increasingly O2deficient, and a lower percentage of that O 2s available to support activities other than breathing. O 2vailability decreases with altitude - An increase in altitude reduces the O 2upply for air breathers. - At all altitudes, 2 makes up 20.9% of the dry air. - As you go up in altitude, the total amount of gas per unit volume decreases. - Since the movement of O ac2oss respiratory gas exchange surfaces and into the body depends on diffusion, its rate of movement depends on the P diO2erence between air and the body fluids. - The drastically reduced P O2 the air at high altitudes constrains O u2take.
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