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Chapter 48

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University of Toronto Scarborough
Biological Sciences
Kamini Persaud

48.1 What Physical Factors Govern Respiratory Gas Exchange? The respiratory gases that animals must exchange are oxygen (O ) and2carbon dioxide (CO 2. Cells need to obtain O 2rom the environment to produce an adequate supply of ATP by cellular respiration. CO 2s an end product of cellular respiration, and it must be removed from the body to prevent toxic effects. 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 respiratory gases across biological membranes. Because diffusion is a physical process, knowing the physical factors that influence rates of diffusion helps us understand the diverse adaptations of gas exchange systems. Diffusion is driven by concentration differences Because diffusion results from the random motion of molecules, the net movement of a molecule is always down its concentration gradient. One way biologists express the concentrations of different gases in a mixture is by theartial pressures of those gases. First, we have to know what the total pressure is, and we measure that with an instrument called a barometer. At sea level, the pressure exerted by the atmosphere will support, and therefore be equal to, a column of mercury in the tube that is about 760 mm high. Therefore, barometric pressure (atmospheric pressure) at sea level is 760 millimeters of mercury (mm Hg). Because dry air is 20.9% O ,2the partial pressure of oxygen (P )O2t sea level is 20.9% of 760 mm Hg. Describing the concentration of respiratory gases in a liquid such as water is a little more complicated because another factor is involvedthe solubility of the gas in the liquid. Thus, the actual amount of a gas in a liquid depends on the partial pressure of that gas in the gas phase in contact with the liquid as well as on the solubility of that gas in that liquid. However, the diffusion of the gas between the gaseous phase and the liquid still depends on the partial pressures of the gas in the two phases. Ficks law applies to all systems of gas exchange Diffusion is a physical phenomenon that can be described quantitatively with a simple equation called Ficks law of diffusion. All environmental variables that limit respiratory gas exchange and all adaptations that maximize respiratory gas exchange are reflected in one or more components of this equation. Ficks law is written as where Q is the rate at which a gas diffuses between two locations. D is the diffusion coefficient, which is a characteristic of the diffusing substance, the medium, and the temperature. (For example, perfume has a higher D than motor oil vapor, and all substances diffuse faster at higher temperatures, as well as diffusing faster in air than in water.) A is the cross-sectional area through which the gas is diffusing. P 1nd P ar2 the partial pressures of the gas at the two locations. L is the path length, or distance, between the two locations. Therefore, (P P1)/L 2s a partial pressure gradient. Animals can maximize D for respiratory gases by using air rather than water as their gas exchange medium whenever possible; doing so greatly increases Q. All other adaptations for maximizing respiratory gas exchange must influence the surface area (A) for gas exchange or the partial pressure gradient [(P P )1L] a2ross that surface area. Air is a better respiratory medium than water Oxygen can be obtained more easily from air than from water for several reasons: The O c2ntent of air is much higher than the O content2of an equal volume of water. The maximum O conte2t of a bubbling stream in equilibrium with air is less than 10 ml of O 2er liter of water. The O co2tent of the air over the stream is about 200 ml of O 2 per liter of air. Oxygen diffuses about 8,000 times more rapidly in air than in water. That is why the O 2ontent of a stagnant pond can be zero only a few millimeters below the surface. 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 is 800 times more dense than air and about 50 times more viscous. The slow diffusion of O mo2ecules in water affects air-breathing animals as well as water-breathing ones. Eukaryotic cells carry out cellular respiration in their mitochondria, which are located in the cytoplasman aqueous medium. Cells are bathed in extracellular fluidalso an aqueous medium. In addition, all respiratory surfaces must be protected from desiccation by a thin film of fluid through which O 2 must diffuse. O 2 The slow rate of O 2 must diffuse in water to reach cells. diffusion in water limits the efficiency of O distribu2ion from gas exchange surfaces to the sites of cellular respiration even in air-breathing animals. Diffusion of O i2 water is so slow that even animal cells with low rates of metabolism can 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 the many species of invertebrates that lack internal systems for distributing O . Mos2 of these species are very small, but some have grown larger by evolving a flat, thin body with a large external surface area. Still others have very thin bodies that are built around a central cavity through which water circulates. A critical factor enabling larger, more complex animal bodies has been the evolution of specialized respiratory systems with large surface areas for enhancing respiratory gas exchange. High temperatures create respiratory problems for aquatic animals Animals that breathe water are in a double bind when environmental temperatures rise. Most water breathers are ectothermstheir body temperatures are closely tied to the temperature of the water around them. As the temperature of the water gets warmer, the ectotherms body temperature and metabolic rate rise. Thus, water breathers need more O a2 the water gets warmer. But warm water holds less dissolved gas than cold water does. In addition, if the animal performs work to move water across its gas exchange surfaces (as fish do), the energy the animal must expend to breathe increases as water temperature rises. Therefore, as water temperature goes up, the water breather must extract more and more O from an2 environment that is increasingly O 2 deficient, and a lower percentage of that O is a2ailable to support activities other than breathing. O 2vailability decreases with altitude Just as a rise in temperature reduces the supply of O availa2le to water breathers, an increase in altitude reduces the O sup2ly for air breathers. For example, at an altitude of 5,800 m, barometric pressure is only half what it is at sea level, so the P at that O2 altitude is only about 80 mm Hg (760/2 x 0.21). Since the movement of O across 2 respiratory gas exchange surfaces and into the body depends on diffusion, its rate of movement depends on the P O2 difference between the air and the body fluids. Therefore, the drastically reduced P O2in the air at high altitudes constrains O upt2ke. Because of these constraints, mountain climbers who venture to the heights of Mount Everest usually breathe O fr2m pressurized bottles. CO i2 lost by diffusion Respiratory gas exchange is a two-way process: CO diffuses o2t of the body as O 2 diffuses in. The direction and rate of diffusion of the respiratory gases across the respiratory exchange surfaces depend on the partial pressure gradients of the gases. The partial pressure gradients of O and CO across these gas exchange surfaces are quite 2 2 different. The amount of CO in th2 atmosphere is extremely low (0.03%), so for air- breathing animals there is always a large concentration gradient for diffusion of CO 2 from the body to the environment. Whereas the partial pressure gradient for O 2 decreases with altitude, the partial pressure gradient for CO does not. The 2 partial pressure of CO in2the atmosphere is close to zero both at sea level and on top of Mount Everest. In general, getting rid of CO is not 2 problem for water-breathing animals because CO is m2ch more soluble in water than is O . Even in stagnan2 water, where the P CO2is higher than in fresh water, the lack of O beco2es a problem for the animal long before CO ex2hange difficulties arise. 48.1 RECAP Respiratory gases are exchanged by diffusion only. Air is a better respiratory medium than water because there is more O in a given v2lume of air than in the same volume of water, O diffuses faster in air than in water, and it 2 requires less work to move air over respiratory exchange surfaces than water. 48.2 What Adaptations Maximize Respiratory Gas Exchange? Some common ways the respiratory systems of different organisms maximize the exchange of O a2d CO with2the environment include adaptations for increasing the surface area over which diffusion of gases can occur; for maximizing partial pressure gradients; and for minimizing the diffusion path length through an aqueous medium. Respiratory organs have large surface areas Many anatomical adaptations maximize the specialized body surface area (A) over which respiratory gases can diffuse. External gills are highly
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