BGYA02 - Lec4 (text+lec combined notes)

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Biological Sciences
Mary Olaveson

481 What Physical Factors Govern Respiratory Gas ExchangeRespiratory gases animals must exchange are O and CO Diffusion is the only 22means by which respiratory gases are exchanged between the internal body fluids of an animal and the outside medium air or water No active transport mechanisms move respiratory gases across biological membranesDiffusion is driven by concentration differences oBarometric pressure at sea level is 760 mmHg high Because dry air is 209 percent O the partial pressure of O at sea level is 209 22percent of 760 mmHg 159 mmHgoDescribing concentration of gases in liquid is different The actual amount of gas in a liquid depends on the partial pressure of that gas in the gas phase in contain with the liquid as well as on the solubility of that gas in the liquidoThe diffusion of gas between the gaseous phase and the liquid still depends on the partial pressures of the gas in the two phasesFicks Law applies to all systems of gas exchangeoDiffusion described by Ficks law of diffusionoQ DAPPL12Q is the rate at which a gas such as O diffuses between two 2locationsD is the diffusion coefficient which is a characteristic of the diffusing substance the medium and the temperatureA is the crosssectional area through which the gas is diffusingP and P are the partial pressures of the gas at the two 12locationsL is the path length or distance between the two locationsoTherefore PP L is a partial pressure gradient Animals can 12maximize D for respiratory gases by using air rather than water as their gas exchange medium whenever possible which greatly increases QoAll other adaptations for maximizing respiratory gas exchange must influence the surface area A for gas exchange or the partial pressure gradient across that surface areaAir is a better respiratory medium than wateroO can be obtain more easily from air than water for several reasons2The O content of air is much higher than in an equal volume of 2waterOdiffuses about 8000 times more rapidly in air than in water2 When an animal breathes it does work to move water or air over its specialized gas surfaces More energy is required to move water than airoThe slow diffusion of O molecules in water affects airbreathing and 2waterbreathing animalsoDiffusion of O in water is so slow that even animal cells with low rates 2of metabolism can be no more than a couple of mm away from a good source of environmental O Therefore there are severe size and shape 2limits on the many species of invertebrates that lack internal systems for distributing O Most of these species are small but some have 2grown larger by developing flat think body with a large external surfaceHigh temperature create respiratory problems for aquatic animalsoMost water breathers are ectotherms So as the temperature of the water gets warmer the ectotherms body temperature and metabolic rate rise Thus water breathers need more O as the water gets 2warmer As water temperature rises the waterbreather must extract more and more O from an environment that is increasingly O22 deficient and a lower percentage of that O is available to support 2activities other than breathingO availability decreases with altitude2oAs you go up in altitude the total amount of gas per unit volume decreasesoSince the movement of O across respiratory gas exchange surfaces 2and into the body depends on diffusion its rate of movement depends on the P difference between the air and the body fluidsO2CO is lost by diffusion2oThe amount of CO in the atmosphere is extremely low so for air2breathing animals there is always a large concentration gradient for
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