TAILIEUCHUNG - Entomology 3rd edition - C.Gillott - Chapter 15

15 Gas Exchange 1. Introduction In all organisms gas exchange, the supply of oxygen to and removal of carbon dioxide from cells, depends ultimately on the rate at which these gases diffuse in the dissolved state. The diffusion rate is proportional to the surface area over which diffusion is occurring. | 15 Gas Exchange 1. Introduction In all organisms gas exchange the supply of oxygen to and removal of carbon dioxide from cells depends ultimately on the rate at which these gases diffuse in the dissolved state. The diffusion rate is proportional to 1 the surface area over which diffusion is occurring and 2 the diffusion gradient concentration difference of the diffusing material between the two points under consideration divided by the distance between the two points . Diffusion alone therefore as a means of obtaining oxygen or excreting carbon dioxide can be employed only by small organisms whose surface area volume ratio is high . where all cells are relatively close to the surface of the body and organisms whose metabolic rate is low. Organisms that are larger and or have a high metabolic rate must increase the rate at which gases move between the environment and the body tissues by improving 1 and or 2 above. In other words specialized respiratory structures with large surface areas and or transport systems that bring large quantities of the gas closer to the site of use or disposal thereby improving the diffusion gradient have been developed. For most terrestrial animals prevention of desiccation is another important problem and this has had a major influence on the development of their respiratory surfaces through which considerable loss of water might occur. Typically respiratory surfaces of terrestrial animals are formed as invaginated structures within the body so that evaporative water loss is greatly reduced. In insects the tracheal system a series of gas-filled tubes derived from the integument has evolved to cope with gas exchange. Terminally the tubes are much branched forming tracheoles that provide an enormous surface area over which diffusion can occur. Furthermore tracheoles are so numerous that gaseous oxygen readily reaches most parts of the body and equally carbon dioxide easily diffuses out of the tissues. Thus in most insects in contrast .

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