Several systems of gas transport have developed during evolution, all of which are able to sufficiently supply oxygen to the tissues and eliminate the CO2 produced by the metabolism, in spite of great distances between the environment and the individual cells of the tissues. Almost all these systems utilize a combination of convection and diffusion steps. Convection achieves an efficient transport of gas over large distances, but requires energy and cannot occur across tissue barriers. Diffusion, on the other hand, achieves gas transport across barriers, but requires optimization of diffusion paths and diffusion areas. When two convectional gas flows are linked via a diffusional barrier (gas/fluid in the case of the avian lung, fluid/fluid in the case of gills), the directions in which the respective convectional movements pass each other are important determinants of gas exchange efficiency (concurrent, countercurrent and cross-current systems). The tracheal respiration found in insects has the advantage of circumventing the convective gas transport step in the blood, thereby avoiding the high energy expenditure of circulatory systems. This is made possible by a system of tracheae, ending in tracheoles, that reaches from the body surface to every cell within the body. The last step of gas transfer in these animals occurs by diffusion from the tracheoles ("air capillaries") to the mitochondria of cells. The disadvantage is that the tracheal system occupies a substantial fraction of body volume and that, due to limited mechanical stability of tracheal walls, this system would not be able to operate under conditions of high hydrostatic pressures, i. e. in large animals. Respiration in an "open" system, i. e. direct exposure of the diffusional barrier to the environmental air, eliminates the problem of bringing the oxygen to the barrier by convection, as is necessary in the avian and mammalian lung, in the insects' tracheal system and in the gills. An open system is found in the respiration via the skin, which is of significance in some amphibians, but is limited by the thickness of the skin that constitutes a substantial diffusion path for O2 and CO2. The thick skin, on the other hand, provides mechanical protection as well as flexibility for the animals' body and helps avoid massive water loss via the body surface. The gills of fishes, in contrast, exhibit rather short diffusion distances, are located in a mechanically protected space, and the problem of water loss does not exist. The flows of blood and water occur in opposite direction (countercurrent flow) and this situation makes an arterial PO2 approaching the environmental PO2 possible. A major disadvantage is constituted by the environmental medium since water contains little O2 compared to air and, to compensate this, much energy is expended to maintain a high flow rate of water through the gills. In the mammalian lung ("pool system"), the presence of a dead space and the rhythmic ventilation that replaces only a small fraction of the gas volume of the lung per breath, are responsible for an arterial PO2 (2/3 of the atmospheric PO2) that cannot reach the expiratory PO2. However, an advantage of this feature is the constantly high alveolar and arterial PCO2, which provides a highly effective H(+) buffer system in the entire body. The apparent disadvantage of the mammalian lung is avoided by the avian lung, which uses an extended system of airways to establish continuous equilibration of a part of the capillary blood with fresh air (cross current system), during inspiration as well as during expiration. In this system, arterial PO2 can significantly exceed expiratory PO2. A disadvantage here is the enormous amount of space taken up by the avian lung, in animals of 1 kg body weight three times as much as taken up by the mammalian lung. All respiratory exchange systems considered here exhibit high degrees of optimization - yet follow highly diverse construction principles. There is no such thing as an ideal gas exchange system. The system that has evolved in each species depends to an impressive extent on environmental conditions, on body build and size, on the animal's patterns of movement and on its energy consumption.