Phylogenese der Gasaustauschsysteme

Several systems of gas transport have developed during evolution, all of which are able to sufficiently supply oxygen to the tissues and eliminate the CO 2 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 tranfer 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 O 2 and CO 2 . 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 PO 2 approaching the environmental PO 2 possible. A major disadvantage is constituted by the environmental medium since water contains little O 2 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 PO 2 (2/3 of the atmospheric PO 2 ) that cannot reach the expiratory PO 2 . However, an advantage of this feature is the constantly high alveolar and arterial PCO 2 , which provides a highly effective H + buffer system in the entire body.