Altitude adaptation: organisms without lungs

Organisms which depend upon diffusive transfer for their external gas can, by structural modification, adjust to the increase of the diffusion coefficient at altitude. In avian eggs, reduced water loss is achieved by a reduction of the pore area which matches the increase in diffusivity so that the functional conductance remains similar to that at sea level and presumably prevents dessication of the embryo. What modifications occur in the tracheal system of insects is not known. In plants there are observations of increased as well as decreased conductance at altitude which at least appear to rule out temperature and partial pressure of the ambient CO2 as chief determinants for altering the conductance (8). In organisms with convective transport systems, changes in the environment and metabolic demands are satisfied by appropriate changes in ventilation. In organisms with diffusive gas transport, the response is provided by structural changes. Such changes, however, do not result in equivalent transfer rates for all the gases, since the diffusion coefficients for CO2, O2 and water vapor differ by a ratio of 0.78 : 1.0 : 1.2, respectively.