Comparative Pulmonary Morphology and Morphometry: The Functional Design of Respiratory Systems

Abstract

Though there are assertions that life is possible without oxygen, such states can only exist for definite periods and in simplest of life forms (Hochachka et al. 1973; Herreid 1980; Portner et al. 1985). Intestinal parasites have been said to live without molecular oxygen and intertidal molluscan facultative anaerobes survive for days without it (Ghiretti 1966). Extended life in total anoxia is impossible in more advanced animals. Adaptively, however, in unpropitious ambient conditions, some animals drastically reduce their energy needs by adopting latent (ametabolic) life states, variably termed aestivation, lethargy, hibernation, quiescence and dormancy. Cryptobiosis, a state when life virtually stops is the extreme of such low energy retreats (Hochachka and Guppy 1987). Even in such a state an infinitesimal quantity of energy must, nevertheless, be essential to sustain the integrity of the basic molecular life-support processes such as protein turnover and ion pumping. The presence of oxygen is crucial in maintaining what was appropriately termed by Kleiber (1961) “the fire of life”. Metabolic rate is an expression of intensity and speed of life while death is empirically a state of zero cellular energy production. Energy is crucial to all biological events from molecular and biochemical level to ecological and evolutionary processes. To varying extents, the principal factors which appear to determine the metabolic demands of an organism are size, phylogeny, temperature and physical activity (Bennett 1988a)