Structure, function and evolution of the gas exchangers: comparative perspectives

Abstract

Over the evolutionary continuum, animals have faced similar fundamental challenges of acquiring molecular oxygen for aerobic metabolism. Under limitations and constraints imposed by factors such as phylogeny, behaviour, body size and environment, they have responded differently in founding optimal respiratory structures. A quintessence of the aphorism that 'necessity is the mother of invention', gas exchangers have been inaugurated through stiff cost-benefit analyses that have evoked transaction of trade-offs and compromises. Cogent structuralfunctional correlations occur in constructions of gas exchangers: within and between taxa, morphological complexity and respiratory efficiency increase with metabolic capacities and oxygen needs. Highly active, small endotherms have relatively better-refined gas exchangers compared with large, inactive ectotherms. Respiratory structures have developed from the plain cell membrane of the primeval prokaryotic unicells to complex multifunctional ones of the modern Metazoa. Regarding the respiratory medium used to extract oxygen from, animal life has had only two choices -water or air -within the biological range of temperature and pressure the only naturally occurring respirable fluids. In rarer cases, certain animals have adapted to using both media. Gills (evaginated gas exchangers) are the primordial respiratory organs: they are the archetypal water breathing organs.Lungs (invaginated gas exchangers) are the model air breathing organs. Bimodal (transitional) breathers occupy the water-air interface. Presentation and exposure of external (water/air) and internal (haemolymph/blood) respiratory media, features determined by geometric arrangement of the conduits, are important features for gas exchange efficiency: counter-current, cross-current, uniform pool and infinite pool designs have variably developed.

Three‐Dimensional Serial Section Computer Reconstruction of the Arrangement of the Structural Components of the Parabronchus of the Ostrich, Struthio Camelus Lung

“…In addition to thin and extensive respiratory surface area and large pulmonary capillary blood volume Maina, 2000Maina, , 2002a, structural parameters that promote the diffusing capacity of the lung for oxygen (O 2 ), the spatial arrangement of the structural components of a gas exchanger that determine the delivery and presentation of the respiratory media (water/air and blood) at the respiratory site where a thin blood-gas separates them to a large extent contributes to respiratory efficiency. One of the passionate debates of the 1960s and early 1970s on the functional biology of the avian respiratory system concerned the manner that this occurs in the gas exchange of the avian lung.…”

Bioaccumulation potential of air contaminants: Combining biological allometry, chemical equilibrium and mass-balances to predict accumulation of air pollutants in various mammals

“…Additionally, the alveolar surface area per kg body weight is independent of species weight (Weibel 1979;Hughes 1984) and the diffusion distance across the blood-air barrier is comparable across various mammalian species (Mania 2002;Maina and West 2005;Weibel 1979). Consequently, the diffusion resistance scales to the power of 1 instead of 0.75.…”

Membrane mediated development of the vertebrate blood‐gas‐barrier

“…Lung development has been well studied in mammals and to some reasonable extent in birds, but not much has been done in the ectotherms. In reptiles (Maina, 1989a;Perry, 1990;Fleetwood and Munnell, 1996), amphibians (Maina, 1989b), and fish (Maina, 2002), the parenchymal inter-airspace septa do not mature and a double capillary system is retained in the adults of these ectotherms. While controlling molecules may be similar to those in mammals and birds at the inaugural stages of lung development, remarkable differences would be expected in later stages of lung maturation.…”