A scanning and transmission electron microscopic study of the bat lung

Critical appraisal of some factors pertinent to the functional designs of the gas exchangers

Respiration acquires O₂ from the external fluid milieu and eliminates CO₂ back into the same. Gas exchangers evolved under certain immutable physicochemical laws upon which their elemental functional design is hardwired. Adaptive changes have occurred within the constraints set by such laws to satisfy metabolic needs for O₂, environmental conditions, respiratory medium utilized, lifestyle pursued and phylogenetic level of development: correlation between structure and function exists. After the inaugural simple cell membrane, as body size and structural complexity increased, respiratory organs formed by evagination or invagination: the gills developed by the former process and the lungs by the latter. Conservation of water on land was the main driver for invagination of the lungs. In gills, respiratory surface area increases by stratified arrangement of the structural components while in lungs it occurs by internal subdivision. The minuscule terminal respiratory units of lungs are stabilized by surfactant. In gas exchangers, respiratory fluid media are transported by convection over long distances, a process that requires energy. However, movement of respiratory gases across tissue barriers occurs by simple passive diffusion. Short distances and large surface areas are needed for diffusion to occur efficiently. Certain properties, e.g., diffusion of gases through the tissue barrier, stabilization of the respiratory units by surfactant and a thin tripartite tissue barrier, have been conserved during the evolution of the gas exchangers. In biology, such rare features are called Bauplans, blueprints or frozen cores. That several of them (Bauplans) exist in gas exchangers almost certainly indicates the importance of respiration to life.

Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the "oxygen cascade"-step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.

The morphometry of the lung of the African lungfish (Protopterus aethiopicus) : its structural-functional correlations

The lung of the African lungfishProtopterus aethiopicushas been investi­gated by morphometric techniques. The volume of the lung was strongly correlated with body mass. The exchange tissue made up about 50% of the lung. The intrapulmonary air constituted 73% of the volume of the lung, the rest being made up of the interalveolar septa (22%) and the blood capillaries (5%). The surface area of the blood-gas (tissue) barrier per unit body mass was 14.3 cm2g-1and the harmonic mean thickness of the tissue barrier 0.370 μm. The total morphometric pulmonary diffusing capacity per unit body mass was 0.0024 ml O2s-1mbar-1kg-1(1 bar = 105Pa) Of the three existing genera of lungfish, the general structure of the lung ofProtopteruswas similar to that ofLepidosirenand much unlike that ofNeoceratodus. This could be attributed to the fact that bothProtopterusandLepidosirenare obligate air-breathers whileNeoceratodusis an obligate water- breather. A comparison of the pulmonary morphometric data onProtopteruswith those of the gas exchange apparatus of other groups of vertebrates has been made and pulmonary morphometric and design specializations in the evolution of the air-breathing vertebrates from the lungfishes (some of the initial air-breathers) to reptiles through to birds are apparent.