Each system is adapted to deliver enough oxygen and eliminate enough carbon dioxide to allow the species to surmount specific environmental and predatory pressures while simultaneously limiting the energy cost of breathing and cumulative oxidative stress in cells and organelles within an acceptable range. Harnessing the energy from oxidative phosphorylation while minimizing cellular stress and damage is an eternal struggle transcending specific organ systems or species, a conflict that shaped an assortment of gas-exchange systems. While aerobic respiration is essential for efficient metabolic energy production, a prerequisite for complex organisms, cumulative cellular oxygen stress has also made senescence and death inevitable. Oxygen, a vital gas and a lethal toxin, represents a trade-off with which all organisms have had a conflicted relationship. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. 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. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the вЂњoxygen cascadeвЂќ-step-down of PO 2 that balances supply against toxicity. Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). 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. Ambient oxygen tension ( PO 2) 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. 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.

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