The Gas Exchangers: Structure, Function, and Evolution of the Respiratory Processes

"Amongst animals, diversity of form and of environmental circumstances have given rise to a multitude of different adap­ tations subserving the relatively unified patterns of cellular metabolism. Nowhere else is this state of affairs better exem­ plified than in the realm of respiration". Jones (1972). The field of comparative respiratory biology is expanding almost exponentially. With the ever-improving analytical tools and methods of experimentation, its scope is blossoming to fascinating horizons. The innovativeness and productivity in the area continue to confound students as well as specialists. The increasing wealth of data makes it possible to broaden the information base and meaning­ fully synthesize, rationalize, reconcile, redefine, consolidate, and offer empirical validation of some of the earlier anecdotal views and interpretations, helping resolve the issues into adequately realistic and easily perceptible models. Occa­ sional reflections on the advances made, as well as on the yet unresolved prob­ lems, helps chart out new grounds, formulate new concepts, and stimulate inquiry. Moreover, timely assessments help minimize isolation among investiga­ tors, averting costly duplication of effort. This exposition focuses on the diversity of the design of the gas exchangers and gives a critical appraisal of the plausible or constrained the evolvement of respiration. The factors that have motivated cause-and-effect relationship between the phylogenetic, developmental, and en­ vironmental factors, conditions, and states which at various thresholds and under certain backgrounds conspired in molding the gas exchangers is argued.

1 Perspectives on Life and Respiration: How, When, and Wherefore.- 1.1 Life: Diversity, Complexity, and Uniformity Fabricated on Simplicity.- 1.2 The Earth: a Highly Dynamic Planet.- 1.3 Factors that Encouraged the Evolution of Life on Earth.- 1.4 Oxygen: a Vital Molecular Resource for Life.- 1.5 Anaerobic Metabolism and Adaptive Success in Animals.- 1.6 Evolved Mechanisms and Strategies of Procuring Molecular O­2.- 1.7 Explicating the Process of Evolution of Respiration: Limitations.- 1.8 Plans and Performance Measures of the Gas Exchangers.- 1.9 The Early Anoxic Earth and the Evolution of Life.- 1.10 Abundance of Molecular O2 in the Earth’s Biosphere.- 1.11 Shift from Anaerobiotic to Aerobiotic State in the Early Earth.- 1.12 Accretion of Molecular O2.- 1.13 CO2 Pulses in the Biosphere.- 1.14 The Overt and Covert Roles of O2 in Colonization and Extinctions of Biota.- 1.15 Oxygen: a Paradoxical Molecule.- 1.16 The Rise of the Level of Molecular O2: a Curse or a Blessing?.- 1.16.1 The Deleterious Reactive Radicals of Molecular O2.- 1.16.2 Senescence: the Effects Molecular O2.- 1.16.3 Biological Defenses Against O2 Toxicity.- 1.17 The Evolution of Complex Metabolic Processes.- 1.18 Oxygen and CO2 as Biochemical Factors in Respiration.- 1.19 Homeostasis: the Role of Respiration.- 2 Essence of the Designs of Gas Exchangers — the Imperative Concepts.- 2.1 Innovations and Maximization of Respiratory Efficiency.- 2.2 Safety Factors and Margins of Operation of Gas Exchangers.- 2.3 Engineering Principles in the Design of the Gas Exchangers.- 2.4 Scopes and Limitations in the Design and Refinement of the Gas Exchangers.- 2.5 Optimal Designs in Biology and Gas Exchangers in Particular.- 2.5.1 Symmorphosis: the Debate.- 2.5.2 The Operative Strategies for Optimization in the Gas Exchangers.- 2.5.3 Symmorphosis and Optimization: are they Logical Outcomes of Evolution?.- 2.6 Fractal Geometry: a Novel Approach for Discerning Biological Form.- 2.7 From Diffusion, Perfusion, and Ventilation to Respiratory Pigments.- 2.7.1 Diffusion.- 2.7.2 Convective Flows.- 2.8 Blood and the Respiratory Pigments.- 2.8.1 Hemoglobinless Fish.- 2.9 Energetic Cost and Efficiency of Respiration.- 2.9.1 The Requisites for Efficient Gas Exchange.- 2.9.2 Efficient vs. Inefficient - Primitive vs. Advanced Gas Exchangers: the Contention.- 2.10 Modeling: Utility in Study of Integrative Construction of the Gas Exchangers.- 2.10.1 Evaluation of the Functional Efficiency of the Gas Exchangers.- 2.10.2 Modeling the Gas Exchangers.- 3 Gas Exchange Media, Respiratory States, and Environments.- 3.1 Water and Air as Respiratory Media: General Considerations.- 3.2 Physical Charateristics of Water and Air.- 3.3 The Distribution of Water and Air on Earth.- 3.4 Water: a Respirable Medium and an Integral Molecule for Life.- 3.4.1 Oxygen and CO2 Content in Water: Effect on Respiration.- 3.4.2 Density and Viscosity of Water.- 3.4.3 Thermal Capacity and Conductivity of Water.- 3.4.4 Derelict Waters: Respiratory Stress from Hypercapnia and Hypoxia.- 3.5 Terrestrial Habitation and Utilization of Atmospheric O2.- 3.6 Hydrogen Sulfide Habitats. Tolerance and Utilization.- 3.7 The Porosphere and Fossorial Respiration.- 3.7.1 Gaseous Composition in Burrows.- 3.7.2 Burrowing Aquatic Annelids, Crustaceans, and Fish.- 3.8 Living at High Altitude: Coping with Hypoxia and Hypobaria.- 3.8.1 Tolerance of Arterial Hypocapnia in Birds.- 3.8.2 Flying over Mt. Everest: the Bar Headed Goose, Anser indicus.- 3.9 Gravity: Effects on Respiratory Form and Function.- 4 Water Breathing: the Inaugural Respiratory Process.- 4.1 The Design of the Gills.- 4.2 Adaptive Diversity and Heterogeneity of Gill Form.- 4.3 The Functional Innovations of the Gills for Aquatic Respiration.- 4.4 The Simple Gills.- 4.4.1 Morphological Characteristics.- 4.4.2 Ventilation and Functional Capacities.- 4.4.3 Gas Exchange Pathways and Mechanisms.- 4.5 The Complex Gills.- 4.5.1 Structure and Architectural Plans.- 4.6 The Water Lungs.- 4.7 The Placenta: an Ephemeral Liquid to Liquid Gas Exchanger.- 4.7.1 The Functional Reserves of the Placentae.- 5 Bimodal Breathing: Compromise Respiration.- 5.1 The Water-Air Interface: an Abstract Respiratory Rubicon.- 5.2 Strategies and Adaptive Convergence for Air Breathing.- 5.3 Risks, Costs, and Benefits in the Change to Air Breathing.- 5.4 CO2 Elimination: Impediment to Evolution of Air Breathing and Terrestriality.- 5.5 Control and Coordination of the Bimodal Gas Exchange Process.- 5.5.1 Ventilatory Modalities of the Gas Exchangers in the Bimodal Breathers.- 5.5.2 The Circulatory Patterns in the Gas Exchangers of the Bimodal Breathers.- 5.6 Taxa with Notable Propensity for Bimodal Breathing.- 5.6.1 Mollusks.- 5.6.2 Crustaceans.- 5.6.3 Fish.- 6 Air Breathing: the Elite Respiration.- 6.1 Is the Surface of the Lung Dry, Moist, or Wet? Do Real Air Breathers Exist?.- 6.2 Lung and Swim Bladder - Which Developed Earlier and for What Purpose?.- 6.3 Evolution of Air Breathing and Terrestriality: the Limitations.- 6.4 Aerial Gas Exchangers: Structural and Functional Diversity.- 6.5 The Diffusive Type Gas Exchangers.- 6.6 The Mixed Type Gas Exchangers.- 6.6.1 The Insectan Tracheal System.- 6.7 The Convective Type Gas Exchangers.- 6.7.1 Ventilatory Mechanisms and Organization of the Gas Exchangers.- 6.7.2 The Amphibian Lung.- 6.7.3 The Reptilian Lung.- 6.7.4 The Mammalian Lung.- 6.7.5 The Avian Lung.- 6.8 The Internal Subdivision of the Lung: the Functional Implications.- 6.9 The Surfactant: a Versatile Surface Lining of the Gas Exchangers.- 6.10 Nonrespiratory Roles of the Gas Exchangers.- 6.10.1 Trophic, Sensory, and Locomotory Functions.- 6.10.2 Metabolic and Pharmacokinetic Functions of the Lung.- 6.11 The Implications of Liquid Breathing in Air Breathers.- 6.12 Physical Gill and the Plastron: a Unique Underwater Respiratory Strategy.- 6.13 The Cleidoic Egg: a Fascinating Gas Exchanger.- 6.14 The Bottom Line.- References.