Adaptation to Altitude-Hypoxia in Vertebrates

Adaptation to altitude hypoxia is characterized by a variety offunctional changes which collectively facilitate oxygen trans­ port from the ambient medium to the cells of the body. All of these changes can be seen at one time or another in the course of hypoxic exposure. Yet, as already stressed (Hannon and Vogel, 1977), an examination of the literature gives only a sketchy and often conflicting picture of the exact nature of these changes and how they interact as a function of exposure duration. This is partly because of the limited number of variables explored in a given study, but it is also attributable to differences in experimental design, differences among species in susceptibility to hypoxia, nonstandardized experimental conditions, lack of proper control of physical (e. g. , temperature) and physiological variables (e. g. , body mass), failure to take measurements at key periods of exposure, and gaps in knowledge about some fundamental mechanisms. Furthermore the available data on animals native to high altitude are meager and/or inconclusive. Extensive further work under well-controlled experimental conditions is required before a detailed picture can be made. Nevertheless, it has been a guiding principle in the prepara­ tion of this monograph rather to summarize the vastly dis­ persed material that constitutes the comparative physiology of adaptation to high altitude into a coherent picture, than to provide a comprehensive survey of the field.

Contenu

1. General Introduction.- 1.1 High Altitude.- 1.1.1 Environmental Factors.- 1.1.1.1 Hypoxia.- 1.1.1.2 Cold.- 1.1.1.3 Dehydration.- 1.1.1.4 Malnutrition.- 1.1.2 Major Mountain Systems.- 1.1.3 Animal Life at High Altitudes.- 1.1.3.1 Lower Vertebrates.- 1.1.3.2 Birds.- 1.1.3.3 Mammals.- 1.2 Adaptations.- 1.2.1 Problems of Definition.- 1.2.2 Problems in Assessing Adaptation to High Altitude.- 1.2.2.1 What Adapts to What?.- 1.2.2.2 How Do Adaptations Occur?.- 1.2.2.3 Time Courses of Adaptations.- 1.3 A Comparative Account of Respiratory Processes.- 2. The Respiratory Gas Exchange System and Energy Metabolism Under Altitude Hypoxia.- 2.1 Concept of Conductance.- 2.1.1 From External Medium to Mitochondria.- 2.1.2 Basic Equation.- 2.1.3 Diffusive Conductances.- 2.1.4 Convective Conductances.- 2.1.5 Adaptation to Hypoxia.- 2.2 The O2 and CO2 Cascades.- 2.2.1 O2 Loss in the External Gas Exchange Organ.- 2.2.2 Complex Gas/Blood Transfer.- 2.2.3 O2 Transfer into the Tissues.- 2.2.4 Elimination of Carbon Dioxide, and Acid Base Status.- 2.3 Energy Expenditure and O2 Availability.- 2.3.1 Aerobic Metabolism.- 2.3.1.1 Minimal Oxygen Consumption.- 2.3.1.2 Increased Requirements for O2.- 2.3.2 Anaerobic Metabolism.- 2.3.2.1 Alactic Mechanism.- 2.3.2.2 Lactic Mechanism.- 3. Ventilatory Adaptations.- 3.1 Major Features.- 3.1.1 Strategies.- 3.1.1.1 O2 Extraction and Specific Ventilation Basic Equations.- 3.1.1.2 Comparison Among Animals.- 3.1.1.3 Interpretation.- 3.1.1.4 Resulting O2 Driving Pressure.- 3.1.2 Tactics.- 3.1.2.1 Effective Ventilation.- 3.1.2.2 Breathing Pattern.- 3.1.3 Associated Hypocapnia and Alkalosis.- 3.1.4 Time Course.- 3.1.4.1 A Study on Rats.- 3.1.4.2 The $$ {\text{P}}{{\text{O}}{\text{2}} } $$ Criterion.- 3.1.4.3 Interspecies Differences.- 3.2 Mediation.- 3.2.1 Hypoxic Chemoreflex Drive.- 3.2.1.1 Definition.- 3.2.1.2 Comparative Overview.- 3.2.1.3 Magnitude.- 3.2.1.4 Consequence of Its Loss.- 3.2.2 Central Mechanisms.- 3.2.2.1 Hyperventilatory Responsiveness.- 3.2.2.2 Some Unsolved Problems.- 3.2.2.3 A Speculative Overview.- 3.3 Efficiency.- 3.3.1 At Rest.- 3.3.2 During Exercise.- 3.3.3 During Sleep.- 3.3.4 During Pregnancy.- 3.4 Concluding Remarks.- 4. Circulatory Adaptations.- 4.1 An Overview.- 4.2 Blood O2 Capacitance.- 4.2.1 Hemoglobin Concentration.- 4.2.1.1 Major Features.- 4.2.1.2 Efficiency.- 4.2.1.3 Mechanism.- 4.2.1.4 Hyperexis.- 4.2.2 Blood O2 Affinity.- 4.2.2.1 Basic Considerations.- 4.2.2.2 Theoretical Studies.- 4.2.2.3 Experimental Data.- 4.2.2.4 Underlying Mechanisms.- 4.3 Blood Flow.- 4.3.1 Cardiac Output.- 4.3.1.1 Resting Conditions.- 4.3.1.2 Exercise.- 4.3.2 Redistribution of Blood Flow.- 4.3.2.1 Skin.- 4.3.2.2 Renal and Splanchnic Areas.- 4.3.2.3 Skeletal Muscle.- 4.3.2.4 Myocardium.- 4.3.2.5 Brain.- 4.3.2.6 Pulmonary and Branchial Circulation.- 4.4 Summary.- 5. Diffusive Processes.- 5.1 Basic Considerations.- 5.2 Diffusion Across the Eggshell.- 5.2.1 Effects of Hypobaria.- 5.2.2 Sea-Level Eggs at High Altitude.- 5.2.3 Eggs from Birds Breeding at High Altitude..- 5.3 Complex Gas/Blood O2 Transfer.- 5.3.1 Alveolar-Arterial Difference in $$ {\text{P}}{{\text{O}}{\text{2}} } $$.- 5.3.2 Physiological Shunt.- 5.3.2.1 Venous Admixture to Arterial Blood.- 5.3.2.2 Ventilation-Perfusion Inequality.- 5.3.3 Diffusing Capacity.- 5.3.3.1 Methodological Approach.- 5.3.3.2 Acute and Prolonged Hypoxia.- 5.3.3.3 Chronic Hypoxia.- 5.3.3.4 Morphological Studies.- 5.3.3.5 Indirect Evidence.- 5.3.4 Comparative Data.- 5.4 Maternal-Fetal O2 Transfer.- 5.5 Tissue O2 Diffusion.- 5.5.1 Tissue Capillarity.- 5.5.2 Myoglobin.- 5.6 Summary.- 6. Biochemical Changes.- 6.1 Oxygen Utilization in Cells.- 6.2 Bioenergetic Adaptations.- 6.2.1 Introductory Remarks.- 6.2.2 A Typical Experiment.- 6.2.3 Respiratory Chain.- 6.2.3.1 Critical Oxygen Tension.- 6.2.3.2 Hypoxic Changes.- 6.2.4 Krebs Cycle.- 6.2.5 Glycolysis.- 6.3 Nonbioenergetic Adaptations.- 6.3.1 Neurotransmitters.- 6.3.1.1 Monoamines.- 6.3.1.2 Acetylcholine.- 6.3.2 Vasoactive Metabolites.- 6.3.2.1 Adenosine.- 6.3.2.2 Prostaglandins.- 6.3.3 Nucleic Acids.- 6.4 Concluding Remarks.- References.