| Acclimatization |
Reversible physiological changes (ventilation, [Hb], mitochondrial density) that occur within an individual over days to weeks of altitude exposure. |
| Accessory muscles of ventilation |
Muscles (sternocleidomastoid, scalenes, trapezius, pectoralis) that assist breathing during exercise or respiratory distress but are largely inactive during quiet breathing. |
| Acute hypoxia |
A short-duration episode of reduced O2 availability — in diving, caused by breath-holding rather than by reduced inspired PO₂. |
| Aerobic metabolism (oxidative phosphorylation) |
Metabolic pathway that fully oxidizes glucose using O2 in the mitochondria; theoretical yield 36 ATP per glucose, actual ≈ 29–32 ATP. |
| Aerobic scope |
The range (difference or ratio) between resting and maximal aerobic metabolic rate; a measure of capacity for sustained activity. Collapses to ≈ 1 at the summit of Everest. |
| Air sacs |
Thin-walled, compliant sacs in the avian respiratory system that act as bellows to move air unidirectionally through the rigid parabronchial lung; do not themselves participate in gas exchange. |
| Airway resistance |
Opposition to airflow through the airways, determined primarily by airway diameter; increased in asthma, COPD, and other obstructive conditions. |
| Alveolar dead space |
Volume of alveoli that are ventilated but not perfused, and therefore do not contribute to gas exchange. |
| Alveolar gas equation |
An equation that estimates alveolar PO₂ from inspired PO₂ and alveolar PCO₂, accounting for CO2 dilution. |
| Alveolar ventilation ($\dot{V}_A$) |
Volume of air per minute that reaches the gas-exchange surfaces; equals breathing frequency × (tidal volume − dead space). |
| Ambient air |
Atmospheric air surrounding the organism, at environmental temperature, humidity, and barometric pressure; the starting point of the oxygen supply cascade. Dry composition ≈ 21% O2, 0.04% CO2, 78% N2, with variable water vapor. |
| Anaerobic glycolysis |
Metabolic pathway that converts glucose to pyruvate and then lactate without O2; yields 2 ATP per glucose. |
| Anatomical dead space |
Volume of the conducting airways (trachea, bronchi, bronchioles) that do not participate in gas exchange; ≈ 150 mL in healthy adults. |
| Anatomical shunt (in divers) |
Functional redirection of blood away from skeletal muscle by vasoconstriction and reduced capillary recruitment; preserves central-organ perfusion during a dive. |
| Arterial blood gas (ABG) |
A clinical test that directly measures PO₂, PCO₂, and pH in arterial blood. |
| Archosaurs |
Vertebrate clade including birds, crocodilians, and extinct dinosaurs; many members share respiratory features promoting unidirectional pulmonary airflow. |
| a–v O2 difference |
Difference in O2 content between arterial and mixed venous blood; reflects tissue O2 extraction. Widens from ~5 to ~16 mL O2/dL from rest to maximal exercise. |
| Bar-headed goose (high-flyer) |
Migratory goose that routinely crosses the Himalaya; textbook example of convergent adaptation along the avian oxygen supply cascade (larger lungs, leftward-shifted Hb, higher capillarity). |
| Bar-headed goose hemoglobin |
A hemoglobin variant with higher O2 affinity (leftward-shifted dissociation curve) than lowland goose Hb, enabling near-full saturation at low PO₂. |
| Barometric pressure (PB) |
Pressure exerted by the atmosphere at a given point; 760 mmHg at sea level. |
| Blood flow redistribution |
Shift of blood flow from non-essential organs (gut, kidney) to active skeletal muscles during exercise, mediated by sympathetic vasoconstriction and local metabolic vasodilation. |
| Blood-gas barrier |
Thin tissue separating alveolar air from capillary blood (~0.5 μm in mammals; thinner in birds); composed of alveolar epithelium, fused basement membranes, and capillary endothelium. |
| Bohr effect |
Rightward shift of the O2–Hb dissociation curve caused by decreased pH (or increased CO2); promotes O2 unloading at active tissues. |
| Breath-hold diving |
Diving without an external air supply; the diver depends entirely on stored O2 for the duration of the dive. |
| Burst locomotion |
A locomotor strategy of short, high-intensity sprints rather than sustained aerobic activity; typical of lizards and other sprawling tetrapods. |
| Capacitance coefficient ($\beta_{gO_2}$) |
Constant describing the amount of O2 that can be carried per unit volume of medium per unit partial-pressure difference. |
| Cardiac output ($\dot{Q}$) |
Volume of blood pumped per minute by one ventricle; equal to heart rate × stroke volume (L/min). |
| Cardiac shunt |
Mixing of oxygenated and deoxygenated blood in animals with incompletely divided ventricles; reduces arterial O2 saturation at rest but can be adjusted during exercise. |
| Cardiovascular drift |
Progressive rise in heart rate and fall in stroke volume during prolonged exercise with dehydration; cardiac output is maintained. |
| Central chemoreceptors |
Neurons on the ventral medullary surface that detect changes in CSF pH caused by CO2 diffusing across the blood–brain barrier; primary drivers of resting ventilation. |
| Chronic mountain sickness |
A condition affecting long-term altitude residents, caused by chronic hypoxia and marked by excessive red cell production and cardiovascular complications. |
| Clinical alveolar ventilation equation |
An equation that estimates alveolar PCO₂ from CO2 production and alveolar ventilation, using $K = 863$ mmHg. |
| Comparative approach |
A research framework that studies physiological and biomechanical principles across multiple species to uncover fundamental mechanisms and evolutionary adaptations. |
| Conducting zone |
Airway generations 0–16; transport air but do not participate in gas exchange. |
| Convection |
Bulk flow of a fluid (air or blood) that transports gases over large distances; the mechanism of ventilation and circulatory transport. |
| Convergent evolution |
Independent evolution of similar features in unrelated lineages — e.g., bipedal locomotion in humans and birds, or four-chambered hearts in mammals and birds. |
| Cost of transport (CoT) |
Energy required to move a unit of body mass over a unit of distance (J/kg·m); used to compare locomotor efficiency across species and gaits. |
| Countercurrent gas exchange |
Arrangement in which blood and the respiratory medium flow in opposite directions (fish gills), maintaining a diffusion gradient along the entire exchange surface. |
| Crosscurrent gas exchange |
Avian lung arrangement in which air flows through parabronchi perpendicular to capillary blood flow; intermediate in efficiency between countercurrent (fish) and tidal-pool (mammalian) exchange. |
| Dalton’s law |
The total pressure of a gas mixture equals the sum of the partial pressures of its component gases. |
| Dead space ventilation |
Ventilation of lung regions with no blood flow (V/Q → ∞); wastes ventilatory effort because no gas exchange occurs. |
| Diaphragm |
Dome-shaped muscle unique to mammals that separates the thoracic and abdominal cavities and is the primary muscle of inspiration; enables ventilation independent of trunk locomotion. |
| Diffusion |
Passive movement of gas molecules from regions of high partial pressure to low partial pressure across a membrane. |
| Diffusion capacity of the lungs (DLO2) |
Composite measure combining surface area, membrane thickness, and gas-specific diffusion coefficient; determines structural capacity for gas transfer. |
| Diffusion coefficient (D) |
Gas-specific constant describing rate of diffusion through a given medium; CO2 diffuses ~20× faster than O2 in tissue. |
| Excess post-exercise oxygen consumption (EPOC) |
Elevated $\dot{V}O_2$ after exercise ends, reflecting PCr resynthesis, O2-store replenishment, thermoregulation, and gluconeogenesis. |
| Exercise-induced arterial hypoxemia (EIAH) |
Decrease in arterial O2 saturation during high-intensity exercise, observed in 40–50% of elite athletes; indicates a pulmonary (often V/Q) limitation on O2 uptake. |
| Exercise physiology |
Study of the physiological mechanisms that govern movement and responses to physical activity, including respiratory, cardiovascular, and musculoskeletal systems. |
| Exercise pressor reflex |
Reflex increase in cardiovascular drive triggered by muscle chemoreceptors and mechanoreceptors detecting metabolic byproducts and mechanical activity. |
| Factorial aerobic scope (fAS) |
Ratio of $\dot{V}O_2$max to SMR (or BMR); typically 5–10× in vertebrates, > 50× in flying birds. |
| Fick principle |
Conservation-of-mass statement equating O2 uptake to the product of a flow rate (ventilation or cardiac output) and a concentration (or partial-pressure) difference across that step. |
| Fick’s law of diffusion |
A physical law stating that gas flux across a barrier is proportional to area and partial-pressure gradient and inversely proportional to thickness. |
| FiO2 |
Fractional concentration of inspired O2. Normoxia ≈ 0.21; Meir’s bar-headed goose wind-tunnel experiments used 0.105 (~5,500 m) and 0.07 (severe hypoxia). |
| Frank–Starling mechanism |
Intrinsic cardiac property where greater venous return stretches the ventricle during diastole, producing a stronger contraction and higher stroke volume. |
| Gluconeogenesis |
Metabolic conversion of lactate (and other substrates) back to glucose, primarily in the liver; contributes to the slow component of EPOC. |
| Graphical Fick solution |
Visualization of total O2 delivery as a rectangle where width = $\dot{Q}$, height = a–v O2 difference, and area = $\dot{V}O_2$. |
| High-altitude native |
An organism whose lineage has lived at high altitude for many generations and exhibits heritable genetic and developmental adaptations to chronic hypoxia, distinct from the within-lifetime acclimatization responses of lowland-born individuals. |
| Hypaxial muscles |
Muscles of the body wall below the vertebral column (including intercostals and abdominals) used for both ventilation and trunk stabilization during locomotion. |
| Hyperventilation |
Breathing in excess of metabolic demand; lowers PACO2 and raises PAO2, producing respiratory alkalosis. |
| Hypocapnia |
Low arterial PCO₂, a consequence of hyperventilation; drives respiratory alkalosis and altered cerebral blood flow at altitude. |
| Hypoventilation |
Breathing below metabolic demand; raises PACO2, lowers PAO2, and produces respiratory acidosis. |
| Hypoxia |
A condition of reduced O2 availability — at altitude, caused by reduced barometric pressure; in diving, caused by breath-holding. |
| Hypoxic pulmonary vasoconstriction |
Pulmonary arteriolar constriction in response to low alveolar PO₂; normally matches perfusion to ventilation but at altitude can cause high-altitude pulmonary edema (HAPE). |
| Hypoxic threshold |
Arterial PO₂ (~60 mmHg) below which the ventilatory response to hypoxia becomes steeply exponential. |
| Hypoxic ventilatory response (HVR) |
Reflexive increase in ventilation as arterial PO₂ falls below the hypoxic threshold; raises alveolar PO₂ by lowering alveolar PCO₂. |
| Ideal gas law |
The equation $PV = nRT$ relating pressure, volume, moles of gas, and temperature through the universal gas constant R. |
| Incremental exercise test |
Standardized protocol for measuring $\dot{V}O_2$max in which work rate is progressively increased until exhaustion or $\dot{V}O_2$ plateaus. |
| Inspired PO₂ ($P_IO_2$) |
Partial pressure of O2 in inspired air after warming and humidification; falls curvilinearly with altitude as PB decreases. |
| Krogh Principle |
The principle that for any given physiological problem there exists an ideal animal species on which it can be most conveniently studied (Krogh, 1929). |
| Lateral body undulation |
A locomotor pattern in which the trunk bends side-to-side during movement, inherited from aquatic ancestors and retained in modern lizards and salamanders. |
| Mass-specific cost of transport |
Metabolic energy required to move a unit of body mass over a unit of distance, expressed as O2 consumption per kilogram per kilometer (mL O2/kg/km) or in energetic units (J/kg·m); used to compare running economy across individuals and species. Lower values indicate greater locomotor efficiency. |
| Mass-specific O2 stores |
Total body O2 store per unit body mass, partitioned into lung, blood, and muscle compartments; specialized divers have much higher blood and muscle fractions than humans. |
| Mass-specific VO2 |
$\dot{V}O_2$ normalized to body mass (mL/kg/min), used for comparing metabolic rates across individuals and species. |
| Mechanical constraint (on ventilation) |
Physical limitation arising from dual use of trunk muscles for both breathing and locomotion in sprawling tetrapods, limiting simultaneous aerobic capacity. |
| Metabolic vasodilation |
Local dilation of arterioles in active muscles caused by CO2, H+, adenosine, NO, and K+; overrides sympathetic vasoconstriction in working muscles. |
| Minute ventilation ($\dot{V}_E$) |
Total volume of air moved in and out of the lungs per minute; also called expired pulmonary ventilation. |
| Mitochondrial coupling efficiency |
ATP yield per O2 consumed in oxidative phosphorylation; higher coupling efficiency supports aerobic energy supply with lower mitochondrial volume. |
| Mixed venous blood |
Blood sampled from the pulmonary artery, where blood from all regional drainages has mixed; represents whole-body average venous O2 content. |
| Mixed venous PO₂ |
PO₂ of blood returning to the right heart; lower values indicate a wider a–v O2 difference and greater tissue extraction. |
| Muscle chemoreceptors |
Sensory receptors in skeletal muscle that detect local metabolites (H+, K+); fine-tune ventilation during steady-state submaximal exercise. |
| Muscle mechanoreceptors |
Sensory receptors in skeletal muscle (muscle spindles, Golgi tendon organs) that sense mechanical exertion and drive ventilation in proportion to exercise intensity. |
| Myoglobin (Mb) |
Intracellular muscle protein with hyperbolic, high-affinity O2 binding; shuttles O2 from capillary to mitochondrion and buffers muscle O2 demand at exercise onset. |
| Myoglobin net surface charge (ZMb) |
Total electrostatic charge on the myoglobin surface; high values prevent aggregation at the extreme [Mb] found in diving mammals; evolved independently in cetaceans and pinnipeds. |
| Oxygen cascade / Oxygen supply cascade |
Series of alternating convection and diffusion steps through which O2 travels from the atmosphere to the mitochondria, with PO₂ decreasing at each stage. |
| Oxygen deficit |
Difference between total energy demand at exercise onset and the aerobically supplied energy during the period before $\dot{V}O_2$ reaches steady state. |
| Oxygen–hemoglobin dissociation curve |
Sigmoid curve describing the relationship between PO₂ and Hb saturation; determines O2 loading in the lung and unloading at tissues. |
| Parabronchi |
Rigid gas-exchange tubes in the avian lung through which air flows unidirectionally; sites of crosscurrent O2 exchange with capillary blood. |
| Parabronchial lung |
The avian lung structure in which air flows unidirectionally through rigid parabronchi, forming a crosscurrent exchange system with essentially no dead space. |
| Partial pressure |
The pressure exerted by a single gas within a mixture; equals total pressure × fractional concentration. |
| Partial-pressure gradient ($\Delta P_{O_2}$) |
Difference in PO₂ between alveolus and capillary blood; driving force for O2 diffusion into the blood. |
| Peripheral chemoreceptors |
Carotid and aortic bodies that detect arterial PO₂, PCO₂, pH, and K+; carotid bodies are the primary peripheral O2 sensors. |
| Phosphocreatine (PCr) pathway |
Fastest ATP-regeneration pathway; uses PCr stored in muscle to immediately regenerate ATP from ADP. |
| Physiological dead space |
Total non-functional ventilatory volume: anatomical dead space plus alveolar dead space. |
| Pneumotachometer |
Instrument that measures the flow rate of gas during breathing; essential for metabolic rate measurement. |
| Pulse oximetry (SpO2) |
Non-invasive method for monitoring oxygen saturation of hemoglobin in peripheral blood. |
| R (relaxed) state |
Quaternary conformation of hemoglobin with higher O2 affinity; favored in the lung where PO₂ is high. |
| Respiratory exchange ratio (R) |
Ratio of CO2 production to O2 consumption ($\dot{V}CO_2 / \dot{V}O_2$); typically 0.7 (fat) to 1.0 (carbohydrate), ≈ 0.8 default. |
| Respiratory zone |
Airway generations 17–23 containing alveoli where gas exchange occurs by diffusion. |
| Right-to-left cardiac shunt |
Flow of deoxygenated blood from the right heart directly into the systemic circulation without passing through the lungs; present in amphibians and non-crocodilian reptiles with incompletely divided ventricles. |
| Sarcopterygian fish |
Lobe-finned fish; the common aquatic ancestor of all tetrapods, which used paired fins and lateral body undulation for locomotion. |
| Shunt |
Blood flow that bypasses the gas-exchange surface without becoming oxygenated; results from V/Q < 1.0 or, in non-mammalian tetrapods, from incomplete ventricular division. |
| Splenic O2 reservoir |
Store of red blood cells in the spleen, mobilized at dive onset by vasoconstriction; ~60% of RBC mass is stored splenically in Weddell seals (spleens up to ~7% of body mass). |
| Spirometry |
A clinical and research technique for measuring lung volumes and airflow rates by analyzing exhaled air. |
| Sprawling posture |
Limb configuration in which the legs extend laterally from the body (as in lizards), as opposed to the erect posture of mammals and birds. |
| Standard metabolic rate (SMR) |
Resting metabolic rate of an ectotherm measured at a standard ambient temperature; analogous to BMR in endotherms. |
| Stroke volume (SV) |
Volume of blood ejected by one ventricle per heartbeat (mL/beat); increases with training-induced cardiac hypertrophy. |
| Surfactant |
A substance produced by alveolar cells that reduces surface tension, preventing alveolar collapse and preserving surface area for gas exchange. |
| Sympathetic vasoconstriction |
α-adrenergic narrowing of arterioles in non-essential organs during exercise, diverting blood flow to active muscles. |
| Systemic peripheral vasoconstriction (dive response) |
Reflex constriction of arterioles supplying skeletal muscle and non-essential organs at dive onset; preserves O2 delivery to brain and heart. |
| T (tense) state |
Quaternary conformation of hemoglobin with lower O2 affinity; stabilized by H+, CO2, 2,3-BPG, and elevated temperature. |
| Temperature effect (on Hb curve) |
Rightward shift of the O2–Hb dissociation curve at higher temperature; enhances O2 release in exercising, heat-producing muscle. |
| Tetrapod |
A vertebrate animal with four limbs (or descended from four-limbed ancestors), including amphibians, reptiles, birds, and mammals. |
| Tidal-pool exchange |
Mammalian lung arrangement in which alveolar gas does not flow directionally relative to blood, so the O2 gradient equilibrates quickly. |
| Tidal ventilation |
Bidirectional airflow in which the same airways carry air in and out of dead-end alveoli; the mammalian pattern (also amphibians and non-crocodilian reptiles). |
| Tidal volume (VT) |
Volume of air inhaled or exhaled in a single breath during normal breathing. |
| Torr |
Unit of pressure equal to 1 mmHg; 760 Torr = 1 atmosphere. |
| Two-cycle breathing |
Avian breathing pattern in which a single bolus of air requires two full respiratory cycles to pass through: posterior air sacs → lung → anterior air sacs → out. |
| Unidirectional airflow |
Respiratory pattern in which air moves in one direction through the gas-exchange surfaces, regardless of inhalation or exhalation phase; found in birds, crocodilians, and some lizards. |
| Ventilation–perfusion ratio (V/Q) |
Ratio of alveolar ventilation to pulmonary blood flow; V/Q = 1.0 is ideal, V/Q < 1.0 = shunt, V/Q > 1.0 = dead space. |
| Ventilatory threshold |
Exercise intensity (≈ 50–70 % $\dot{V}O_2$max) at which ventilation begins to rise exponentially rather than linearly with work rate. |
| V/Q heterogeneity |
Variation in V/Q ratio across lung regions; greater heterogeneity reduces overall gas-exchange efficiency and contributes to EIAH. |
| Vital capacity |
Maximum volume of air that can be exhaled after a maximum inhalation; equals IRV + VT + ERV. |
| $\dot{V}O_2$ |
Volume rate of oxygen consumption; the standard measure of metabolic rate, typically expressed in mL O2/min or mL/kg/min. |
| $\dot{V}O_2$max |
Maximum rate of oxygen consumption attainable during incremental exercise; represents the upper limit of aerobic metabolism. |
| Water vapor pressure (PH₂O) |
Partial pressure exerted by water vapor in a gas mixture; at body temperature (37 °C), saturated PH₂O = 47 mmHg. |
| Weddell seal |
Model species for diving physiology; long-duration diver with the largest spleen as a fraction of body mass of any mammal (~7%). |