The ventilatory response to environmental hypercarbia in the South American rattlesnake,<Emphasis Type="Italic"> Crotalus durissus</Emphasis>

To study the effects of environmental hypercarbia on ventilation in snakes, particularly the anomalous hyperpnea that is seen when CO₂ is removed from inspired gas mixtures (post-hypercapnic hyperpnea), gas mixtures of varying concentrations of CO₂ were administered to South American rattlesnakes, Crotalus durissus, breathing through an intact respiratory system or via a tracheal cannula by-passing the upper airways. Exposure to environmental hypercarbia at increasing levels, up to 7% CO₂, produced a progressive decrease in breathing frequency and increase in tidal volume. The net result was that total ventilation increased modestly, up to 5% CO₂ and then declined slightly on 7% CO₂. On return to breathing air there was an immediate but transient increase in breathing frequency and a further increase in tidal volume that produced a marked overshoot in ventilation. The magnitude of this post-hypercapnic hyperpnea was proportional to the level of previously inspired CO₂. Administration of CO₂ to the lungs alone produced effects that were identical to administration to both lungs and upper airways and this effect was removed by vagotomy. Administration of CO₂ to the upper airways alone was without effect. Systemic injection of boluses of CO₂-rich blood produced an immediate increase in both breathing frequency and tidal volume. These data indicate that the post-hypercapnic hyperpnea resulted from the removal of inhibitory inputs from pulmonary receptors and suggest that while the ventilatory response to environmental hypercarbia in this species is a result of conflicting inputs from different receptor groups, this does not include input from upper airway receptors.Communicated by G. Heldmaier     

Sensitivity to hypercapnia and elimination of CO2 following diving in Steller sea lions (Eumetopias jubatus)

The diving ability of marine mammals is a function of how they use and store oxygen and the physiological control of ventilation, which is in turn dependent on the accumulation of CO₂. To assess the influence of CO₂ on physiological control of dive behaviour, we tested how increasing levels of inspired CO₂ (hypercarbia) and decreasing inspired O₂ (hypoxia) affected the diving metabolic rate, submergence times, and dive recovery times (time to replenish O₂ stores and eliminate CO₂) of freely diving Steller sea lions. We also measured changes in breathing frequency of diving and non-diving individuals. Our findings show that hypercarbia increased breathing frequency (as low as 2 % CO₂), but did not affect metabolic rate, or the duration of dives or surface intervals (up to 3 % CO₂). Changes in breathing rates indicated respiratory drive was altered by hypercarbia at rest, but blood CO₂ levels remained below the threshold that would alter normal dive behaviour. It took the sea lions longer to remove accumulated CO₂ than it did for them to replenish their O₂ stores following dives (whether breathing ambient air, hypercarbia, or hypoxia). This difference between O₂ and CO₂ recovery times grew with increasing dive durations, increasing hypercarbia, and was greater for bout dives, suggesting there could be a build-up of CO₂ load with repeated dives. Although we saw no evidence of CO₂ limiting dive behaviour, the longer time required to remove CO₂ may eventually exhibit control over the overall time they can spend in apnoea and overall foraging duration.     

Effect of Mechanical Loading on Ventilatory Response to CO2 and CO2 Excretion

The ventilatory response to CO₂ (S) and respiratory exchange ratio have been measured in 10 healthy subjects breathing naturally and through added resistive loads. The changes in these values produced by the added loads were shown to be correlated with the unloaded CO₂ responsiveness. The results indicated that poorly responsive individuals had a greater depression of ventilatory response to CO₂ and were more liable to retain CO₂.These observations raise the possibility that the constitutional CO₂ responsiveness of an individual influences the alveolar ventilation achieved in the presence of airways obstruction. The propensity to develop respiratory failure may thus be conditioned by the premorbid CO₂ responsiveness.     

Ventilatory response of the diamondback water snake,Natrix rhombifera to hypoxia, hypercapnia and increased oxygen demand

Summary Simultaneous measurements of ventilatory frequency, tidal volume, O₂ uptake, CO₂ output and cardiac frequency were made in the diamondback water snake,Natrix rhombifera while breathing hypoxic (15% to 5% O₂ in N₂) or hypercarbic (2% to 10% CO₂ and 21% O₂ in N₂) gases. The snakes responded to hypoxia by increasing tidal volume and decreasing ventilatory frequency resulting in little change in ventilation (50% increase at 5% inspired O₂), or O₂ uptake and only a light increase in CO₂ output. Hypercarbia to 4.2% inspired CO₂ resulted in a slight hyperventilation but ventilation was depressed at 6.3% inspired CO₂ and became erratic at higher concentrations. The resting rate of O₂ uptake was maintained throughout hypercapnia. Heart rate increased during hypoxia and decreased during hypercapnia. Cutaneous O₂ uptake increased during extreme hypoxia (5% inspired O₂) and cutaneous CO₂ output increased during hypercapnia, probably due to changes in the body-to-ambient gas gradients (Crawford and Schultetus, 1970). Both pulmonary oxygen uptake and ventilation were dramatically increased immediately following 10–15 min experimental dives. The increased ventilation was achieved primarily through an increased tidal volume.     

Cardiorespiratory reflexes and aquatic surface respiration in the neotropical fish tambaqui (<Emphasis Type="Italic">Colossoma macropomum</Emphasis>): acute responses to hypercarbia

We examined the cardiorespiratory responses to 6 h of acute hypercarbia (1, 2.5, and 5% CO₂) in intact and gill-denervated (bilateral denervation of branchial branches of cranial nerves IX and X) tambaqui, Colossoma macropomum. Intact fish exposed to 1 and 2.5% CO₂ increased respiratory frequency (f_R) and ventilation amplitude (V_AMP) slowly over a 1- to 3-h period. Denervated fish did not show this response, suggesting that tambaqui possess receptors in the gills that will produce excitatory responses to low levels of hypercarbia (1 and 2.5% CO₂) if the exposure is prolonged. The cardiac response to stimulation of these receptors with this level of CO₂ was a tachycardia and not a bradycardia. During exposure to 5% CO₂, intact fish increased f_R and V_AMP, and showed a pronounced bradycardia after 1 h. After 2 h, the heart rate (f_H) started to increase, but returned to control values after 6 h. In denervated fish, the increase in f_R was abolished. The slow increase in V_AMP and the bradycardia were not abolished, suggesting that these changes arose from extra-branchial receptors. Neither intact nor denervated fish developed the swelling of the lower lip or performed aquatic surface respiration, even after 6 h, suggesting that these are unique responses to hypoxia and not hypercarbia.Abbreviations ASR aquatic surface respiration - f_H heart frequency - f_R respiratory frequency - V_AMP ventilation amplitude - _TOT total ventilation     

Voluntary induced alterations in regional ventilation in normal humans

We tested the hypothesis that voluntary changes of thoraco-abdominal shape can influence regional ventilation via altering regional pleural pressure swings (Ppl). Regional ventilation was measured simultaneously with regional Ppl during tidal volume breathing maneuvers in five normal subjects while they were performing one of three thoracoabdominal patterns of breathing: normal, preferential intercostal (IC), or preferential diaphragmatic (DIA). In every subject, the lower lung region's 133Xe washout rate was faster than the upper region's, regardless of the pattern of thoracoabdominal breathing adopted. Although IC breathing tended to make regional ventilation more homogeneous, DIA breathing tended to augment regional ventilation inhomogenities. On the average, the Ppl values were greatest in the lower lung region, regardless of the thoracoabdominal pattern adopted; however, IC breathing reduced and DIA breathing increased regional Ppl inhomogenities. When the ratios of the Ppl (lower/upper) were plotted vs. the ratios of the regional 133Xe washout decay constants (lower/upper), a significant positive correlation was found. These data suggest that a causal relation between regional tidal Ppl and regional ventilation exist, thus supporting the concept that thoracoabdominal shape changes can influence regional ventilation.     

Effect of upper airway carbon dioxide on ventilation and blood gases in the awake pony

Carbon dioxide concentrations were increased during expiration in the upper one-half of the trachea, pharynx, and nasal sinuses to determine if elevation of upper airway CO2 would alter breathing or arterial blood gases in the awake pony. Carbon dioxide (100%) was injected into the midcervical trachea via a chronically implanted transcutaneous cannula during the first part of the animal's expiration. This maneuver elevated upper airway expiratory CO2 concentrations but prevented any exogenous CO2 from entering the lung and being absorbed into the arterial blood. Twelve experiments were performed on six ponies in which upper airway CO2 was elevated 2, 4, and 6% above the normal expired CO2 concentrations. Tidal volume increased in a dose dependent manner during upper airway CO2 exposure, but total ventilation was unchanged from base-line measurements made while the animal breathed room air. Arterial Po2 also increased during upper airway CO2 administration, reaching a mean value 6 Torr (1 Torr = 133.322 Pa) greater than the base-line values at the +6% CO2 exposure. We conclude that upper airway CO2 exposure alters breathing pattern slightly (increases tidal volume) and increases arterial PO2 in the awake pony.     

Ventilatory responses of the burrowing Owl and Bobwhite to hypercarbia and hypoxia

Summary The fractional concentration of CO₂ within the occupied burrows of burrow-dwelling birds has been measured at 0.065 or higher while the fractional concentration of O₂ can be 0.14 or lower. The ventilatory responses of the Burrowing Owl (Athene cunicularia) and a non-burrowing bird, the Bobwhite (Colinus virginianus), to the levels of hypercarbia, hypoxia and hypoxic hypercarbia likely encountered by this owl within its burrow were assessed. Ventilatory responses toFi _{CO 2} of 0.01, 0.03, 0.05 and 0.07 at normoxia; toFi _{CO 2} of 0.19, 0.17, 0.15 and 0.13 at normocarbia; and to combinedFi _{CO 2} andFi _{O 2} of 0.01:0.19, 0.03:0.17, 0.05:0.15 and 0.07:0.13 were measured by volumetric plethysmography. The Burrowing Owl exhibited a significantly attenuated ventilatory response to these levels of hypercarbia, hypoxia and hypoxic hypercarbia compared to the non-burrow-dwelling Bobwhite. A reduced ventilatory response to the hypoxic and hypercarbic stimuli has been previously observed in fossorial mammals, and is reported here for the first time in a burrow-dwelling bird. This reduced response is believed to represent an adaptation to burrow atmospheric conditions.Abbreviations F fractional concentration (of O₂ and CO₂) - f respiratory frequency - V _T tidal volume - V _D dead space (tracheal volume)     

Cyclic model of respiration applied to asymmetrical ventilation and periodic breathing

A model taking into account the cyclic character of respiration in humans is developed using two classical simplifications: CO₂ is the only respiratory gas involved; and respiration is regulated only by a CO₂ linear controller. The model is used to investigate two important clinical aspects of respiratory disease: asymmetrical ventilation and periodic breathing. We show that asymmetry in ventilation significantly influences the time course of the CO₂ partial pressure in the expired alveolar air at the mouth and the elimination of CO₂ through the lungs. Furthermore, the CO₂ controller delay plays a major role in periodic breathing.     

Respiratory evaporation in panting fowl: Partition between the respiratory and buccopharyngeal pumps

Summary Ventilation (V) and respiratory water loss were measured in domestic fowlGallus gallus subjected to raised environmental temperatures (33±2°C) and breathing air, 8% O₂ in N₂, 3% CO₂ in air or 5% CO₂ in air. Birds breathing air underwent an 18.6-fold increase in respiratory frequency and a 5-fold reduction in tidal volume and panting was accompanied by vigorous gular flutter. Hypoxic and hypercapnic birds breathed more slowly and deeply and gular flutter was strongly inhibited. The ratio was similar to that predicted on the basis of the measured ventilation assuming saturation of expired gas at measured gular mucosal temperature in hypoxic and hypercapnic birds but 54% greater than the predicted value in birds panting in air. It is concluded that the excess water loss during normal panting results from tidal airflow generated independently by the buccopharyngeal pump and that buccopharyngeal ventilation is equivalent to 54% of the respiratory ventilation.     

CO2‐metabolism in early tetrapods revisited: inferences from osteological correlates of gills,skin and lung ventilation in the fossil record

One of the most important physiological changes during the conquest of land by vertebrates was the increasing reliance on lung breathing, with the concomitant decrease in importance of gill breathing. The main problem involved here was to cope with the excessive accumulation of CO₂ in the body and to avoid respiratory acidosis. In the past, several often mutually contradicting hypotheses of CO₂‐elimination via skin, lungs and gills in early tetrapods have been proposed, based on theoretical physiological considerations and comparison with extant air‐breathing fishes and amphibians. This study proposes a revised scenario of CO₂‐elimination in early tetrapods based on fossil evidence, that is recently identified osteological correlates of gills, skin structure and mode of lung ventilation. In stem tetrapods of the Devonian and Carboniferous, O₂‐uptake via the lungs by buccal pumping was decoupled from CO₂‐release via internal gills, and the rather gas‐impermeable skin played a minor role in gaseous exchange. The two main lineages of crown‐group tetrapods, the amphibian and amniote lineage, used different strategies of CO₂‐elimination. As in stem tetrapods, O₂‐uptake and CO₂‐release remained always largely decoupled in temnospondyls, which ventilated their lungs via buccal pumping and relied mainly on their internal gills for CO₂‐release. Temnospondyls were not able to reduce their internal gills before their skin became more gas permeable and their body size was reduced, to shift from internal gills to the skin as the major site of CO₂‐elimination, a pattern that is retained in most lissamphibians. In contrast, internal gills were lost very early in stem amniote evolution. This was associated with the evolution of the more effective aspiration pump that allowed the elimination of the bulk of CO₂ via the lungs, leading to a coupled O₂‐uptake and CO₂‐loss in stem amniotes and later in amniotes.     

Ventilatory responses to temperature variation in the fresh water turtle,Mauremys caspica leprosa

A study of lung gas exchange in the fresh water turtle Mauremys caspica leprosa at normal physiological body temperatures (15, 25 and 35 °C) was extended to extreme temperatures (5 and 40 °C) to determine whether the direct relationship between body temperature and ventilatory response found in many lung-breathing ectotherms including other chelonian species was maintained. From 5 to 35 °C the lung ventilation per unit of O₂ uptake and CO₂ removed declined with temperature. Consequently, lung CO₂ partial pressure increased with temperature. Its value was maintained within narrow limits at each thermal constant, suggesting a suitable control throughout the complete ventilatory cycle. At 40 °C the ventilatory response showed the opposite trend. The ratios of ventilation to lung gas exchange increased compared to their values at 35 °C. The impact of this increased breathing-lowering the estimated mean alveolar CO₂ partial pressure-was nevertheless less than expected due to an increase in calculated physiological dead space. This suggests that the relative hyperventilation in response to hyperthermia found in Mauremys caspica leprosa is related to evaporative heat loss.Abbreviations BTPS body temperature, ambient pressure, saturated with water vapour - CTM critical thermal maximum - F_N2 fractional concentration of nitrogen - PA _CO2or PL _CO2 alveolar or lung CO₂ pressure - PA_O2or PL_O2 alveolar or lung O₂ pressure - PI_O2 inspired O₂ pressure - R respiratory exchange ratio - STPD standard temperature, standard pressure, dry - T _a ambient temperature - T _b body temperature - VA alveolar ventilation - VA/VCO₂ relative alveolar ventilation (alveolar ventilation per unit of CO₂ removed) - VO₂ O₂ uptake - VCO₂ CO₂ output - V _D anatomical dead space volume - V _D physiological dead space volume - VE/VO₂ ventilatory equivalent for O₂ - VE pulmonary ventilation or expiratory minute volume - VE/VCO₂ ventilatory equivalent for CO₂ - V _T tidal volume     

Parasympathetic Stimuli on Bronchial and Cardiovascular Systems in Humans

Background

It is not known whether parasympathetic outflow simultaneously acts on bronchial tone and cardiovascular system waxing and waning both systems in parallel, or, alternatively, whether the regulation is more dependent on local factors and therefore independent on each system. The aim of this study was to evaluate the simultaneous effect of different kinds of stimulations, all associated with parasympathetic activation, on bronchomotor tone and cardiovascular autonomic regulation.

Methods

Respiratory system resistance (Rrs, forced oscillation technique) and cardio-vascular activity (heart rate, oxygen saturation, tissue oxygenation index, blood pressure) were assessed in 13 volunteers at baseline and during a series of parasympathetic stimuli: O₂ inhalation, stimulation of the carotid sinus baroreceptors by neck suction, slow breathing, and inhalation of methacholine.

Results

Pure cholinergic stimuli, like O₂ inhalation and baroreceptors stimulation, caused an increase in Rrs and a reduction in heart rate and blood pressure. Slow breathing led to bradycardia and hypotension, without significant changes in Rrs. However slow breathing was associated with deep inhalations, and Rrs evaluated at the baseline lung volumes was significantly increased, suggesting that the large tidal volumes reversed the airways narrowing effect of parasympathetic activation. Finally inhaled methacholine caused marked airway narrowing, while the cardiovascular variables were unaffected, presumably because of the sympathetic activity triggered in response to hypoxemia.

Conclusions

All parasympathetic stimuli affected bronchial tone and moderately affected also the cardiovascular system. However the response differed depending on the nature of the stimulus. Slow breathing was associated with large tidal volumes that reversed the airways narrowing effect of parasympathetic activation.     

Development of dependence on aerial respiration in Polypterus senegalus (Cuvier)

The respiratory behaviour and partitioning of O₂ uptake between air and water were investigated in Polypterus genegalus using continuous-flow and two-phase respirometers and lung gas replacement techniques P. senegalus rarely resorts to aerial respiration under normal conditions. Partitioning of O₂ consumption depends on the activity and age of fish and the availability of aquatic oxygen. Immature fish (12–22 g) cannot utilize aerial O₂ but older fish exhibit age-dependent reliance on aerial respiration in hypoxic and hypercarbic waters. Pulmonary respiration accounts for 50% of the total requirement at aquatic O₂ concentrations of about 3.5 mg · l^–1 (or CO₂ of about 5%) and fish rely exclusively on aerial respiration at O₂ concentrations of less than 2.5 mg · l^–1. Branchial respiration is initially stimulated by hypercarbia (CO₂: 0.5–0.8%) but increased hypercarbia (CO₂ – 1%) greatly depresses (by over 90%) brancial respiration and initiates (CO₂: 0.5%) and sustains pulmonary respiration.     

Changes in oxygen partial pressure of brain tissue in an animal model of obstructive apnea

Background

Prolonged weaning from mechanical ventilation has a major impact on ICU bed occupancy and patient outcome, and has significant cost implications. There is evidence in patients around the period of extubation that helium-oxygen leads to a reduction in the work of breathing. Therefore breathing helium-oxygen during weaning may be a useful adjunct to facilitate weaning. We hypothesised that breathing helium-oxygen would reduce carbon dioxide production during the weaning phase of mechanical ventilation.

Materials/patients and methods

We performed a prospective randomised controlled single blinded cross-over trial on 19 adult intensive care patients without significant airways disease who fulfilled criteria for weaning with CPAP. Patients were randomised to helium-oxygen and air-oxygen delivered during a 2 hour period of CPAP ventilation. Carbon dioxide production (VCO₂) was measured using a near patient main stream infrared carbon dioxide sensor and fixed orifice pneumotachograph.

Results

Compared to air-oxygen, helium-oxygen significantly decreased VCO₂ production at the end of the 2 hour period of CPAP ventilation; there was a mean difference in CO₂ production of 48.9 ml/min (95% CI 18.7-79.2 p = 0.003) between the groups. There were no significant differences in other respiratory and haemodynamic parameters.

Conclusion

This study shows that breathing a helium-oxygen mixture during weaning reduces carbon dioxide production. This physiological study supports the need for a clinical trial of helium-oxygen mixture during the weaning phase of mechanical ventilation with duration of weaning as the primary outcome.

Trial registration

ISRCTN56470948     

Osmoregulation by the broad-snouted caiman, Caiman latirostris, in estuarine habitat in southern Brazil

The broad-snouted caiman Caiman latirostris, of South America mostly frequents freshwater but occurs also in estuaries. Nothing of substance is known of its osmoregulatory physiology but, in the light of accumulating evidence that alligatorids lack specialised adaptations for life in hyperosmotic waters, we anticipated its physiology would be more similar to that of Alligator mississippiensis than the euryhaline Crocodylus porosus, which has both lingual salt glands and a more complex renal:cloacal system. This proved to be the case. Caiman captured in estuaries of the Ilha do Cardoso in southern Brazil were effective hypo-osmotic osmoregulators in salinities of 0–24 ppt (seawater = 35 ppt). Plasma osmolarity, sodium and chloride were similar to those in other crocodilians and not influenced by salinity. Plasma urea was low and did not vary with salinity. We found no evidence of lingual or other salt glands. Urinary electrolyte concentrations varied considerably with salinity and in ways reminiscent of A. mississippiensis but very different from C. porosus. Ca. latirostris dehydrated in seawater more rapidly than C. porosus and had substantially higher integumental permeability to water. Caiman did not drink seawater but rehydrated rapidly when returned to freshwater (FW). We found small caiman (<500 g) only in very low salinities (<3 ppt) and larger caiman closer to the sea. We postulate that medium to large Ca. latirostris can take advantage of the feeding opportunities presented by the estuarine mangal despite lacking the physiological specialisations of crocodylids. Two individuals which we re-sighted by chance had travelled at least 600 m in 2–3 days, showing that every caiman we captured or saw was within easy reach of FW. Most likely their habitation of the estuary and its mangal is achieved through a combination of low surface area:volume ratio, relatively impermeable skin, and periodic access to FW. Accepted: 11 May 1998     

Comparison of responses to CO2 under the conditions of natural respiration and controlled artificial pulmonary ventilation

The relationship between the sensation of lack of air caused by the addition of carbon dioxide to inhaled air and minute pulmonary ventilation was studied in eight healthy male volunteers. Under the conditions of both natural respiration and controlled artificial ventilation of the lungs, ventilation increased in proportion to the increase in the partial pressure of CO₂ in the end-tidal exhaled air. The individual differences in the ventilation responses to CO₂ under the conditions of controlled ventilation and natural ventilation are regarded as a property of voluntary control.     

Chronic hypoxia and chronic hypercapnia differentially regulate an NMDA-sensitive component of the acute hypercapnic ventilatory response in the cane toad (<Emphasis Type="Italic">Rhinella marina</Emphasis>)

This study addressed the hypotheses that exposure to chronic hypoxia (CH) and chronic hypercapnia (CHC) would modify the acute hypercapnic ventilatory response in the cane toad (Rhinella marina; formerly Bufo marinus or Chaunus marinus) and its regulation by NMDA-mediated processes. Cane toads were exposed to 10 days of CH (10% O₂) or CHC (3.5% CO₂) followed by acute in vivo hypercapnic breathing trials, conducted before and after an injection of the NMDA-receptor channel blocker, MK801 into the dorsal lymph sac. CH, CHC and MK801 did not alter ventilation under acute normoxic normocapnic conditions. CH blunted the increase in breathing frequency during acute hypercapnia while CHC had no effect. The effect of CH on breathing frequency was mediated by a decrease in the number of breaths per breathing episode. Neither CH nor CHC altered breath area (volume). MK801 augmented breathing frequency (via an increase in breaths per episode) and total ventilation during acute hypercapnia in control toads and toads exposed to CH; there was no effect of MK801 on the increase in breathing frequency or total ventilation, during acute hypercapnia in toads exposed to CHC. The results indicate that CH and CHC differentially alter breathing pattern. Furthermore, they indicate an absence of NMDA-mediated glutamatergic tone during normoxic normocapnia but that NMDA-mediated processes attenuate the increase in breathing frequency during acute hypercapnia under control conditions and following CH but not following CHC. Given that MK801 was administered systemically, the effects could be acting anywhere in the reflex pathway from CO₂-sensing to respiratory motor output.     

Effect of venous (gut) CO2 loading on intrapulmonary gas fractions and ventilation in the tegu lizard

Summary Studies were conducted to determine regional pulmonary gas concentrations in the tegu lizard lung. Additionally, changes in pulmonary gas concentrations and ventilatory patterns caused by elevating venous levels of CO₂ by gut infusion were measured.It was found that significant stratification of lung gases was present in the tegu and that dynamic fluctuations of CO₂ concentration varied throughout the length of the lung. Mean was greater and less in the posterior regions of the lung. In the posterior regions gas concentrations remained nearly constant, whereas in the anterior regions large swings were observed with each breath. In the most anterior sections of the lung near the bronchi, CO₂ and O₂ concentrations approached atmospheric levels during inspiration and posterior lung levels during expiration.During gut loading of CO₂, the rate of rise of CO₂ during the breathing pause increased. The mean level of CO₂ also increased. Breathing rate and tidal volume increased to produce a doubling ofV _E.These results indicate that the method of introduction of CO₂ into the tegu respiratory system determines the ventilatory response. If the CO₂ is introduced into the venous blood a dramatic increase in ventilation is observed. If the CO₂ is introduced into the inspired air a significant decrease in ventilation is produced. The changes in pulmonary CO₂ environment caused by inspiratory CO₂ loading are different from those caused by venous CO₂ loading. We hypothesize that the differences in pulmonary CO₂ environment caused by either inspiratory CO₂ loading or fluctuations in venous CO₂ concentration act differently on the IPC. The differing response of the IPC to the two methods of CO₂ loading is the cause of the opposite ventilatory response seen during either venous or inspiratory loading.Abbreviations IPC intrapulmonary chemoreceptors - UAC upper airway chemoreceptors - V _T inspiratory tidal volume - CO₂ gas fraction - O₂ gas fraction - V _E minute ventilation     

Ventilatory accommodation of oxygen demand and respiratory water loss in a large bird,the emu (Dromaius novaehollandiae), and a re-examination of ventilatory allometry for birds

Ventilation was studied in the emu, a large flightless bird of mass 40kg, within the range of ambient temperatures from-5 to 45°C. Data for the emu and 21 other species were used to calculate allometric relationships for resting ventilatory parameters in birds (breath frequency=13.5 mass^-0.314; tidal volume=20.7 mass^1.0). At low ambient temperatures the ventilatory system must accommodate the increased metabolic demand for oxygen. In the emu this was achieved by a combination of increased tidal volume and increased oxygen extraction. Data from emus sitting and standing at-5°C, when metabolism is 1.5x and 2.6x basal metabolic rate, respectively, indicate that at least in the emu an increase in oxygen extraction can be stimulated by low temperature independent of oxygen demand. At higher ambient temperatures ventilation was increased to facilitate respiratory water loss. The emu achieved this by increased respiratory frequency. At moderate heat loads (30–35°C) tidal volume fell. This is usually interpreted as a mechanism whereby respiratory water loss can be increased without increasing parabronchial ventilation. At 45°C tidal volume increased; however, past studies have shown that CO₂ washout is minimal under these conditions. The mechanism whereby this is possible is discussed.Abbreviations BMR basal metabolic rate - BTPS body temperature, ambient pressure, saturated - EO ₂ oxygen extraction - EWL evaporative water loss - f _R ventilation frequency - RH relative humidity - RHL respiratory heat loss - SEM standard error of the mean - SNK student-Newman-Keuls multiple range test - STPD standard temperature and pressure, dry - T _a ambient temperatures(s) - T _b body temperature(s) - T _ex expired air temperature(s) - T _rh chamber excurrent air temperature - V _J ventilation - VO₂ oxygen consumption - V _T tidal volume - V/Q air ventilation to blood perfusion ratio