Comparative studies in lung mechanics based on a survey of literature data

Sniffing speeds up chemical detection by controlling air-flows near sensors

Most mammals sniff to detect odors, but little is known how the periodic inhale and exhale that make up a sniff helps to improve odor detection. In this combined experimental and theoretical study, we use fluid mechanics and machine olfaction to rationalize the benefits of sniffing at different rates. We design and build a bellows and sensor system to detect the change in current as a function of odor concentration. A fast sniff enables quick odor recognition, but too fast a sniff makes the amplitude of the signal comparable to noise. A slow sniff increases signal amplitude but delays its transmission. This trade-off may inspire the design of future devices that can actively modulate their sniffing frequency according to different odors.

Breathing frequency in ruminants: a comparative analysis with non-ruminant mammals

In cattle, sheep and goats, during resting conditions, breathing frequency (f) greatly exceeds the value expected for their body weight (W, kg) from the allometric curve of other land mammals. It was postulated that this was the effect of the production of CO2 during rumen fermentation. If this were the case, a high f should be a general characteristic of ruminants. Measurements of f have been obtained in 106 awake and resting animals of 56 species, of which 30 were non-ruminant land mammals, and 26 were ruminants. The ruminant species comprised 15 Bovidae (B), 6 Cervidae (CR), 3 Camelidae (CM), and 2 Giraffidae (G). Additional data of 14 species were available from the literature. The allometric function (f, breaths/min; W, kg) for non-ruminant species was f=53W(-0.251) and not different between males and females. The allometric curve remained unaltered with the inclusion of the 11 species of CR, CM and G, but it was greatly modified by the B species. The significantly higher f in ruminants, as a group, was contributed by B (P<0.001), whereas the f of CR, CM and G did not differ significantly from the non-ruminant values. We conclude that the high f of cattle and other domestic ruminants is a characteristic shared by many other species of the Bovidae family, but is not a general feature of ruminants. Out of various possibilities, it seems most probable that the tachypnea of Bovidae is in response to the elastic load imposed on the respiratory system by the rumen, the size of which exceeds that of other families of ruminants.

Pleural mechanics and fluid exchange

The pleural space separating the lung and chest wall of mammals contains a small amount of liquid that lubricates the pleural surfaces during breathing. Recent studies have pointed to a conceptual understanding of the pleural space that is different from the one advocated some 30 years ago in this journal. The fundamental concept is that pleural surface pressure, the result of the opposing recoils of the lung and chest wall, is the major determinant of the pressure in the pleural liquid. Pleural liquid is not in hydrostatic equilibrium because the vertical gradient in pleural liquid pressure, determined by the vertical gradient in pleural surface pressure, does not equal the hydrostatic gradient. As a result, a viscous flow of pleural liquid occurs in the pleural space. Ventilatory and cardiogenic motions serve to redistribute pleural liquid and minimize contact between the pleural surfaces. Pleural liquid is a microvascular filtrate from parietal pleural capillaries in the chest wall. Homeostasis in pleural liquid volume is achieved by an adjustment of the pleural liquid thickness to the filtration rate that is matched by an outflow via lymphatic stomata.

Effects of posture on the respiratory mechanics of the chick embryo

In the chicken embryo, pulmonary ventilation and pulmonary gas exchange begin approximately one day before the completion of hatching. We asked to what extent the posture inside the egg, and the presence of the eggshell and membranes, may alter the mechanical behaviour of the respiratory system. The passive mechanical properties of the respiratory system were studied in chicken embryos during the internal pipping phase (rupture of the air cell) or the external pipping phase (hole in the eggshell). Tracheal pressure and changes in lung volume were recorded during mechanical ventilation, first, with the embryo curled up inside the egg, then again after exteriorization from the eggshell. In the internal pippers, respiratory system compliance increased and expiratory resistance decreased after exteriorization, whereas the mean inspiratory impedance did not change. In the external pippers, exteriorization had no significant effects on respiratory compliance, resistance, or impedance, and the values were similar to those of newly hatched chicks. We conclude that, in the chicken embryo, at a time when pulmonary ventilation becomes an important mechanism for gas exchange, the curled up posture inside the egg does not provide any significant mechanical constraint to breathing.

Geometric determinants of airway resistance in two isomorphic rodent species

The flow resistance of the pulmonary airway tree (Raw) is disproportionately larger in large animals than in smaller ones. This is thought to be due to the fact that smaller animals have relatively wider central airways than larger animals. However, Raw is not determined solely by the diameter of the main bronchi or trachea. It depends on the dimensions of all the individual airways and how they are connected. We investigated how the degree of asymmetry of the airway tree, the number of airway orders, and dimensions of the individual airways combine to determine Raw. We performed this investigation using computer models of the airway trees of two different sized isomorphic rodents that differ in weight by more than two orders of magnitude-the harvest mouse and the giant pouched rat. We calculated airway resistance in these models under various conditions, and confirmed that the smaller species has a lower Raw relative to body size than the larger species. We also showed that these differences are due to a combination of differences both in relative airway diameters and in the degree of asymmetry of the airway trees.

Comparison of rat and mouse pulmonary tissue mechanical properties and histology

The present study compares the dynamic mechanical properties and the contents of collagen and elastic fibers (oxytalan + elaunin + fully developed elastic fibers) of mice and rat lung strips. Resistance, elastance (E), and hysteresivity (eta) were obtained during sinusoidal oscillations. The relative amounts of blood vessel, bronchial, and alveolar walls, as well as the mean alveolar diameter were determined. In both species, resistance had a negative and E a positive dependence on frequency, whereas eta remained unchanged. Mice showed higher E and lower eta than rats. Although collagen and elastic fiber contents were similar in both groups, mice had more oxytalan and less elaunin and fully developed elastic fibers than rats. Rats showed less alveolar and more blood vessel walls and higher mean alveolar diameter than mice. In conclusion, mice and rats present distinct tissue mechanical properties, which are accompanied by specific extracellular fiber composition.

Monitoring respiratory function and sleep in the obese Vietnamese pot-bellied pig

Development of drug treatments for obstructive sleep-disordered breathing has been impeded by the lack of animal models. The obese pig may be a suitable animal model, as it has been reported to experience sleep-disordered breathing resembling human obstructive sleep apnea. The purpose of this paper is to describe in detail techniques for chronic instrumentation of the obese Vietnamese pot-bellied pig and to study respiratory function during sleep. Under general anesthesia, four obese pigs were instrumented for long-term recording of intrapleural and tracheal pressures, genioglossal EMG, and bioelectric signals related to sleep. A custom-fitted face mask was used to record respiratory variables including airflow, snoring, and expired CO(2). Most chronic instrumentation provided robust signals for up to 6 wk after installation. All pigs displayed sleep-disordered breathing characterized by increased resistance to airflow, snoring, inspiratory flow limitation, and possible sleep disruption. Apneas and hypopneas were not a feature of breathing during sleep in these animals. Nonetheless, this animal preparation may be useful for exploring possible drug treatments for obstructive sleep-disordered breathing.

How stiff is the armadillo? A comparison with the allometrics of mammalian respiratory mechanics

Static respiratory mechanics were examined in the armadillo (Dasypus novemcinctus) and compared with allometric relationships newly derived for adult mammals from values in the literature. Normalised by body weight, chest wall compliance (Cw) in the armadillo is lower than predicted. Lung compliance (Cl) is also low in the armadillo, however it is appropriately matched to the resting lung volume (Vr) (ie. Cl/Vr infinity Mass0.0) and the ratio of Cw/Cl is appropriate for the size of the animal. Respiratory system resistance is high in the armadillo, presumably because of smaller airways associated with the small lung. The power of breathing in the armadillo is comparatively high, mainly due to the high resistive forces. Indeed, the oxidative cost of breathing is approximately double that of a mammal with similar Vr. Hypoxia or hypercapnia are known to invoke an attenuated ventilatory response in the armadillo and one that relies more on changes in frequency rather than volume. While such a breathing pattern helps to reduce the power of breathing it also compromises the degree of hyperventilation achieved.

Cardiorespiratory response to bicuculline during resistive loaded breathing in anesthetized rabbits

We hypothesized that hypoventilation induced by resistive loaded breathing may result in part from an increase in gamma-aminobutyric acid (GABA) concentration in the central nervous system. Accordingly, ventilatory depression should be minimized by GABA receptor blockade. The effects of subseizure doses of the GABAA receptor antagonist bicuculline on the cardiorespiratory variables and cortical activities were evaluated in two groups of urethane anesthetized rabbits, breathing either through an inspiratory resistive load (IRL) or not. Bicuculline induced cardiorespiratory changes which consisted mainly of an augmented respiratory rate, through shortening of expiratory duration, and of bradycardia. Bicuculline effects did not significantly differ between both groups and were accompanied by high amplitude delta rhythmic cortical activities. These data show that GABAA receptors may exert a tonic depressive effect on the respiratory circuit and suggest that endogenous GABA release is not augmented by an acute increase in the work of breathing.

Role of vagal afferents in the ventilatory response to naloxone during loaded breathing in the rabbit

It was previously shown that inspiratory resistive loading (IRL) increases the cerebrospinal fluid (CSF) level of beta endorphin in awake goats, and also that the slower ventilation induced by injection of this substance into the CSF of anesthetized dogs is suppressed after vagotomy. In the present study, performed on anesthetized rabbits, we evaluated the part played by vagal afferents in the ventilatory response to IRL after opioid receptor blockade by naloxone. During unloaded breathing, naloxone injection did not modify baseline ventilation. Conversely, naloxone partially reversed IRL-induced hypoventilation through an increase in respiratory rate. This effect was abolished after either vagotomy or cold blockade of large vagal fibers, but it persisted after procaine blockade of thin vagal fibers. These results suggest that pulmonary stretch receptors, which are connected to some large vagal afferent fibers, would play a major role in the ventilatory response to IRL under opioid receptor inhibition.

Effects of serotonin receptor blockade by methysergide on loaded breathing in the rabbit

Previous works demonstrated that the excitatory role of serotonin (5-hydroxytryptamine, 5-HT) on ventilation is mediated by 5-HT1,2 receptors stimulation. We hypothesized that load-induced hypoventilation could be minimized by a central release of 5-HT. Conversely, blockade of 5-HT receptors should accentuate hypoventilation. We compared the ventilatory effects of methysergide (MS), a 5-HT1,2 receptors antagonist, in 3 groups of anesthetized, spontaneously breathing rabbits: (1) a group of animals breathing for 60 min through a 370 cm H2O.liter-1.s inspiratory resistive load (IRL group), whose paCO2 increased with IRL; (2) a Control group; (3) a Control + CO2 group, made hypercapnic to assess the possible effect of this stimulus on the ventilatory responses to MS. In the 3 groups, i.v. injection of MS induced the same ventilatory changes, characterized by a rapid shallow breathing with a shorter integrated diaphragmatic activity. This confirms the tonic facilitatory effect of 5-HT on ventilation and suggests that IRL would not increase 5-HT release in the sites close to the respiratory nuclei.

Preservation of pulmonary function by an outer membrane protein F vaccine. A study in rats with chronic pulmonary infection caused by Pseudomonas aeruginosa

This study investigated the ability of a protein F vaccine to reduce macroscopic evidence of lung damage and preserve pulmonary function in immunized animals in a rat model of chronic pulmonary infection caused by Pseudomonas aeruginosa. Other membrane protein F of P aeruginosa was purified by extraction from polyacrylamide gels of cell envelope proteins of the PAO1 immunotype 7 strain. Rats were immunized intramuscularly with either 25 micrograms of the purified protein F or bovine serum albumin on days 0 and 14 and then challenged on day 28 via intratracheal inoculation of agar beads containing cells of an immunotype 3 clinical isolate of P aeruginosa. Also, included was a noninfected control group which received only sterile agar beads. On day 35, the lungs were excised, pulmonary compliance measured, and the lungs examined macroscopically for the presence and severity of lesions. The protein F-immunized rats had a significant (p < 0.01) reduction in the number of severe pulmonary lesions as compared with bovine serum albumin-immunized rats. Lung compliance (CL) was significantly (p < 0.001) reduced in rats which were immunized with bovine serum albumin (n = 17, CL = 0.12 +/- 0.008), whereas CL of protein F-immunized rats (n = 12, CL = 0.17 +/- 0.006) was similar to that of noninfected control rats (n = 5, CL = 0.15 +/- 0.008). This study demonstrated that a protein F vaccine has the ability to decrease macroscopic lung lesions from infection and preserve pulmonary function in actively immunized rats upon subsequent challenge with P aeruginosa in this model of chronic lung infection.

Resonant frequency of the chest-lung system by analysis of the respiratory flow curve

1. Resonant frequency of the chest-lung system in six tracheotomized, spontaneously breathing dogs was determined by analyzing the shape of the respiratory flow curve. 2. The resonant frequency was calculated from the periodic deviations from a sinusoidal flow pattern observed in the inspiratory phase of the breathing cycle. 3. Mean (+/- S.D.) resonant frequency was 6.1 +/- 0.9 Hz which was very close to the panting frequency (5.7 Hz) of the same dogs. 4. Resonant frequencies of the respiratory system in various species are compared.

Respiratory mechanics determined by flow interruption during passive expiration in cats

We used the interrupter technique to measure the resistance Rinit (equal to the initial change delta Pinit in tracheal pressure divided by flow at interruption) during expiration in six normal anaesthetized-paralyzed cats. By performing interruptions at different points in expiration we found Rinit in each cat to be linearly dependent on flow. By allowing the cats to expire through two different resistances we were also able to demonstrate a volume dependence of Rinit in four of the cats. In addition, we obtained a secondary pressure change delta Pdif in each cat, as the magnitude of the slow change in tracheal pressure in the 2 sec following interruption of flow. delta Pdif was approximately constant over most of the expired volume range, and represented the difference between the static elastic recoil pressure of the respiratory system and the pressure driving flow at any volume during a passive expiration. delta Pdif became larger than delta Pinit towards the end of expiration. Since previously used methods for measuring respiratory system resistance have employed varying combinations of delta Pinit and delta Pdif as the resistive pressure drop, it is clear that measurements of resistance must be made with standard techniques under standard conditions if they are to be compared.

An inexpensive servo-respirator based upon regulation of a shunt resistance

We have designed and built an inexpensive servo-respirator for use in investigations of respiratory control in small animals. The device uses a butterfly valve to alter the resistance of an outflow shunt from a manifold that connects the animal's tracheal cannula to a pressure source. Tracheal pressure is regulated in response to a command provided by a suitably processed neural signal, often the integrated phrenic neurogram. As the valve opens, tracheal pressure approaches atmospheric; as it closes, tracheal pressure approaches the source pressure. An electronic controller circuit was developed to permit experimental procedures that include withholding volume delivery while maintaining a desired level of positive end-expiratory pressure. The device is able to track the neural command signal satisfactorily, and its performance appears to be limited primarily by the constraints applied by the respiratory system mechanics.

Mechanics of the pleural space: fundamental concepts

The transmission of forces from the respiratory muscles to the lung across the extremely thin pleural space has been poorly understood because of the difficulty of accurately measuring pleural liquid and pleural surface pressure (lung static recoil or transpulmonary pressure). Recent results using relatively noninvasive techniques have indicated that the vertical gradient in pleural liquid pressure is not hydrostatic, that pleural liquid pressure is closely related to lung recoil, and that there exists a very thin but continuous pleural liquid layer. These findings contradict concepts based on hydrostatic equilibrium and on the distinction between pleural liquid and pleural surface pressure due to pleural contact. Pleural liquid pressure is not in hydrostatic equilibrium because the difference between the vertical gradient in pleural liquid pressure and the effect of gravity is always balanced by a pressure loss due to a viscous flow within the pleural space. Fluid lubrication of the pleural surfaces is the primary function of the pleural space. The mechanical interaction between the lung and the chest wall is coupled to the dynamics of liquid within the pleural space, which is viewed as a flow-through system. Homeostasis is achieved in such a system by the adjustment of the viscous flow within the pleural space and the outflow absorption rate by lymphatics to the microvascular filtration rate across pleural capillaries.

Lung mechanical properties in the West Indian Manatee (Trichechus manatus)

Airway dimensions, extent of cartilage reinforcements and lung mechanical properties were examined in excised lungs of the West Indian Manatee (Trichechus manatus). All airways with a mean inside diameter of more than 0.6 mm had complete cartilaginous rings in their walls, and cartilage reinforcement extended into the respiratory bronchi. Expiratory flow rates over the vital capacity range were higher than those encountered in humans varying between 20 and 50% of peak flow at 25% of vital capacity. They were, however, lower than in other marine mammals. The ratio of lung compliance to lung weight was found to be 0.67 ml/(g X cm H2O) compared to 0.18 ml/(g X cm H2O) in terrestrial mammals. This together with a high gas-tissue ratio of 11 mg/g for the inflated manatee lung indicates that strong airway reinforcements are necessary to prevent airway collapse during exhalation.

The pattern of sympathetic neurone activity during expiration in the cat

The properties of sympathetic preganglionic neurone activity during expiration were studied in pentobarbitone-anaesthetized (n = 26) and in non-anaesthetized, mid-collicular decerebrate (n = 5), paralysed, artificially ventilated cats in which the electrical activity of the phrenic nerve and of the cervical sympathetic trunk was recorded. In control conditions (end-tidal PCO2 between 35 and 40 mmHg, zero end-expiratory pressure) sympathetic activity during expiration was either steady at a low level (n = 11) or showed a modest progressive increase from a low level in early expiration (n = 17). Very infrequently (n = 3), it showed a transient increase during the second half of expiration. Artificial ventilation with positive end-expiratory pressures in the range from 2.1 +/- 0.4 (mean +/- S.D.) to 6.7 +/- 0.6 cmH2O caused, in cats with intact vagus nerves, an increase in sympathetic neurone activity during the second half of expiration. Within this range of pressures, the magnitude of the increase was related to the magnitude of the positive end-expiratory pressure. This effect reversed at higher positive end-expiratory pressures. Pressures in excess of 10.2 +/- 1.8 cmH2O caused inhibition of sympathetic activity. The sympatho-excitatory effect of positive end-expiratory pressure disappeared after bilateral cervical vagotomy. With intact vagus nerves, it also disappeared at levels of systemic hypocapnia (end-tidal PCO2 less than or equal to 15 mmHg) which abolished phrenic nerve activity. In hypocapnia, artificial ventilation with peak tracheal pressures greater than 7.2 +/- 1.1 cmH2O caused inhibition of sympathetic activity, while ventilation with lower end-expiratory pressures had no effect on sympathetic activity. It may be concluded that the sympatho-excitatory effect of positive end-expiratory pressure is mediated by vagal afferents and requires a certain level of brain-stem respiratory neurone activity. Sympatho-excitation during expiration was also observed, in normocapnic conditions, during short-duration static lung inflation with tracheal pressures in the range from 2.5 +/- 0.3 to 7.0 +/- 0.8 cmH2O as well as during artificial ventilation with zero end-expiratory pressure when lung inflation occurred in expiration. These responses were abolished by bilateral cervical vagotomy and during systemic hypocapnia. Sympatho-excitation during expiration was also observed when systemic hypercapnia was produced in vagotomized cats by artificial ventilation with gas mixtures containing 5 or 10% CO2.(ABSTRACT TRUNCATED AT 400 WORDS)

Exponential analysis of the lung pressure-volume curve in newborn mammals

The compliance of the lung (per unit of lung weight) is less in newborn mammals than in adults. This could result from a smaller volume of airspaces per unit weight and/or a lower lung distensibility. The isolated role of lung distensibility was evaluated by using a mathematical description of the pressure-volume (P-V) curve during lung deflation. Deflation limbs of static P-V curves in newborns of six species (four experimentally obtained and two taken from the literature) ranging from total lung capacity to the resting volume (Vr) were fitted by a monoexponential function of the type V = B - Ae-KP, where B equals Vmax at infinite P, A equals the difference between Vmax and V at P = O, and K is a constant representing lung distensibility. Unlike in adults, in newborns the monoexponential fitting provided an adequate description of the P-V curve for only a relatively small range of transpulmonary pressure (from P at Vr to 10-15 cm H2O). The K value of this portion of the curve was similar among species but higher than in adult mammals, averaging 0.240 cm H2O-1. This suggests a similar lung structure in the different species. Since lung distensibility in newborns is larger than in adults, the fact that a unit mass of lung in the newborn is less compliant should be due to the smaller volume of its airspaces.

Mass transport in mammalian lungs: comparative physiology

Comparative physiology of mass transport of gases in mammalian lungs is surveyed in terms of the use of experimental mammals in inhalation toxicology. Principles of similarity, scaling, and the relationship of metabolism to body size are touched on, with reference to the wide variability among mammals of similar size. Mechanisms that influence the magnitude and distribution of pulmonary ventilation are reviewed, including mechanical differences associated with variation in body size. More systematic and complete descriptions and understanding are needed. Recent advances in the understanding of gas mixing and transport in airways and in the pulmonary acinus have applications in comparative physiology and inhalation toxicology that are worth exploring.

Pulmonary mechanics in conscious Fischer 344 rats: multiple evaluations using nonsurgical techniques

An adaptation of the method reported by Amdur and Mead (1958, Amer. J. Physiol. 192, 364-368) was used to evaluate pulmonary mechanics in conscious rats, utilizing a flow plethysmograph and intraesophageal catheter to determine compliance, resistance, respiratory flow, thoracic pressure, tidal volume, frequency, and minute volume. The procedure did not require surgery, and repeat evaluations were made in conscious rats. Real time calculations of pulmonary parameters were accomplished with an analog computer interfaced with a digital computer for display and data storage. Multiple evaluations (1, 3, 6, 13, 18, 24, 30, and 36 weeks) in 40 rats are reported. The utility of the system in establishing a base line against which pulmonary impairment, related to the effects of known pulmonary toxicants, e.g., ANTU and NO2, could be measured, was evaluated.

Ventilatory functions of normal rats of different ages

1. Lung volume, respiratory mechanics and flow-volume curve were measured in 96 male Wistar rats weighing from 102 to 521 g. Their ages ranged from 31 to 137 days and therefore, most of them were in the rapidly growing phase. 2. Data except for functional residual capacity were obtained under conditions of artificial ventilation. 3. Total lung capacity and the fractions of lung volume of these animals were found to relate linearly with their body weights. 4. Most of the parameters concerning respiratory mechanics and flow-volume curve also changed generally in proportion to their body weights, but those standardized for lung volume were not constant but varied depending on the weights. 5. These results suggest that the mechanical properties of lungs of young rats are age-related and comparisons of ventilatory functions among those animals should be carefully done.

Comparative mechanics of mammalian respiratory system

Compliances (C) of lung, thorax and respiratory system as well as resistances (R) of the respiratory system (lower airways + lung + chest wall) and of upper airway (primarily laryngeal and nasal) were measured in 5 species of mammals, spanning a 1000-fold range of body weights (mouse to dog), immediately following sacrifice with an overdose of sodium pentobarbital. The results indicate that compliance is proportional to BW1.0, while respiratory resistance and upper airway resistance have exponents of -0.819 and -0.702, respectively. The reciprocal of the time constant, tau -1 = (RC)-1 is proportional to BW-0.298 for respiratory resistance alone and BW-0.326 when upper airway resistance is included. Since breathing frequency varies as BW-0.28, these results indicate a proportional relationship between breathing frequency and passive emptying time. This suggests that passive respiratory mechanics play a major role in determining TE and therefore TTOT for animals during quiet breathing. Changes in volume history were found not to affect the slope of the relationships between compliance and body weight.

Impedance of the lower respiratory system in ducks measured by forced oscillations during normal breathing

The lower respiratory system of 10 conscious Pekin ducks breathing normally was subjected to superimposed oscillations of 1.3 to 16 Hz by a small volume piston pump. Induced sinusoidal flow (VO) and pressure (PO) signals were measured in late expiration, when the gas flow ventilated by the animal was minimum. The modulus of the respiratory impedance was calculated as the ratio PO/VO. The values obtained at the various oscillatory frequencies were compared to those predicted on the basis of a series mechanical network model consisting of resistive, inertial and compliant elements (RIC) using a least squares non linear regression method. The experimental data fitted well the frequency response of a simple RIC mathematical model. After correcting for the effects of the endotracheal tube, the mean values +/- SE of the optimized parameters were: resistance 4.8 +/- 0.4 cm H2O . L-1 . sec; inertance: 0.05 +/- 0.01 cm H2O . L-1 . sec2; compliance: 7.7 +/- 0.5 ml . cm H2O-1; natural frequency: 8.0 +/- 0.4 Hz. It is concluded that: (1) the lower respiratory system in ducks can be closely modeled by a RIC mechanical series network model; (2) the forced oscillation method can be used to investigate avian mechanics of breathing, with the birds awake and breathing spontaneously.

Statics of the respiratory system in newborn mammals

The static mechanical properties of the respiratory system have been studied in newborn mammals within the first two weeks of life, ranging in body size from rat to piglet, and compared to the corresponding adult. Animals were supine, paralyzed, and passively ventilated. The pressure-volume curves of the respiratory system and of the lung have been constructed by changing volume a known amount and measuring the tracheal pressure before and after opening the rib cage. In the newborns, the mean elastic recoil pressure of the lung at functional residual capacity (PLFRC) is 1.9 cm H2O with no clear correlation with body weight. The dry lung weight (LW)/body weight (BW) ratio is greater in newborns than in adults and progressively decreases with age. FRC (measured directly with the saline displacement method) is proportional to LW1.09 and BW1.03. In newborns, lung compliance per lung weight (CL/LW) is similar between species (Cl alpha LW1.07). Chest wall compliance per body weight is also constant (CW alpha BW1.00). By comparing the newborns with the corresponding adults we found that PLFRC is 35-57% of the adult value. FRC/LW in the newborn is less than or equal to the adult value, while FRC/BW is generally larger in the newborn. CL/LW is usually smaller in the newborn than in the adult while CW/BW is larger.