Mechanical coupling of upper and lower canine rib cages and its functional significance

We studied rib cage distortability and reexamined the mechanical action of the diaphragm and the rib cage muscles in six supine anesthetized dogs by measuring changes in upper rib cage cross-sectional area (Aurc) and changes in lower rib cage cross-sectional area (Alrc) and the respective pressures acting on them. During quiet breathing in the intact animal the rib cage behaved as a unit (Aurc: 14.6 +/- 7.9 vs. Alrc: 15.1 +/- 9.6%), whereas considerable distortions of the rib cage occurred during breathing after bilateral phrenicotomy (Aurc: 21.0 +/- 5.1 vs. Alrc: 7.0 +/- 4.8%). These distortions were even more pronounced during phrenic nerve stimulation and separate stimulation of the costal and crural parts of the diaphragm (e.g., phrenic nerve stimulation; Aurc: -7.1 +/- 5.1 vs. Alrc: 6.9 +/- 3.5%). During the latter maneuvers the upper rib cage deflated along the relationship between upper rib cage dimensions and pleural pressure obtained during passive deflation, whereas the lower rib cage inflated close to the relationship between lower rib cage dimensions and abdominal pressure obtained during passive inflation. The latter relationship is expected to differ between costal and crural stimulation, since costal action has both an appositional and insertional component and crural action only has an appositional component. The difference between costal and crural stimulation, however, was relatively small, and the slopes were only slightly steeper for the costal than for the crural stimulation (2.9 +/- 1.2 vs. 2.2 +/- 1.0%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of unilateral vagal stimulation on intrapulmonary neuroepithelial bodies

We studied the influence of unilateral vagal stimulation on intrapulmonary neuroepithelial bodies (NEB) in rabbits. The left vagus nerve was cut and electrically stimulated for 10 min. Animals were killed and the lungs studied with fluorescence and electron microscopy. Intensity of formaldehyde-induced fluorescence, which reflects the serotonin content in NEB, was higher on the stimulated side than on the nonstimulated side (118 +/- 7 vs. 100%, n = 8, P less than 0.001). The latter difference was found to correlate with the stimulus amplitude (r = 0.9, P less than 0.05). Ultrastructurally a decrease in the number of exocytotic dense-cored vesicle (DCV) profiles at the level of the NEB basal epithelial cell membrane was found on the stimulated side (0.32 +/- 0.10 vs. 0.45 +/- 0.16 DCV/micron of basal epithelial cell membrane, n = 8, P less than 0.05). Section of the left vagus nerve without electrical stimulation affected neither the fluorescence intensity nor the number of exocytotic DCV profiles. In animals with supranodosal or infranodosal chronic vagotomy the observed effects of unilateral vagal stimulation were no longer present. We conclude that 1) vagal stimulation increases the serotonin content of NEB; 2) it decreases the number of exocytotic DCV profiles; 3) this effect depends on the amplitude of the stimulus; 4) it is obtained through efferent vagal fibers; 5) these results are the opposite of the effects seen after exposing normal NEB to acute hypoxia; and 6) these physiological experiments corroborate a vagal innervation of NEB, which may play an important role in modulating the sensitivity and reaction of NEB to various stimuli.

Effects of acute hyperinflation on chest wall mechanics in dogs

We studied chest wall mechanics at functional residual capacity (FRC) and near total lung capacity (TLC) in 14 supine anesthetized and vagotomized dogs. During breathing near TLC compared with FRC, tidal volume decreased (674 +/- 542 vs. 68 +/- 83 ml; P less than 0.025). Both inspiratory changes in gastric pressure (4.5 +/- 2.5 vs. -0.2 +/- 2.0 cmH2O; P less than 0.005) and changes in abdominal cross-sectional area (25 +/- 17 vs. -1.0 +/- 4.2%; P less than 0.001) markedly decreased; they were both often negative during inspiration near TLC. Parasternal intercostal shortening decreased (-3.0 +/- 3.7 vs. -2.0 +/- 2.7%), whereas diaphragmatic shortening decreased slightly more in both costal and crural parts (costal -8.4 +/- 2.9 vs. -4.3 +/- 4.1%, crural -22.8 +/- 13.2 vs. -10.0 +/- 7.5%; P less than 0.05). As a result, the ratio of parasternal to diaphragm shortening increased near TLC (0.176 +/- 0.135 vs. 0.396 +/- 0.340; P less than 0.05). Electromyographic (EMG) activity in the parasternals slightly decreased near TLC, whereas the EMG activity in the costal and crural parts of the diaphragm slightly increased. We conclude that 1) the mechanical outcome of diaphragmatic contraction near TLC is markedly reduced, and 2) the mechanical outcome of parasternal intercostal contraction near TLC is clearly less affected.

Relationship between diaphragm length and abdominal dimensions

We examined the relationship between changes in abdominal cross-sectional area, measured by respiratory inductive plethysmography, and changes in length in the costal and crural parts of the diaphragm, measured by sonomicrometry, in nine supine, anesthetized dogs. During passive inflation, both parts of the diaphragm shortened and abdominal cross-sectional area increased. During passive deflation, both parts of the diaphragm lengthened and abdominal cross-sectional area decreased. We subsequently used the relationship between costal and crural diaphragmatic length, respectively, and abdominal cross-sectional area during passive inflation-deflation to predict the length changes in the costal and crural diaphragm during quiet breathing before and after bilateral phrenicotomy. In the intact animal the inspiratory shortening in the crural diaphragm was almost invariably greater than predicted from the relationship during passive inflation. During inspiration after phrenicotomy the crural diaphragm invariably lengthened, whereas the costal diaphragm often shortened. In general there was a good correlation between the measured and predicted length change for the crural diaphragm (r = 0.72 before and 0.79 after phrenicotomy) and a poor one for the costal diaphragm (r = 0.05 before and 0.19 after phrenicotomy).

Respiratory resistance with histamine challenge by single-breath and forced oscillation methods

Relaxed expirations were obtained from five anesthetized dogs under control conditions and during various rates of intravenous infusion of histamine. All volume vs. time curves obtained from 20 ms to 2 s after the start of expiration were poorly described by a single exponential function but were fitted very well by a biexponential function. The resistance of the respiratory system as a function of frequency from 2 to 26 Hz was also determined by the forced oscillation method in the same dogs. Three two-compartment models of the respiratory system were identified from the exponentials fitted to the relaxed expiration data, and the one that had the most plausible parameter values under control conditions consisted of a homogeneous lung compartment connected to a viscoelastic compartment. Although a two-compartment model is arguably appropriate for describing relaxed expirations in normal dogs, physiological considerations suggest that there should be more than two interacting components with histamine infusion. We cannot identify all these components from our data, however. The equivalent complex impedance of the respiratory system was also calculated from the biexponential curves and showed significant variation in resistance over the frequency range from 0 to 2 Hz and negligible variation above 2 Hz. The calculated resistances at 2 Hz were consistently higher than those obtained by the forced oscillation method, which may be due to the nonlinear behavior of the respiratory system during relaxed expiration. We conclude that the single-breath and forced oscillation methods should be viewed as providing complimentary information about respiratory resistance.

Coupling between triangularis sterni and parasternals during breathing in dogs

The purpose of the present studies was to assess the functional coupling between the parasternal intercostals and the triangularis sterni (transversus thoracis) muscles during resting breathing, and we measured the electrical activity and the respiratory changes in length of these two muscles in 13 supine anesthetized dogs. The changes in muscle length were defined relative to their respective in situ relaxation length (Lr). During inspiration, the parasternal intercostals were active and shortened below Lr, causing the triangularis sterni to be passively stretched above Lr. Shortly after the cessation of parasternal contraction, the triangularis sterni became active and shortened below Lr, and in nine animals this active shortening was associated with a forcible distension of the parasternal intercostals above Lr. Deactivation of the triangularis sterni at end expiration caused both muscles to return to their respective Lr. This pattern was essentially unchanged after supplemental anesthesia and bilateral phrenicotomy. We conclude that in dogs breathing quietly the length of the rib cage muscles during the expiratory pause is not passively determined as conventionally thought.

Respiratory and postural changes in intercostal muscle length in supine dogs

In an attempt to assess the physiological function(s) of the external (E) and internal interosseous (I) intercostal muscles, we measured the changes in intercostal muscle length during spontaneous breathing, during passive inflation, and during passive rotation of the trunk. Studies were performed on 46 muscles from 16 supine anesthetized dogs, and changes in muscle length were assessed by sonomicrometry. The changes were small during spontaneous breathing, whether before or after bilateral phrenicotomy, and the pattern was variable among animals and among interspaces. The E, however, particularly in the lower interspaces, often lengthened with inspiration, and the I, in particular in the upper interspaces, often shortened with inspiration. Only occasionally did the E and I in one interspace change in length in opposing directions. This was also true during passive inflation, where both E and I usually shortened in the upper interspaces and lengthened in the lower interspaces. By contrast, during passive rotation of the trunk, the E and I systematically changed in length in opposing directions, and either muscle could successively lengthen and shorten a substantial amount depending on the side of rotation. These results suggest that 1) the E and I in supine dogs do not behave as antagonistic muscles during moderate respiratory efforts; and 2) they do behave as antagonistic muscles during rotation of the trunk. A primary function of these muscles as rotators of the trunk, unlike breathing, may explain why two layers of intercostal muscles with different fiber orientation exist between the ribs.

Volume-time profile during relaxed expiration in the normal dog

Airway opening pressure, esophageal pressure, and flow were obtained during relaxed expirations in two normal anesthetized paralyzed dogs. The signal-to-noise ratio in the flow signals was greatly increased by averaging 10 different signals obtained with the same lung inflation volume. Numerical integration of an averaged flow signal then yielded the time course of the volume of the respiratory system above functional residual capacity (the elastic equilibrium volume). Comparison of volume signals obtained with different inflation volumes suggests that the resistance of the respiratory system increases with flow. The flow-volume and semilog volume curves show that expiration is induced by two apparently separate mechanisms: one causes emptying of most of the expired volume over a time interval of much less than 1 s, whereas the other contributes a relatively small amount to the expired volume over a significantly longer time (greater than or equal to 1 s). We postulate the first mechanism to be due to that of the respiratory system behaving like a single unit, with an elastance that is slightly volume dependent, emptying through a single airway which has a resistance that increases with flow. From the nature of airway opening pressure and esophageal pressure measured after occlusion in midexpiration, we conclude that the second mechanism is due to the viscoelastic properties (i.e., creep) of the respiratory system. The properties are manifest mainly in the chest wall.

Contractile properties of intercostal muscles and their functional significance

To have some insight into the functional coupling between the parasternal intercostals (PS) and the diaphragm (DPM), we have examined the isometric contractile properties of bundles from canine PS and DPM muscles. Bundles of external (EXT) and internal (INT) interosseous intercostals were studied for comparison. In addition we have related sonometrically measured length of the intercostals in vivo at supine functional residual capacity (FRC) to in vitro optimal force-producing length (Lo). We found that 1) intercostal twitch speed is significantly faster than DPM, thus displacing their relative force-frequency curve to the right of that of the DPM; 2) the ascending limb of the active length-tension curve of all intercostals lies below the DPM curve; i.e., at 85% Lo, PS force is 46% of maximal force (Po), whereas DPM force is still 87% Po; 3) for any given length change beyond Lo, all intercostals generate greater passive tension than the DPM; 4) Po is greater for the intercostals than the DPM; and 5) at supine FRC, both EXT and INT in dogs are nearly operating at Lo, whereas the PS are operating at a length greater than Lo. We conclude that 1) PS produce less force than DPM during breathing efforts involving low- (10-20 Hz) stimulation frequencies, but they generate more force than DPM when high- (greater than 50 Hz) stimulation frequencies are required; and 2) the pressure-generating ability of the PS is better preserved than that of the DPM with increases in lung volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory resistance in dogs by the single-breath and the forced oscillation methods

Total respiratory resistance (Rrs) was measured in six anesthetized dogs with two different methods: the single-breath (SB) method, which provides the time constant of the system during a relaxed expiration and the forced oscillation (FO) method, which uses a pseudorandom noise signal applied at the airway opening. The comparison was made in three conditions: before muscle paralysis (A), after muscle paralysis (B), and after tracheal banding (C). In conditions A and B the two computed resistances correlated very well with each other (r = 0.98). No systematic difference between Rrs values obtained with the two methods was found. In condition C the respiratory resistance was clearly nonlinear from the flow-volume curves during SB and could be described with Rohrer's equation: Rrs = K1 X V + K2 X V2, where K1 and K2 are Kohrer's constant and V is flow. Rrs measured with FO was not frequency dependent during tracheal banding (C) and was virtually equivalent to K1. Since the FO method uses low flows as the input of the respiratory system and K1 could be ascribed to laminar flow, the numerical matching appears reasonable and tends to reinforce the validity of both methods of measurement. We conclude that, for the normal respiratory system, FO and SB methods are approximately equivalent. In the presence of a markedly alinear central airway resistance with normal lungs, the SB method appears to provide a more adequate description of the flow-resistive properties of the system.

Relationship between parasternal intercostal length and rib cage displacement in dogs

The relationship between parasternal intercostal length and rib cage cross-sectional area was examined in nine supine dogs during passive inflation and during quiet breathing before and after phrenicotomy. Parasternal intercostal length (PSL) was measured with a sonomicrometry technique, and rib cage cross-sectional area (Arc) was measured with a Respitrace coil placed around the middle rib cage. During active inspiration as well as during passive inflation, PSL decreased as Arc increased. However, the relationship between PSL and Arc during active inspiration, whether in the intact or phrenicotomized animal, was almost invariably different from that during passive inflation, so that the same increase in Arc was associated with a greater decrease in PSL in the former than in the latter instance. This difference between passive inflation and active inspiration is probably due to the active contraction of the parasternals during inspiration and the consequent caudal displacement of the sternum. In upright humans, the sternum moves cephalad and not caudad during inspiration, so the relationship between PSL and Arc during active breathing might be similar to that during passive inflation.

Mechanical coupling between the ribs and sternum in the dog

We measured the axial (cranio-caudal) displacements of the sternum and the second and seventh bony ribs using linear displacement transducers in five supine anesthetized dogs during passive inflation and deflation, during quiet breathing and static inspiratory efforts before and after bilateral phrenicotomy, and during tetanic stimulation of either the sternocleidomastoids or the sternal fibers of the rectus abdominis. Quiet inspiration before and after phrenicotomy was always associated with a caudal displacement of the sternum and a cranial displacement of the seventh rib; the second rib, however, was either motionless or also showed an inspiratory caudal displacement. During static inspiratory efforts, the second rib was always moving in concert with the sternum in the caudal direction, while the seventh rib, in particular after phrenicotomy, usually moved in the cranial direction. Finally, for any given axial (cranial or caudal) displacement of the sternum, stimulation of the sternocleidomastoid or rectus abdominis muscles invariably caused the second rib to move disproportionately more than the seventh. These results indicate that the upper ribs are more tightly linked to the sternum than the lower ribs. This presumably results from the fact that the costal cartilages increase in length from above downwards, and it implies that the upper portion of the rib cage behaves more as a unit with the sternum than the lower portion.

Regional differences in abdominal pressure swings in dogs

The pressure swings under the costal (Pcos) and crural diaphragms (Pcru) and between the intestinal loops (Pint) were compared with the swings in gastric pressure (Pga) in 13 supine anesthetized dogs. Pcos, Pcru, and Pint were measured with air-filled latex balloons in eight dogs and saline-filled catheters in five. Pga was measured with an air-filled balloon in all dogs. During quiet breathing differences were often present, the directions of which were variable from animal to animal. During mechanical ventilation, all pressures increased, but both Pcos and Pcru increased more than Pga, whereas only a small change was observed in Pint. During bilateral stimulation of the costal diaphragm, Pcos invariably increased more than Pga and Pint, whereas almost no change was observed in Pcru. During bilateral stimulation of the crural diaphragm, Pcru invariably increased more than Pga, Pint, and Pcos. During abdominal muscle stimulation as during external abdominal compression, Pint always increased more than Pcos and Pcru. During lower rib cage compression, Pga, Pcos, and Pcru increased more than Pint. During sternocleidomastoid stimulation, all pressure swings were negative, but the change in Pint was always smaller than in Pcos, Pcru, or Pga. Inhomogeneities observed with balloons and saline-filled catheters were similar. After the abdomen was filled with 2 liters of saline all pressure swings became much more homogeneous.

Respiratory changes in parasternal intercostal length

In an attempt to understand the role of the parasternal intercostals in respiration, we measured the changes in length of these muscles during a variety of static and dynamic respiratory maneuvers. Studies were performed on 39 intercostal spaces from 10 anesthetized dogs, and changes in parasternal intercostal length were assessed with pairs of piezoelectric crystals (sonomicrometry). During static maneuvers (passive inflation-deflation, isovolume maneuvers, changes in body position), the parasternal intercostals shortened whenever the rib cage inflated, and they lengthened whenever the rib cage contracted. The changes in parasternal intercostal length, however, were much smaller than the changes in diaphragmatic length, averaging 9.2% of the resting length during inflation from residual volume to total lung capacity and 1.3% during tilting from supine to upright. During quiet breathing the parasternal intercostals always shortened during inspiration and lengthened during expiration. In the intact animals the inspiratory parasternal shortening was close to that seen for the same increase in lung volume during passive inflation and averaged 3.5%. After bilateral phrenicotomy, however, the parasternal intercostal shortening during inspiration markedly increased, whereas tidal volume diminished. These results indicate that 1) the parasternal intercostals in the dog are real agonists (as opposed to fixators) and actively contribute to expand the rib cage and the lung during quiet inspiration, 2) the relationship between lung volume and parasternal length is not unique but depends on the relative contribution of the various inspiratory muscles to tidal volume, and 3) the physiological range of operating length of the parasternal intercostals is considerably smaller than that of the diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical arrangement of costal and crural diaphragms in dogs

To assess the mechanical arrangement of the costal and crural parts of the diaphragm, we studied changes in diaphragmatic length with piezoelectric crystals in 17 supine anesthetized dogs. During control resting inspiration, the crural part usually shortened more and earlier than the costal part. After phrenicotomy, the crural part always lengthened during inspiration, whereas the costal part shortened or lengthened. These interanimal differences disappeared after opening of the abdomen; the costal part then always lengthened during inspiration. During stimulation of one part, the relaxed nonstimulated part always lengthened. However, when compared with the relationship between length and transdiaphragmatic pressure (Pdi) obtained during passive deflation, the lengthening of the relaxed part during stimulation of either part was small. This difference between predicted and measured Pdi-length relationship decreased in magnitude as lung volume increased above functional residual capacity (FRC) and increased as residual volume was approached. These results indicate that 1) even during quiet breathing the diaphragm in the dog is not a single functional entity; 2) at FRC the costal and crural portions of the diaphragm behave as if they were mechanically arranged partly in parallel and partly in series; and 3) they gradually move into a pure mechanical series arrangement as lung volume increases.

Isocapnic hyperventilation with cold air in healthy non-smokers, smokers and asthmatic subjects

Isocapnic hyperventilation with subfreezing air was performed by 15 healthy non-smokers, 10 asymptomatic smokers and 9 asthmatics. All subjects had normal ventilatory function and airway resistance (Raw) before challenge. The hyperventilation was performed twice. In one session, total respiratory resistance (Rrs) and reactance (Xrs) were measured at various frequencies, using a forced oscillation technique; in another session, vital capacity (VC), forced expiratory volume in 1 s (FEV1), maximal expiratory flow rates (FEF) and Raw were determined. In non-smokers, no changes in FEV1, FEF nor Raw were observed, whereas Rrs increased significantly (+ 20% of the prechallenge value), without change in resonant frequency nor in the Rrs-frequency relationship. This suggests a constrictory effect on central airways (possibly a narrowing of the glottis) only. In smokers, Rrs showed a similar, though longer lasting, increase than in non-smokers. Besides, a significant change of the Rrs-frequency relationship and a reduction in FEF at 50% of VC was found, suggesting an involvement of peripheral airways also. In asthmatics, bronchial reactivity was more pronounced, resulting in significant changes in all parameters: Rrs increased by about 100% of the prechallenge value, and became highly frequency dependent; Xrs decreased markedly, resulting in an increase in resonant frequency of the respiratory system. Similarly, VC, FEV1 and FEF decreased. These alterations are compatible with a more generalized constriction of the peripheral as well as central airways.

Salutary effect of fall in abdominal pressure during diaphragm paralysis

To examine the mechanical effects of the fall in abdominal pressure (Pab) that occurs during inspiration in diaphragmatic paralysis, we studied lung inflation and rib cage expansion before and after the abdomen was opened in nine spontaneously breathing dogs with bilateral phrenicotomy . We measured Pab, tidal volume, and parasternal electromyographic (EMG) activity during quiet breathing and CO2-induced hyperpnea. In six dogs, we also measured changes in anteroposterior and transverse rib cage diameters, the resting length of the parasternal intercostal muscles, and the amount of shortening of these muscles during inspiration. Opening the abdomen caused a marked reduction in the fall in Pab during inspiration and invariably resulted in a decrease in tidal volume (mean decrease, 13%), which contrasted with marked increases in inspiratory rib cage expansion and in the amount of parasternal intercostal shortening. The procedure, however, did not affect the resting length or inspiratory EMG activity of the parasternals . These findings indicate that although the fall in Pab, which occurs during inspiration in diaphragmatic paralysis, causes paradoxical inward displacement of the ventral abdominal wall, it has a salutary effect on tidal volume. This phenomenon is probably due to the fact that the diaphragm is part of the abdominal wall.

Effects of unilateral hypoxia on neuroepithelial bodies in rabbit lungs

The present study was undertaken to investigate the influence of regional (unilateral) alveolar hypoxia on the intrapulmonary neuroepithelial bodies (NEB), which contain bioactive substances and are distinctly innervated. Eight (4-wk-old) rabbits were subjected to unilateral hypoxia. The animals were anesthetized by an intramuscular injection of Hypnorm and breathed spontaneously during the experiment. The right lung received a hypoxic gas mixture (10% O2-90% N2) and the left lung a hyperoxic mixture. Blood gas measurements indicated that no systemic hypoxemia or acidosis occurred under these conditions. Lung samples were examined by electron microscopy to determine morphometrically the extent of a secretory exocytosis at the basal cell pole of the NEB and by a microspectrographical analysis of the formaldehyde-induced fluorescence to quantify the NEB serotonin content. After 20 min of unilateral hypoxia the NEB in the right hypoxic lung exhibited an increased exocytosis and a lower serotonin content in comparison with the left hyperoxic lung NEB. These results indicate that NEB react to regional alveolar hypoxia by secreting serotonin and/or peptides to the surrounding lung tissue (blood vessels, smooth muscle, nerve endings, etc.).

Radiographic evaluation of regional pulmonary dimensions and volumes: effect of age

On chest radiographs performed at spirometrically controlled residual volume (RV), functional residual capacity (FRC) and total lung capacity (TLC), several distances were measured in 8 young and 7 elderly male subjects. Regional volumes above and below the minor fissure were calculated using Barnhard's method for thoracic gas volume determination. When the volumes above and below the fissure were expressed as a percentage of their volume at TLC, and related to overall lung volumes (in percent of TLC), graphs similar to the ones obtained in scintigraphic studies were obtained. This illustrates that the present radiographic method may provide information on regional pulmonary volumes. Regional TLC of the upper zone was larger in elderly subjects than in young ones, while the opposite was true for regional TLC of the lower zone, indicating that maximal regional expansion changes with age. For the upper zone, the changes in diameters between RV and TLC were largest in antero-posterior direction and smallest in cranio-caudal direction. For the lower zone, changes were largest in cranio-caudal direction. This suggests an anisotropic expansion of the upper and lower zone. This anisotropy was more pronounced in the young than in the elderly subjects.

Evaluation of bedside myocardial scintigraphy with 201Tl in acute myocardial infarction

Bedside myocardial scintigraphy was performed on 149 patients admitted to the Coronary Care Unit (CCU), after IV injection of 74 MBq 201Tl, using a mobile gamma camera (Dynamo). The study was displayed on Polaroïd pictures, without any image treatment, and read by two independent readers. Clinical history, findings, and final diagnosis were assessed by an independent clinician. The following conclusions were reached: 1) The sensitivity of the study for the detection of a recent myocardial infarction (MI) was 0.84 with a specificity of 0.87. 2) There was a good correlation between scintigraphic and ECG localization. 3) No firm correlation was found between scintigraphic and enzymatic estimates of infarct size. 4) Abnormal visualization of the right ventricle was probably associated with more extensive infarction.

Model of elasticity of the human lung

A model of the elasticity of the human lung has been developed to evaluate the relative importance of the characteristics of the lung parenchyma, of thorax configuration, and of gravity on the vertical gradients of pleural pressure and regional volumes, and on the linear displacements of lung tissue, of various lung volumes. The predictions of the model are compared with available experimental data. It is suggested that the bulk elasticity modulus of the human lung is high with respect to that of canine lungs, that the shearing forces are low ("effective" Poison's ratio of about 0.4-0.45), and that the variations of regional pleural pressures and volumes during deflation are determined primarily by the interaction between lung weight and changes in thorax configuration.