Effect of hypersensitivity on protein uptake across the air-blood barrier of isolated rabbit lungs.

In previous studies with isolated perfused rabbit lungs, we observed that human serum albumin (HSA) and ovalbumin, introduced into the isolated lungs as an aerosol, entered the pulmonary circulation antigenically intact. The "inhaled" proteins were also broken down in the lung. When lungs from animals immunized with one protein inhaled the two proteins simultaneously, absorption of intact antigen was specifically reduced, and there was a nonspecific increase in the appearance of metabolites of both proteins in the blood. In the present study, we investigated the antigen-specific and nonspecific effects of two types of hypersensitivity responses on protein absorption across the air-blood barrier of isolated rabbit lungs. In one group of lungs, an acute hypersensitivity response was induced by introducing HSA into the blood perfusing lungs from HSA-immunized rabbits. In another, the rabbits had been previously exposed to chronic HSA aerosol until their lungs exhibited a chronic immunologic inflammatory response. Lungs from both groups were insufflated simultaneously with HSA, and a nonspecific protein, ovalbumin. Lungs in which the acute anaphylactic response was induced showed no alteration in the absorption of either intact protein compared with HSA-immunized controls, but absorbed a somewhat larger quantity of breakdown products of the specific antigen. Lungs undergoing the chronic alveolar inflammation were more permeable to nonspecific protein than were noninflamed lungs. Despite the increased permeability to nonspecific protein, the absorption of antigen was blocked as effectively as in immune but noninflamed controls. In these chronically inflamed lungs, the absorption of antigen breakdown products was enhanced. The results indicate that both immunologic and inflammatory mechanisms may control the amounts of inhaled soluble proteins that reach the blood via the alveolocapillary barrier. Alterations in the absorption of inhaled proteins and their metabolites across the air-blood barrier during certain types of hypersensitivity responses may be of immunologic and pathologic significance.

Absorption of inhaled antigen into the circulation of isolated lungs from normal and immunized rabbits.

The purpose of this study was to determine whether the absorption of inhaled antigen (Ag) across the pulmonary air-blood barrier of the isolated perfused lung can be modulated by immunologic mechanisms. Lungs from immunized or nonimmunized rabbits were removed, ventilated, and perfused with autochthonous blood. Radioiodinated Ag (human serum albumin or ovalbumin) was introduced as an aerosol into the isolated lung for 15 min and blood samples were taken over a 4-h period. The results showed that radioactivity fom inhaled Ag entered the perfusing blood as two fractions. One fraction was precipitable by 5% trichloroacetic acid or antiserum. The TCA-soluble fraction chromatographed differently from iodide and may have represented metabolites of the Ag. Immunization specifically reduced the amount of antigenically intact protein entering the blood. On the other hand, the metabolite reached higher concentrations in the blood of immunized lungs. We conclude that the alveolar capillary barrier of the normal rabbit lung could provide a significant route of entry for inhaled antigen into the systemic circulation and that immunization reduces absorption via this route and enhances pulmonary metabolism of the Ag.

Influence of inflation and atelectasis on the hypoxic pressor response in isolated dog lung lobes.

Apnoeic left lower lobes of dog lungs were inflated by increasing alveolar pressure or decreasing pleural pressure, or the lobes were collapsed and exposed to decreasing pleural pressure with the bronchus occluded. Under each of these conditions the lobe could be made 'hypoxic' by perfusion with mixed venous blood or 'normoxic' by perfusion with systemic arterial blood. Inflation of the lobes diminished the hypoxic presor response. The relative influence of decreasing pleural pressure on inflated and collapsed lobes was such that at low pleural pressures resistance to flow through the hypoxic atelectatic lobe was no greater than that through the inflated normoxic lobe. The results indicated that the level of lung inflation can alter the effectiveness of the hypoxic pressor response in reducing perfusion to underventilated regions.

Influence of plasma protein on the inhibitory effects of indocyanine green and bromcresol green on pulmonary prostaglandin E1 extraction.

The purpose of this study was to examine the influence of plasma protein on the inhibitory effects of the anionic dyes indocyanine green and bromcresol green on prostaglandin E1 (PGE1) uptake by the lungs. Dog lung lobes were isolated and perfused with either autologous plasma or Krebs-Ringer bicarbonate solution (KRB) containing no protein but with dextran used as a colloid. PGE1 uptake was determined by injecting a bolus, containing radiolabelled PGE1 into the lobar artery and then analysing ethanolic extracts of the venous effluent for radioactivity in PGE1 and PGE1 metabolites by thin layer chromatography and scintillation counting. When the lobes were perfused with KRB, bromcresol green at an average initial concentration of 28.5 microM, reduced PGE1 by an average of 56%. When the lobes were perfused with plasma, similar concentrations of bromcresol green reduced the uptake by less than 2%. A similar result was obtained with indocyanine green, which at an average initial concentration of 17.5 microM reduced uptake by about 70% when the lobes were perfused with KRB, but when the lobes were perfused with plasma similar concentrations of the dye reduced uptake by less than 3.5%. The results suggest that plasma protein binding interferes with the inhibitory effects of these dyes on PGE1 uptake in the lungs.

Cyclophilin-facilitated bradykinin inactivation in the perfused rat lung.

Cis and trans isomers of X-proline (X-Pro) bonds can influence some aspects of the kinetics of peptide metabolism. We previously used the peptidyl-prolyl cis-trans isomerase, cyclophilin, to show that angiotensin converting enzyme (ACE) preferentially hydrolyzes the trans isomer of a synthetic tripeptide that contains a C-terminal proline (Dawson et al., Am J Physiol 257:H853-H865, 1989; Merker et al., J Appl Physiol 75: 1519-1524, 1993). Bradykinin (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) exists as both cis and trans isomers at all three X-Pro bonds, and although its inactivation in the lung by pulmonary endothelial peptidases is extensive, commonly a small fraction of the peptide survives passage through the lung. To determine whether the presence of cis X-Pro bonds might limit the extent of bradykinin metabolism in the lung, we studied inactivation of bradykinin by the isolated perfused rat lung using the rabbit jugular vein superfused with the pulmonary venous effluent as a bioassay for bradykinin. A large fraction (> 90%) of the bradykinin in a bolus injection was inactivated in a single transit through the pulmonary circulation, but a detectable fraction emerged in the venous effluent. The addition of cyclophilin to the bradykinin in the bolus reduced the bradykinin emerging from the lungs to virtually undetectable levels. When the isomerase inhibitor cyclosporin A was included with bradykinin and cyclophilin in the injectate, this effect of cyclophilin was reversed. These observations suggest that the fraction of bradykinin that normally survives passage through the lungs contains isomers that have at least one X-Pro bond that is refractory to enzymatic inactivation and whose isomerization time constant is significantly longer than the pulmonary capillary transit time.

Lung perfusion with chemotherapy in patients with unresectable metastatic sarcoma to the lung or diffuse bronchioloalveolar carcinoma.

Eight patients with metastatic sarcoma to the lung (n = 4) or diffuse bronchioloalveolar carcinoma of the lung (n = 4) underwent isolated lung perfusion with chemotherapy in a pilot study. Ages ranged from 18 to 60 years and half were female. The left lung was perfused in three patients (single lung perfusion) and both lungs in five patients (total lung perfusion). Perfusions ranged from 45 to 60 minutes at ambient or normothermic temperatures. One patient received perfusion at moderate hyperthermia (40 degrees C). Escalating doses of doxorubicin (1 to 10 micrograms/ml perfusate) was used in six patients, whereas two received cisplatin (14 and 20 micrograms/ml perfusate). There were two major complications and no objective responses. The isolated perfusion systems gave excellent separation between systemic and pulmonary circulations with zero to 15% of the measured peak drug concentration of the pulmonary perfusate detected in the systemic circulation. Drug concentrations in normal lung and tumor generally increased with higher drug dosages and drug was detectable in mediastinal lymph nodes of three out of four patients in whom sampling was done. Isolated lung perfusion with chemotherapy can be done safely in patients with lung malignancies and evidence suggests that higher drug dosages should be well tolerated.

Tolerance of the isolated perfused lung to hyperthermia.

With the use of in vivo isolated lung perfusion for targeting antitumor therapy in the treatment of lung cancer, tolerance of normal lung tissue to the tumoricidal conditions becomes the limiting factor. This study was performed to determine the short-term tolerance of the lung to hyperthermia. Isolated dog lung lobes were perfused with autologous blood or an artificial salt solution at constant flow. Measurements of lung weight, extravascular water, vascular volume, serotonin uptake, urea permeability surface area product, perfusion pressure, and lung compliance were made with the temperature at about 37 degrees C. The temperature was then set at between 37 degrees and 45 degrees C, and at the end of the subsequent 2 hours the measurements were repeated. When the temperature was less than about 44.4 degrees C, hyperthermia had no detectable influence on the measured variables. Thus on the time frame consistent with in vivo perfusion therapy the normal lung appears to tolerate a fairly severe hyperthermia.

Systemic lobar shunting induces advanced pulmonary vasculopathy.

OBJECTIVES: We characterized the morphology and vasomotor responses of a localized, high-flow model of pulmonary hypertension. METHODS: An end-to-side anastomosis was created between the left lower lobe pulmonary artery and the aorta in 23 piglets. Control animals had a thoracotomy alone or did not have an operation. Eight weeks later, hemodynamic measurements were made. Then shunted and/or nonshunted lobes were removed for determination of vascular resistance and compliance by occlusion techniques under conditions of normoxia, hypoxia (FIO (2) = 0.03), and inspired nitric oxide administration. Quantitative histologic studies of vessel morphology were performed. RESULTS: Eighty-three percent of animals having a shunt survived to final study. Aortic pressure, main pulmonary artery and wedge pressures, cardiac output, blood gases, and weight gain were not different between control pigs and those receiving a shunt. Six of 9 shunted lobes demonstrated systemic levels of pulmonary hypertension in vivo. Arterial resistance was higher (24.3 +/- 12.0 vs 1.3 +/- 0. 2 mm Hg. mL(-1). s(-1), P =.04) and arterial compliance was lower (0. 05 +/- 0.01 vs 0.16 +/- 0.03 mL/mm Hg, P =.02) in shunted compared with nonshunted lobes. Hypoxic vasoconstriction was blunted in shunted lobes compared with nonshunted lobes (31% +/- 13% vs 452% +/- 107% change in arterial resistance, during hypoxia, P <.001). Vasodilation to inspired nitric oxide was evident only in shunted lobes (34% +/- 6% vs 1.8% +/- 8.2% change in arterial resistance during administration of inspired nitric oxide, P =.008). Neointimal and medial proliferation was found in shunted lobes with approximately a 10-fold increase in wall/luminal area ratio. CONCLUSIONS: An aorta-lobar pulmonary artery shunt produces striking vasculopathy. The development of severe pulmonary hypertension within a short time frame, low mortality, and localized nature of the vasculopathy make this model highly attractive for investigation of mechanisms that underlie pulmonary hypertension.

Activation by systemic angiotensin II of neurochemically identified neurons in rat hypothalamic paraventricular nucleus.

Using the immunohistochemical localization of the protein product of the immediate early gene, c-fos, to localize activated neurons in the paraventricular nucleus of the hypothalamus (PVN), we studied the chemical phenotypes of neurons activated by circulating angiotensin II (AII). We determined the proportions of activated PVN neurons that expressed AII type I receptor-like immunoreactivity (AT1-L) or the neurohormones vasopressin (VP) and oxytocin (OXY). In addition, we identified activated PVN neurons that putatively produce nitric oxide (NO) on the basis of histochemical staining for nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d). Conscious rats received intravenous AII infusions at a rate sufficient to elevate mean arterial pressure by 40-60 mmHg for 90 min; control rats received infusions of vehicle. Brains were prepared for double immunohistochemistry [Fos-like immunoreactivity (FLI)/AT1-L, FLI/VP or FLI/OXY] or FLI/ NADPH-d histochemistry. Systemic AII infusions led to activation of 149+/-14 PVN neurons per section. In contrast, control animals showed activation of 21+/-6 PVN neurons per section. AII infusions elicited the activation of the following numbers of chemically identified PVN neurons per section: AT1-L, 24+/-5; VP, 26+/-5; OXY, 11+/-2; NADPH-d, 22+/-4. Control animals had few activated PVN neurons per section. For each of the chemically identified populations of PVN neurons, the following proportions were activated: AT1-L, 12.5%; VP, 15.2%; OXY, 7.2%; NADPH-d, 17.3%. The results suggest that PVN neurons producing the AT1 receptor, VP, OXY, and NO, participate in the mediation of the central responses to circulating AII.

Late response to whole-lung irradiation alone and with whole-body hyperthermia in dogs.

The late effects of whole-lung irradiation with and without whole-body hyperthermia were studied in beagle dogs. The reference doses ranged from 18 to 49.5 Gy given in 1.5-Gy fractions over 6 weeks. Whole-body hyperthermia was given in three 2-h treatments to a deep rectal temperature of 42.0 degrees C. Radiation was given simultaneously with hyperthermia on those days. Physiological and histopathological responses were evaluated. Physiological changes included decreases in cardiac output, systemic blood pressure, dynamic compliance and serotonin uptake. Early changes included an increase in extravascular water and total protein in the lavage. These changes were considered mild, were compensated for and occurred only in dogs receiving doses of 40.5 Gy or greater given in 1.5-Gy fractions over 6 weeks. Histopathological changes were typical of irradiated lung and included pleural fibrosis, interstitial fibrosis, fibrotic foci, and peribronchial and perivascular fibrosis. There was no enhancement of late injury to lung by hyperthermia seen in this study.

Evaluation of Mast-ID 15 system for identifying Enterobacteriaceae, some Vibrionaceae, and Acinetobacter.

Six hundred and twenty one strains (555 Enterobacteriaceae, 46 Vibrionaceae, and 20 Acinetobacter) were examined in the Mast system. The results were consulted in the code book supplied by the manufacturer and those not listed were processed through the manufacturer's full database held on an Apricot microcomputer in our laboratory. The proportion of strains correctly identified was 88%, with 9% not identified, and 3% incorrectly identified.

Misuse and interlaboratory test reproducibility of API 20E system.

One hundred strains were referred to us for identification because they apparently could not be identified satisfactorily with the API 20E system (appareils et procédés d'identification). The inability to identify 31 strains was due primarily to failure to follow the manufacturer's instructions. Twenty six further strains were found to have been correctly identified by the sender's own API 20E results, so that only the remaining 43 strains definitely fell into the category for which our identification service was intended. Eighteen of the 43 strains not identified by the sender were identified by us using the API 20E system, and several possible reasons are given to explain the differences in these results. The remaining 25 strains either could not be identified by us on the API 20E system or, in the case of 13, they could not be identified by our conventional system and therefore no comparison could be made. The average interlaboratory probability of errors for the API 20E tests was 6.1%.

Isolated total lung perfusion as a means to deliver organ-specific chemotherapy: long-term studies in animals.

The objectives of this study were to develop a surgical procedure that would allow for bilateral isolated lung perfusion in vivo as a means of delivering organ-specific chemotherapy and to evaluate the influence of the procedure on certain pulmonary physiologic parameters. The sterile surgical procedure that was carried out in dogs involved the setting up of two separate perfusion circuits. Once standard systemic cardiopulmonary bypass was established, a second circuit was devised to perfuse the lungs by placing an inflow cannula into the main pulmonary artery and collecting venous effluent in the left atrium. Cross-contamination between perfusion circuits was determined in acute studies with labeled plasma protein or red blood cells and was found to be in an acceptable range if the aorta was cross-clamped and the heart arrested. Only about 0.4 ml/min of pulmonary perfusate leaked into the systemic circulation, indicating that systemic toxicity should not be a major concern when chemotherapy agents are added to the pulmonary perfusate. Chronic studies demonstrated that hemodynamic parameters, lung water, pulmonary endothelial serotonin extraction, and histologic findings all showed minimal changes after 50 minutes of isolated lung perfusion. Five days after perfusion, lung dynamic compliance and peak serotonin extraction showed significant decreases. However, all of the measured parameters had returned toward baseline levels by the end of the 8-week postoperative study period. The procedure offers significant advantages over the previously described single lung perfusion and may provide a method of delivering immediate high-concentration adjuvant chemotherapy to coincide with resection of primary or metastatic lung tumors.

Influence of size of emboli on extravascular lung water.

We examined the influence of the size of emboli on the vascular volume (QL) and extravascular volume (Qev) accessible to 3HOH during a single pass through an isolated dog lung lobe using the double indicator-dilution method with 125I-human serum albumin as the vascular indicator. As successively more beads of a given diameter (58, 548, or 3,175 microns) were introduced into a lung lobe, a linear relationship between QL and Qev was obtained as they both decreased. The slope of the graph of QL vs. Qev with progressive embolism was directly proportional to the bead diameter. This suggested an approach for estimating the total vascular volume in vessels smaller than the diameter of the beads before embolization, referred to as Qm. If it is assumed that most of the transvascular diffusional exchange of 3HOH occurs in vessels smaller than the smallest beads (mainly capillaries) and that vessel obstruction does not change the ratio of Qev to the perfused capillary volume, the slope of the plot of QL vs. Qev is an estimate of the fraction, Qm/QL, of the total vascular volume in vessels smaller than the bead diameter. In the dog lung lobes studied, Qm/QL was approximately 0.64 for 58-microns vessels, 0.75 for 548-microns vessels, and 0.82 for 3,175-microns vessels. The results suggest that, with occlusion of vessels greater than or equal to 58 microns, 3HOH does not diffuse significantly into unperfused regions.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for analysis of pulmonary arterial and venous occlusion data.

Recently, we presented a compartmental model of the pulmonary vascular resistance (R) and compliance (C) distribution with the configuration C1R1C2R2C3 (J. Appl. Physiol. 70: 2126-2136, 1991). This model was used to interpret the pressure vs. time data obtained after the sudden occlusion of the arterial inflow (AO), venous outflow (VO), or both inflow and outflow (DO) from an isolated dog lung lobe. In the present study, we present a new approach to the data analysis in terms of this model that is relatively simple to carry out and more robust. The data used to estimate the R's and C's are the steady-state arterial [Pa(0)] and venous [Pv(0)] pressures, the flow rate (Q), the area (A2) encompassed by Pa(t) after AO and the equilibrium pressure (Pd) after DO, and the average slope (m) of the Pa(t) and Pv(t) curves after VO. The following formulas can then be used to calculate the 2 R's and 3 C's: [Pa(0) - Pv(0)]/Q = R1 + R2 = RT, R1C1 congruent to to A2/[Pa(0) - Pd], R1 congruent to [Pa(0) - Pd]/Q, Q/m = C1 + C2 + C3 = CT, and C2 = CT - (RTC1/R2).

A fractal continuum model of the pulmonary arterial tree.

The extant morphometric data from the intrapulmonary arteries of dog, human, and cat lungs produce graphs of the log of the vessel number, (N) or length (l) in each level vs. the log of the mean diameter (D) in each level that are sufficiently linear to suggest that a scale-independent self-similar or fractal structure may underlie the observed relationships. These data can be correlated by the following formulas: Nj = a1Dj-beta 1, and lj = a2Dj beta 2, where j denotes the level (order or generation) number measured from the largest vessel at the entrance to the arterial tree to the smallest vessel at the entrance to the capillary bed. With the hemodynamic resistance (R) represented by Rj = 128 microliterj/(Nj pi Dj4) and the vascular volume (Q) by Qj = Nj pi Dj2lj/4, the continuous cumulative distribution of vascular resistance (Rcum) vs. cumulative vascular volume (Qcum) (where Rcum and Qcum represent the total resistance or volume, respectively, upstream from the jth level) can be calculated from [formula: see text] where r = Dj/Dj+1 is a constant independent of j. Analogous equations are developed for the inertance and compliance distributions, providing simple formulas to represent the hemodynamic consequences of the pulmonary arterial tree structure.

Cat lung hemodynamics: comparison of experimental results and model predictions.

Commonly, attempts have been made to learn about the structure and function of the pulmonary vascular bed from measurements of arterial and venous pressures and blood flow rate under steady-state conditions (e.g., from pressure vs. flow data) or dynamic conditions (e.g., from vascular occlusion data). Zhuang et al. (J. Appl. Physiol. 55: 1341-1348, 1983) have presented a detailed model of steady-state cat lung hemodynamics based on direct measurements of anatomical and elasticity data. This model provides an opportunity to better understand the information content of the hemodynamic data. Therefore, in the present study we carried out a series of steady-state and dynamic experiments on isolated cat lungs. We then compared the results with those predicted by the model. We found that the model provided a good fit to the steady-state data. However, to fit the dynamic data, some modifications were necessary to account for the viscous behavior of the vessel walls and to move the first moment of the distribution of vascular resistance toward the arterial end of the vascular bed relative to that of the distribution of vascular compliance. Due to the sensitivity of the vascular resistance to small changes in vessel diameters and branching ratio, the modifications in morphometry represent small changes in morphometric data and are probably within the range of uncertainty in such data. The modifications had little effect on the steady-state model simulations but substantially improved the dynamic model simulations, suggesting that the dynamic data are quite sensitive to small changes in the relative distributions of vessel diameters and elasticity.

Impact of parallel heterogeneity on a continuum model of the pulmonary arterial tree.

Model arterial trees were constructed following rules consistent with morphometric data, Nj = (Dj/Da)-beta 1 and Lj = La(Dj/Da)beta 2, where Nj, Dj, and Lj are number, diameter, and length, respectively, of vessels in the jth level; Da and La are diameter and length, respectively, of the inlet artery, and -beta 1 and beta 2 are power law slopes relating vessel number and length, respectively, to vessel diameter. Simulated heterogeneous trees approximating these rules were constructed by assigning vessel diameters Dm = Da[2/(m + 1)]1/beta 1, such that m-1 vessels were larger than Dm (vessel length proportional to diameter). Vessels were connected, forming random bifurcating trees. Longitudinal intravascular pressure [P(Qcum)] with respect to cumulative vascular volume [Qcum] was computed for Poiseuille flow. Strahler-ordered tree morphometry yielded estimates of La, Da, beta 1, beta 2, and mean number ratio (B); B is defined by Nk + 1 = Bk, where k is total number of Strahler orders minus Strahler order number. The parameters were used in P(Qcum) = Pa [formula: see text] and the resulting P(Qcum) relationship was compared with that of the simulated tree, where Pa is total arterial pressure drop, Q is flow rate, Ra = (128 microLa)/(pi D4a (where mu is blood viscosity), and Qa (volume of inlet artery) = 1/4D2a pi La. Results indicate that the equation, originally developed for homogeneous trees (J. Appl. Physiol. 72: 2225-2237, 1992), provides a good approximation to the heterogeneous tree P(Qcum).

Hypoxia-induced activation in small isolated pulmonary arteries from the cat.

Effects of hypoxia on force development and membrane potential were studied in isolated small (less than 300 microns diam) and large (greater than 500 microns diam) pulmonary arteries from cats. There was a consistent and reproducible hypoxic constrictor response in small pulmonary arteries that began at PO2 values between 350 and 300 Torr and reached a maximum at PO2 between 50 and 30 Torr. In the small artery smooth muscle cell the membrane potential, which was -51 +/- 1.4 mV at a PO2 of 400 Torr, was depolarized to -37 +/- 2 mV at a PO2 of 50 Torr. In contrast, larger arteries did not exhibit significant hypoxic constriction or depolarization upon exposure to low PO2. Constriction in small arteries was not blocked by phentolamine. Treatment with a low dose of indomethacin (10(-9) M) augmented the response; however, a larger dose of indomethacin (10(-3) M) blocked the constriction to hypoxia but not to 30 mM KCl. Depolarization during hypoxia was not blocked by ouabain. Results of this study suggest that the hypoxic response of these isolated small pulmonary vessels may be like that seen in the intact lung. Furthermore, these data suggest that hypoxic vasoconstriction may be mediated by electrical events occurring at the pulmonary arterial muscle cell membrane either directly or via mediators released from the vessel wall.

Location and mechanisms of pulmonary vascular volume changes.

We examined the influence of changing outflow pressure, P out, on the vascular and extravascular volumes (QV and QEV, respectively, as measured by indicator dilution) and on the outflow occlusion pressures in isolated dog lung lobes perfused with constant flow. Changing P out had a substantial effect on QV, but not on QEV, whether P out was less than or greater than alveolar pressure, PA. Since QEV did not change with QV, recruitment of previously unperfused vessels did not appear to contribute substantially to the increases in QV when P out was increased. The rapid jump in P out immediately following outflow occlusion was virtually independent of the difference between PA and P out suggesting that the alveolar vessels were an important volume storage site when P out was low relative to PA. We conclude that, over a certain range of pressures, alveolar vessel volume can be controlled by venous pressure even when the change in venous pressure has little effect on arterial pressure (zone 2). Further, we conclude that in zone 3 and within the transition from zone 2 to zone 3 increases in the intralobar blood volume occurring within the alveolar vessels may not require recruitment in the sense of opening of previously unperfused vessels.