Comparison of diffusion and perfusion limitations in alveolar gas exchange.
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Animals↗
Biomedical subjects
P Scheid
Publications and source records attributed to P Scheid.
Gas mixing in the airways of dog lungs during high-frequency ventilation.
Washout of insoluble inert test gases of different diffusivity (He and SF6 or He and Ar) from dog lungs was studied during high-frequency ventilation (HFV). Test gas equilibrium and subsequent washout were performed with HFV, succeeding measurements being performed at different stroke volumes (1.5-2.5 ml/kg body wt), oscillation frequencies (10-30 Hz), and with different lung volumes (32-74 ml X kg-1). Test gas concentrations were continuously measured by a mass spectrometer. The time course of washout could be described as the sum of two exponentials. There were no consistent differences in the time courses of washout between He and SF6 or between He and Ar. It is concluded that gas mixing in the airways during HFV is not significantly limited by diffusion, and this is suggested to apply during HFV to steady-state transport of respiratory gases (e.g., O2 and CO2) as well as to the transient state of inert gas washout.
Animals↗
Buffering and CO2 dissociation of body fluids in the pupa of the silkworm moth, Hyalophora cecropia.
To assess the extent of CO2 storage and the changes in the acid-base status that occur during intermittent CO2 excretion in insects, total CO2 content and pH were measured in whole body homogenates (= tissue homogenates) of Hyalophora cecropia pupae at various levels of Pco2 at 20 degrees C. The CO2 dissociation curve, i.e. plot of total CO2 content in tissue homogenates against Pco2 was nearly linear in the Pco2 range from 15 to 50 Torr, the mean slope being 0.138 mM . Torr-1. This value, which constitutes the effective CO2 solubility, was nearly three times the physical solubility in the tissue homogenate which averaged 0.053 mM . Torr-1. Plots of bicarbonate concentration in whole body tissue water against pH yielded an average buffer value of 75 mmol . pH-1 per kg tissue water. The high buffer value results in a small pH change, about 0.04 units, when Pco2 varies between 20 and 45 Torr in the respiratory cycle. The absolute value of mean tissue pH at Pco2 = 30 Torr predicted from the buffer line, 6.57, agrees well with direct measurement in hemolymph samples.
Animals↗
Intrapulmonary CO2 receptors and control of breathing in ducks: effects of prolonged circulation time to carotid bodies and brain.
The role of intrapulmonary chemoreceptors in the breath-to-breath control of spontaneous breathing was studied in anesthetized ducks by stimulating these receptors with changes in mixed venous CO2 loads during prolonged circulation time to the carotid bodies and brain by vascular loops placed in both brachiocephalic arteries. Blood equilibrated with gas mixtures of high (85% CO2-15% O2) or low CO2 (air) was infused into the right ventricle at 100 ml . min-1, while simultaneously withdrawing blood from the entrance of the right atrium at the same rate. A variety of cardiopulmonary and blood gas variables were measured. Infusing blood of high PCO2 increased both respiratory frequency and tidal volume long before the altered blood could have reached the carotid bodies or brain. The increase in ventilation was not enough to prevent a rise in PaCO2. Infusing blood of low PCO2 decreased both respiratory frequency and tidal volume. Again, the changes in respiration occurred before the infused blood had reached the carotid bodies or the brain. Infusion of blood similar in PCO2 to mixed venous blood did not significantly alter ventilation or arterial blood gases. The rapidity of the ventilation response to a change in mixed venous CO2 load led us to conclude that the intrapulmonary chemoreceptors can detect changes in mixed venous CO2 loads and that they initiate a ventilatory change appropriate to minimize alterations in PaCO2. These receptors, thus, can control breathing on a breath-to-breath basis in birds.
Animals↗
A method for localizing intrapulmonary chemoreceptors in the parabronchial mantle of the duck.
1. A method is presented for estimating the location of avian intrapulmonary chemoreceptors within the parabronchial mantle. 2. By determining the discharge frequency of a receptor at known receptor site Pco2's in a nonventilated but perfused lung, the receptor discharge could be calibrated to indicate the receptor site Pco2 during both ventilation and perfusion. 3. The relation among receptor site Pco2, mixed venous Pco2 and inspired Pco2 may be compared with calculated Pco2 profiles along the contact between air capillaries and blood capillaries and the receptor location may be determined as the relative distance between the luminal and peripheral ends of the air capillaries. 4. Of four receptors at the caudal end of a parabronchus, two were located at the terminal end of the air capillary and two along the peripheral half of the air capillary.
Animals↗
Blood-gas CO2 equilibration in lungs of unanesthetized dogs during hypercapnia.
We have reinvestigated the problem of blood-gas equilibration of CO2 in lungs during hypercapnia. Six dogs with chronic tracheostomy and exteriorized carotid artery were subjected in acute experiments to hypercapnic inspired mixtures [CO2 fraction of expired gas (FICO2) = 0.06; 0.08; 0.10]. Expired CO2 partial pressure (PCO2) was continuously measured with a respiratory mass spectrometer and compared with arterial PCO2 determined in blood samples that were collected during apparent steady-state conditions. Particular care was taken in using continuously recorded temperature in the right heart for correction of blood PCO2 measured by CO2 electrodes. In no animal was there a significant difference between arterial and alveolar PCO2. On the average, this difference was -0.1 Torr at FICO2 = 0.06; 0.0 Torr at FICO2 = 0.08, and -0.2 Torr at FICO2 = 0.10. The results are in agreement with the conventional view that PCO2 in pulmonary capillary blood approaches PCO2 in alveolar gas.
Animals↗
Interactions between lung stretch and PaCO2 in modulating ventilatory activity in dogs.
The effects of changes in airway pressure (Paw) and arterial PCO2 (PaCO2) on ventilatory activity were studied in anesthetized thoracotomized dogs in which both lungs were ventilated separately. Pulmonary artery occlusion on one side and contralateral vagotomy allowed the reflex effects on ventilation of changes in Paw and PaCO2 to be elicited independently of each other. Ventilatory activity was assessed from integrated efferent phrenic activity, analyzed with respect to burst amplitude (Phr), burst frequency (f), and inspiratory TI) and expiratory duration (TE). While Phr increased linearly with PaCO2, it was independent of Paw. Both PaCO2 and Paw affected f in a complex nonadditive way; this response was entirely mediated by effects on TE, TI being unaffected by either stimulus. The analog of ventilation, estimated as Phr x f, increased linearly with PaCO2 and decreased linearly with Paw, but the effects of both stimuli appeared to be additive. It is concluded that the apparently simple effect of Paw and PaCO2 on ventilation results from more complex effects these stimuli exert on its components.
Airway Resistance↗
Models for a comparative functional analysis of gas exchange organs in vertebrates.
The analysis of external gas exchange in the various respiratory organs of vertebrates is based on models with gas transport properties determined by ventilatory, diffusive, and perfusive conductances and by the geometric arrangement of medium and blood flows. The following factors are examined: water vs. air as external medium; gas transport properties of blood; diffusive gas transfer between medium and blood; problems in assessing diffusion limitation in fish gills, amphibian skin, avian lungs, and mammalian lungs. Finally the limitations to the analysis imposed by various physiological and anatomical complexities are discussed.
Amphibians↗
Diffusion in gas exchange of insects.
The air-filled tracheal system constitutes the organ for gas exchange in terrestrial insects-its finest branches, the tracheoles, contacting individual cells. In the pupal stage, in which the animal lacks significant ventilatory movement, diffusion in the gas phase of the tracheal system constitutes the only mechanism for gas transfer between the environment and the tissues, transport in the hemolymph being insignificant. We have attempted to identify the main sites of diffusional resistance in the tracheal gas system by measuring the evolution of inert gases of low solubility from the pupa of the giant silkworm moth (Hyalophora cecropia). The results are compatible wih a single model in which the resistance to diffusional gas transfer in the tracheal system is concentrated at its opening at the body surface (spiracle).
Animals↗
Role of diffusion in the external gas exchange of animals.
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Amphibians↗
Inert gas elimination from lungs with stratified inhomogeneity: theory.
The effects of diffusion limitation in alveolar space (stratified inhomogeneity) on steady state inert gas elimination by the lung from blood are studied in theory. The lung models used for the calculations are characterized by a diffusion barrier in the alveolar space, which is quantified by a diffusive conductance (diffusing capacity), D'. In both homogeneous and inhomogeneous lung models (with log-normal distribution of VA, Q and D') increasing stratified inhomogeneity (decreasing D') is shown to diminish inert gas elimination. The effect, which depends on the solubility of the gases, is similar to the effect of alveolar dead space ventilation.
Lung↗
Airflow in the avian respiratory tract: variations of O2 and CO2 concentrations in the bronchi of the duck.
Variations of CO2 and O2 concentrations within a respiratory cycle were recorded at various sites in the bronchial system of anesthetized, spontaneously ventilating ducks, using small metal cannulae introduced into the main bronchus (MB), a medioventral (MV) or mediodorsal (MD) secondary bronchus and connected to a mass spectrometer for continuous gas analysis. The following results were obtained and conclusions drawn. (1) Since during inspiration, CO2 concentration (FCO2) was close to zero all along MB and since FCO2 was nearly constant throughout the respiratory cycle in MV, it must be inferred that on inspiration, no significant amount of air passes directly either from MV to MB or in the opposite direction, there being thus a complete functional valving of the MV orifices. In particular the Hazelhoff loop mechanism (inspiratory reflux of lung gas into the MB) is not operative. (2) During expiration, FCO2 in MV was only slightly higher than that in the trachea, but substantially above FCO2 deep in MB. This suggests that most of the expiratory flow from caudal air sacs is diverted through the paleopulmo and only little exits directly through MB. It is shown that the functional valving of bronchial air flow is advantageous for gas exchange as it reduces air shunts and provides a nearly steady lung ventilation.
Animals↗
Model for capillary-alveolar equilibration with special reference to O2 uptake in hypoxia.
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Capillaries↗
Interpretation of inert gas retention and excretion in the presence of stratified inhomogeneity.
The effects of diffusion limitation in alveolar space (stratification) for inert gas retention and excretion by lungs with log-normal VA/Q distribution are calculated using the approach of Scheid et al. (Respir. Physiol. 44, 299-309, 1981). Since gases used in the multiple inert gas elimination technique have widely varying molecular weights (between 30 and 197), and therefore varying diffusivities, the effects of stratification are different for each gas. The result is a perturbation in the recovered VA/Q distributions that is calculated neglecting stratification effects. Application to inert gas elimination data obtained in the anesthetized rat by Truog et al. (J. Appl. Physiol. 47, 1112-1117, 1979) yields a value for the diffusion resistance in alveolar gas which would give rise to a PO2 difference of 5 Torr, suggesting that stratification may exert a significant limitation to pulmonary O2 transfer in rats.
Animals↗
Pulmonary diffusing capacity for CO independent of alveolar CO concentration.
Pulmonary diffusing capacity for CO (DCO) was measured in human subjects at various CO concentrations using a rebreathing procedure. Two stable CO isotopes, 12C18O and 13C18O, were used. These isotopes could be simultaneously and continuously recorded by a respiratory mass spectrometer. For 13C18O the initial concentration in the rebreathing bag was constant at 0.00016, whereas it was varied for 12C18O from 0 to 0.00224. DCO was calculated for both isotopes. In five normal subjects, both D12C18O and D13C18O were independent of the rebreathing CO concentration, with mean values of 31.0 and 30.2 ml . min-1 . Torr-1, respectively. These results, which are at variance with those of Mendoza et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43: 880-884, 1977), are compatible with the assumption that diffusion is the sole mechanism of alveolocapillary CO transport; in particular, there is no evidence for facilitated transport.
Adult↗
Pulmonary diffusion capacities for O2 and CO measured by a rebreathing technique.
Pulmonary diffusion capacity (D) for O2 and CO was determined from alveolar-mixed venous equilibration kinetics of 16O2, 18O2, and C18O measured during rebreathing by mass spectrometry. During the rebreathing maneuver (15 s) the ventilation was extremely high (about 100 1 X min-1) and PO2 and PCO2 in lung gas were close to their mixed venous values (aveolar hypoxia and hypercapnia). The following mean values (+/- SD) were found in six healthy males (20-33 yr) sitting on a bicycle ergometer (in ml X min-1 X Torr-1): 1) without work load, D18O2 = 54 +/- 10, Dc18O = 47 +/- 11; 2) with the highest work load tested (150 W, O2 uptake 2.1 l X min-1), D18O2 = 62 +/- 12, Dc18O = 54 +/- 8. The ratio D16O2/D18O2 averaged 1.07. The ratio D18O2/Dc18O, averaging 1.2 at rest and at all exercise levels, was close to the estimated O2/CO ratio of Krogh's diffusion constants for tissue and, therefore, was in agreement with the diffusion limitation model. An analysis of the various factors affecting the DO2/DCO ratio does not allow to reliably assess the role of diffusion in red blood cells and reaction with hemoglobin in limiting alveolar-capillary O2 transfer.
Adult↗
Blood/gas equilibrium of carbon dioxide in lungs. A critical review.
(1) The scope of this review is to examine the experimental evidence for the existence of negative PCO2 differences between pulmonary capillary blood and lung gas, [delta PCO2(b-G)], which have been observed both during rebreathing, when CO2 was at equilibrium, and during steady state gas exchange, particularly in hypercapnia. (2) The mechanism that have been invoked to explain negative delta PCO2(b-G) include (i) slow equilibration of the system CO2/HCO3-/H+ in blood, and (ii) effects of a negatvely charged surface of the pulmonary capillary endothelium. While the first postulated mechanism appears to be quantitatively insufficient to explain the results, the second seems to lead to serious qualitative difficulties. (3) Existence of negative delta PCO2(b-G) in CO2 equilibrium would invalidate the basis of the conventional analysis of alveolar gas exchange. (4) A critical analysis of the experimental evidence for the existence of negative delta PCO2(b-G) is presented. It includes the identification of directional experimental errors leading to spurious negative delta PCO2(b-G), and a critical review of the literature data in this regard. (5) Results of own experiments, conducted in an attempt to consider all possible sources of error, are reported, revealing (i) perfect PCO2 equality between alveolar gas and blood in rebreathing equilibrium of CO2; (ii) absence of negative delta PCO2 (b-G) during steady state gas exchange in hypercapnia. (6)Both experiments and model calculations show that negative delta PCO2 between mixed venous blood and end-expired gas observed in birds at steady state of gas exchange are explained by a particular action of the Haldane effect in avian parabronchial lungs with cross-current arrangement of gas and blood flow. (7) It is concluded that the negative delta PCO2(b-G) reported in the literature are probably artifactual and that there is no adequate evidence to invalidate the traditional view according to which blood/gas CO2 equilibration in lungs leads to equal PCO2 in both media.
Animals↗
Effects of intrapulmonary CO2 and airway pressure on phrenic activity and pulmonary stretch receptor discharge in dogs.
Intrapulmonary CO2 and stretch sensitivity were studied in anesthetized, thoracotomized dogs in which both lungs were independently ventilated. The left pulmonary artery was occluded so that changes in left (Test) lung CO2 did not alter systemic arterial PCO2. Adequate gas exchange was maintained in the right lung, which was vagally denervated. In one series of experiments, neural activity in the C5 root of the phrenic nerve was integrated to assess ventilatory drive at various levels of Test lung CO2 and airway pressure both during cyclic ventilation and static lung inflation. In a second series, single unit activity in vagal afferents from pulmonary stretch receptors (PSR) was recorded. Both phrenic activity and PSR discharge were strongly affected by changes in airway pressure but not by changes in Test lung CO2 between 2 and 7%. However, when test lung CO2 was decreased below 2%, phrenic activity decreased and PSR activity increased. These changes were invariably accompanied by an increase in peak airway pressure during cyclic ventilation suggesting that lung mechanics had been altered. The results indicate that, in this preparation, the effects of lung stretch on ventilatory drive spans a wide range whereas intrapulmonary CO2 exerts an effect only at very low levels. It appears that the reflex effects of both stimuli can be accounted by an effect on PSR activity.
Afferent Pathways↗