Peter Scheid

Model for capillary-alveolar equilibration with special reference to O2 uptake in hypoxia

Abstract This paper presents a simple model for the study of diffusion limitation effects in capillary-alveolar gas transfer. The decisive parameter to determine the extent of diffusion limitation is shown to be the dimensionless ratio D/(Qβ) (D, diffusing capacity, Q, blood flow; β, capacitance coefficient or effective solubility in blood). The model predicts a considerable extent of diffusion limitation for pulmonary O 2 uptake in hypoxia, which sets an upper limit to the level of exercise at high altitude.

Gas transport efficacy of gills, lungs and skin: Theory and experimental data

The general functional principles encountered in respiratory organs of vertebrates are investigated. Generally three steps are involved in external gas exchange in vertebrates: (1) convective transport by flow of external respiratory medium, air or water (=ventilation); (2) transfer of gas between external respiratory medium and blood by diffusion (=medium/blood transfer); (3) convective transport by blood flow (=perfusion). According to the arrangement of external medium flow relative to capillary blood flow four construction principles may be distinguished: (a) counter-current system (fish gills), (b) cross-current system (avian lungs), (c) ventilated pool system (mammalian lungs), and (d) infinite pool system (amphibian skin). The gas transfer performance of these systems is analyzed in terms of conductances, relative partial pressure differences and limitations attributable to ventilation. to medium/blood transfer and to perfusion. The theory is applied to analysis of gas exchange data obtained in an elasmobranch fish, domestic fowl, dog and a lungless salamander. The analysis shows that, despite distinct differences in maximum efficiencies of these systems, the differences in efficiency values actually attained are much less pronounced, and may be even less marked when taking functional inhomogeneities into account which are neglected in this study.

Analysis of gas exchange in the avian lung: theory and experiments in the domestic fowl.

Abstract Gas exchange in the parabronchial lung of birds is analysed in theory using a model suggested by Zeuthen (1942), consisting of a continuously ventilated tube, representing the parabronchi, surrounded by blood-perfused capillaries. Equations describing how partial pressures of CO2 and O2 in pulmonary gas and blood depend upon ventilation, perfusion and diffusing capacity are derived. The theory was applied to measurements of gas exchange in unanesthetized White Leghorn hens, of 1.6 kg average weight. In order to confine the pulmonary gas exchange to a PCO2 and PO2 range in which the blood dissociation curves were relatively straight, a combination of inspiratory hypoxia and hypercapnia was used ( F i CO 2 = 0.03 , F i O 2 = 0.13 ). Arterial PCO2 was lower and arterial PO2 was higher than the corresponding values in end-expired gas. This finding is explainable by the theory. From the experimental data the pulmonary diffusing capacity for O2 (DLO2) was estimated at 1.1 to 1.5 ml·min−1·mm Hg−1. As the calculations were based on a homogeneous lung model, these values represent a lower limit for dLO2.

Cross-current gas exchange in avian lungs: Effects of reversed parabronchial air flow in ducks☆

Abstract Experiments have been performed in ducks in order to determine which of both previously proposed physical models, the counter-current or the cross-current system, is best suited for analysis of gas exchange in parabronchial lungs of birds. Mesobronchi of both sides of the body were blocked between the origins of ventrobronchi and dorsobronchi using inflatable catheters, and both postthoracic air sacs were opened by introduction of large bore tubing through the body wall into their lumen. In this preparation a continuous parabronchial air flow could be achieved in either of both directions, from ventrobronchi to dorsobronchi by blowing air into the trachea, or in the opposite direction, from dorsobronchi to ventrobronchi, by blowing air from the air sacs into the bronchial system. During steady state, the pulmonary gas flow and PCO2 and PO2 in air as well as in arterial and mixed venous blood were measured. The results showed no dependence of partial pressures on the direction of parabronchial air flow. In particular, arterial PCO2 was found to be lower than expired (=end-parabronchial) PCO2 with both directions of parabronchial air flow. These findings cannot be easily explained on the basis of a counter-current gas exchange system, which has been proposed by Schmidt-Nielsen (1971), but are in good agreement with the assumption of a cross-current gas exchange system operative in the avian lung.

Gas exchange in the domestic fowl during spontaneous breathing and artificial ventilation

In unanesthetized White Leghorn hens, mean weight 1.6 kg, gas exchange was studied during spontaneous breathing and artificial ventilation delivered by a Starling pump. The trachea was cannulated and connected to an open spirometric system. Arterial and mixed venous blood samples were obtained by catheters introduced into a brachial artery and into the right ventricle, respectively. Air sac gas was sampled. During spontaneous breathing the following mean values were measured: total ventilation, 760 ml BTPS·min−1 respiratory frequency, 23 min−1; O2 uptake 24 ml STPD·min−1; arterial PO2, 87 mm Hg; arterial PCO2, 29 mm Hg; cardiac output (Pick Principle), 430 ml·min−1. When the unanesthetized hens were relaxed with succinylcholine and artificially ventilated, using the same tidal volume and frequency as observed during spontaneous respiration, the same values for O2 uptake and for CO2 and O2 partial pressures in arterial blood were measured, indicating unchanged gas exchange efficiency of the respiratory system. However, the gas composition of the air sacs in respect to expired CO2 and O2 was changed, suggesting alterations in the pattern of gas flow through the respiratory system.

Intrapulmonary CO2 receptors in the duck: I. Stimulus specificity☆

Single unit vagal recordings were made from intrapulmonary CO2 receptors in five domestic ducks. Gas, containing various concentrations of CO2, was unidirectionally passed through the respiratory system either from the trachea to the surgically opened, right caudal thoracic air sac or vice versa. Intrapulmonary gas pressure (Pip) and CO2 concentration in the ventilatory gas (FiCO2 were varied independently. The units were identified and studied during unidirectional ventilation and during phasic ventilation with a Starling pump. The static discharge frequency of these receptors increased as Fico, decreased. In most receptors, step changes in FICO2 also produced dynamic responses. The static discharge frequency at various levels of FiCO2 did not significantly diner when Pip was low (about 2 cm H2O) or high (about 10–20 cm H2O). Likewise, the peak dynamic discharge frequency, elicited when FICO2 was suddenly decreased to 0%, was not affected by these levels of Pip. The receptor discharge frequency was also unaltered by transient changes in Pip provided FiCO2 and gas flow remained constant. All receptors exhibited cyclic discharge patterns during pump ventilation and peak frequencies occurred either during inspiration, or during expiration or during both phases of the respiratory cycle. Their discharge frequency decreased when CO2 was added to the inhaled gas. The results indicate that intrapulmonary CO2 receptors are not stretch-sensitive and therefore are not mechanoreceptors. Their cyclic discharge patterns during pump ventilation were similar to those seen during spontaneous breathing and appear to result from changes in intrapulmonary CO2 concentration at the receptor site. These receptors may be able to monitor alterations in intrapulmonary CO2 concentration resulting from changes in the metabolic activity of the bird, and may thereby be involved in the control of breathing.

Direct measurement of the pathway of respired gas in duck lungs

Abstract For measurement of gas flow, a flowmeter consisting of a heated wire anemometer combined with a thermocouple was implanted into a dorsobronchus of the lungs in unanesthetized ducks. The direction of gas flow recorded in the dorsobronchus during spontaneous breathing and during artificial ventilation was from the mesobronchus to the parabronchi both in inspiration and expiration. This result is in accordance with the unidirectional flow hypothesis propounded by Bethe and by Hazelhoff, and is incompatible with the alternating respiratory flow hypothesis of Zeuthen.

Mechanisms of unidirectional flow in parabronchi of avian lungs: Measurements in duck lung preparations☆

Abstract To investigate the pathway of respired air in birds, lungs of ducks, fixed by means of glutaraldehyde, were separated from the air sacs and from all other organs. When this isolated lung was ventilated by applying pressure or suction to the main bronchus, the dorsobronchial flow was shown to be unidirectional in both respiratory phases, leading from the mesobronchus through the dorsobronchi to the parabronchi, as has previously been shown for the living duck. Flow direction could also be measured in other parts of the bronchial tree including ventrobronchi where the measurements were not in agreement with the hypothesis of Hazelhoff (1943). To analyze the mechanisms of this rectification of flow, the aerodynamical properties of some structures of the bronchial tree, especially the openings of the secondary bronchi into the mesobronchus, were investigated. These openings offered a direction-dependent resistance to air flow, presumably due to detachment of flow at sites of sharp edges. These results lend support to the hypothesis that the particular pathway of respired air in duck lungs is effected by “aerodynamical valving”.

Maximum gas transfer efficacy of models for fish gills, avian lungs and mammalian lungs☆

Abstract The performance limits of the following models for vertebrate gas exchange organs have been investigated in theory: (1) counter-current model, for fish gills, (2) cross-current model, for avian lungs, and (3) uniform pool model, for mammalian lungs. In “ideal” conditions, i.e. when diffusion limitation and distributional inhomogeneities are absent, partial pressures of respiratory gases in the external medium and blood leaving the gas exchange organ relative to those in the medium and blood entering it are shown to depend, in a specific manner for each model, on the medium-to-blood conductance ratio X = ( V β) m ( V β) b , (V, flow rate; β, “capacitance coefficient” ; m refers to respiratory medium; b, to blood). Generally, the effectiveness of the models, in terms of extent of gas transfer for same conductance values, increases in the sequence, uniform pool → cross-current → counter-current. The enhanced efficacy of the counter-current and cross-current models occurs only inside a limited range of X values (not far from 1.0). Measurements have shown that in fishes and birds X values for CO2 and O2 are close to their optimum values.

Determination of diffusivity of oxygen and carbon dioxide in respiring tissue: Results in rat skeletal muscle

SummaryGas transfer rates for O2 and CO2 through freshly excised respiring rat abdominal muscle were measured. The tissue separated as a thin membrane two chambers, one of which was ventilated with a constant gas mixture. The other chamber was closed and the time course of changes ofPO2andPCO2, initially set at varied levels, was followed by electrodes. A plot of rate of change ofPO2 andPCO2 in the closed chamber against the partial pressure difference across the tissue yielded both Kroghs diffusion constant,KO2 andKCO2, and metabolic rate of tissue, i. e. specific O2 consumption and CO2 production,