Significance of the Bohr and Haldane effects in the pulmonary capillary

Pulmonary gas exchange and acid-base balance during exercise

As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO2 difference (A-aDO2), worsens with incremental exercise, and typically reaches an A-aDO2 of approximately 25 mmHg at peak exercise. While there is great individual variability, A-aDO2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. Inert gas data has shown that the increase in A-aDO2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. At the same time, multisystem mechanisms regulate systemic acid-base balance in integrative processes that involve gas exchange between tissues and the environment and simultaneous net changes in the concentrations of strong and weak ions within, and transfer between, extracellular and intracellular fluids. The physicochemical approach to acid-base balance is used to understand the contributions from independent acid-base variables to measured acid-base disturbances within contracting skeletal muscle, erythrocytes and noncontracting tissues. In muscle, the magnitude of the disturbance is proportional to the concentrations of dissociated weak acids, the rate at which acid equivalents (strong acid) accumulate and the rate at which strong base cations are added to or removed from muscle.

The blood oxygen transport system. A numerical simulation of capillary-tissue respiratory gas exchange

A theoretical model of the process of respiratory gas exchange between capillary and tissue is described, with special reference to the importance of variations in blood properties. The volume of tissue supplied with oxygen from a single vessel, as a function of the blood flowrate (u), hematocrit (h), and 2,3 diphosphoglycerate concentration (DPG), is calculated from a solution to the set of equations governing species distributions in the blood and tissue. The results, which are presented in the form of crossplots of the three blood parameters (u, h, and DPG) at a constant oxygen supply rate, show the possible significance of in vivo variations in the oxygen affinity of hemoglobin as a compensatory mechanism. Of further physiological interest is the sharp increase in venous erythrocyte pH in response to decreases in hematocrit, once the hematocrit is below a certain level. These results, and those relating DPG to hematocrit at constant O2 supply, are consistent with experimental observations of elevated DPG and pH levels in anemic individualts, and the dependence of erythrocyte DPG concentration upon pH.

A simulation model of the oxygen alveolo-capillary exchange in normal and pathological conditions

This paper presents a mathematical model of the oxygen alveolo-capillary exchange to provide the capillary oxygen partial pressure profile in normal and pathological conditions. In fact, a thickening of the blood-gas barrier, heavy exercise or a low oxygen partial pressure (PO2) in the alveolar space can reduce the O2 alveolo-capillary exchange. Since the reversible binding between haemoglobin and oxygen makes it impossible to determine the closed form for the mathematical description of the PO2 profile along the pulmonary capillaries, an approximate analytical solution of the capillary PO2 profile is proposed. Simulation results are compared with the capillary PO2 profile obtained by numerical integration and by a piecewise linear interpolation of the oxyhaemoglobin dissociation curve. Finally, the proposed model is evaluated in a large range of physiopathological diffusive conditions. The good fit to numerical solutions in all experimental conditions seems to represent a substantial improvement with respect to the approach based on a linear approximation of the oxyhaemoglobin dissociation curve, and makes this model a candidate to be incorporated into the integrated descriptions of the entire respiratory system, where the datum of primary interest is the value of end capillary PO2.

Effect of nitrogen on carbon dioxide elimination during continuous flow apneic ventilation in dogs

Continuous endobronchial insufflation of air in paralyzed animals (continuous flow apneic ventilation - CFAV) has been shown to maintain adequate oxygenation and carbon dioxide removal. CFAV in patients using oxygen resulted in adequate oxygenation but a mean rise in PaCO2 of 0.6 mmHg/min (0.08 kPa/min). This experiment compared carbon dioxide removal in dogs with air and oxygen. Ten dogs were anesthetized and paralyzed, and CFAV was used for 1 h with either air or oxygen in a randomized fashion. Adequate oxygenation was obtained with air and oxygen. Normal PaCO2 levels were obtained with air; however, in the animals where oxygen was used, PaCO2 levels rose to a mean of 6.45 +/- s.e. mean 0.4 kPa (48.5 +/- s.e.mean 3.2 mmHg).

Analysis of the effects of pulsatile capillary blood flow and volume on gas exchange

Blood flow into the pulmonary capillaries and the volume of blood within the capillary bed are both pulsatile with the cardiac cycle. We have developed a quantitative model of diffusional gas exchange in the lung to investigate the effects of coupling between these two time-varying parameters on lung O2 and CO2 exchange. For normal man breathing room air at rest, the computed results agree well with previous predictions for the constant flow and volume case, and for the case of pulsatile flow alone. When coupled time-varying pulmonary capillary blood flow and volume are included, using the best data available in the literature to define these parameters, diffusional O2 exchange is improved over the cases of pulsatile flow or volume alone, and closely approximates that obtained for the hypothetical constant flow and volume case. CO2 exchanges, and O2 exchange during hypoxia, are not affected by pulsatile flow and/or volume. These results suggest that O2 exchange is efficient in the presence of coupled blood flow and blood volume pulsations as they exist in the lung capillaries, and that these conditions may be optimal for gas exchange under certain physiological (or pathological) conditions.

Effects of pulmonary gas embolism on circulation and respiration in the dog. II. Effects on respiration

In mongrel dogs weighing 15--25 kg and anesthetized with thiopental-gamma-hydroxybutyric acid the effects of venous gas infusion and injection on several respiratory variables were investigated. During spontaneous respiration pulmonary gas embolism caused an increase of the ventilatory minute volume depending on the degree of embolization. The contribution of breathing frequency and tidal volume to the increase of ventilatory minute volume varied from one animal to the other. During constant artificial ventilation pulmonary gas embolism impaired the pulmonary gas exchange depending on the degree of embolization. How far a steady-state phase in gas exchange can be reached during continuous venous gas infusion depended on the adaptation of the circulation. At severe degrees of embolization circulation as well as gas exchange became deficient. Increasing artificial ventilation during pulmonary gas embolism improved the wash-out of carbon dioxide, but hardly affected the uptake of oxygen. The most important origin of the disturbed gas exchange in pulmonary gas embolism seemed to be an increased inequality of the ventilation-perfusion ratio distribution.