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Transport of Gases between the Environment and Alveoli—Theoretical Foundations

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Abstract

The transport of oxygen and carbon dioxide in the gas phase from the ambient environment to and from the alveolar gas/blood interface is accomplished through the tracheobronchial tree, and involves mechanisms of bulk or convective transport and diffusive net transport. The geometry of the airway tree and the fluid dynamics of these two transport processes combine in such a way that promotes a classical fractionation of ventilation into dead space and alveolar ventilation, respectively. This simple picture continues to capture much of the essence of gas phase transport. On the other hand, a more detailed look at the interaction of convection and diffusion leads to significant new issues, many of which remain open questions. These are associated with parallel and serial inhomogeneities especially within the distal acinar units, velocity profiles in distal airways and terminal spaces subject to moving boundary conditions, and the serial transport of respiratory gases within the complex acinar architecture. This article focuses specifically on the theoretical foundations of gas transport, addressing two broad areas. The first deals with the reasons why the classical picture of alveolar and dead space ventilation is so successful; the second examines the underlying assumptions within current approximations to convective and diffusive transport, and how they interact to effect net gas exchange. © 2011 American Physiological Society. Compr Physiol 1:1301‐1316, 2011.

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Figure 1. Figure 1.

Single breath N2 washout experiment, demonstrating the fractionation of the expirate into dead space and alveolar portions. Upper panel shows flow and N2 concentration following an O2 inspiration. Lower panel shows three phases of the washout, and the fractionation of ventilation into dead space and alveolar volumes. From Fowler .

Figure 2. Figure 2.

Cast of the human bronchial and vascular trees. White resin corresponds to airways; red and blue resins correspond to pulmonary arterial and venous vascular trees, respectively. From Weibel ; color photograph courtesy Weibel .

Figure 3. Figure 3.

Sharply increasing net area of the bronchial tree (closed symbols) with generation number in a Weibel model, and the reciprocal fall in gas velocities (open squares at rest, open triangles at moderate exercise) proceeding from proximal to distal airways. Data modified from .

Figure 4. Figure 4.

Casts of the peripheral conducting airways in (A): human and (B): rat. Scale bar = 5 mm. Reprinted from Weibel et al. , with permission from Elsevier.

Figure 5. Figure 5.

A scanning EM picture of terminal bronchioles branching into respiratory bronchioles, ducts, and alveoli. Specimen from Weibel , image courtesy Weibel .

Figure 6. Figure 6.

Peclet number, representing the relative magnitude of convective or bulk transport to diffusive transport, versus generation number. As in Figure , open squares represent conditions at rest, and open triangles moderate exercise. Note the transition point around generation 16 at rest, and generation 19 at exercise. Data taken from Figure .

Figure 7. Figure 7.

N2 concentrations at the first and 20th breath of a multibreath washout test. The slopes of phase III reflect the combined effects of convection‐dependent parallel inhomogeneities in ventilation and diffusion‐dependent inhomogeneities within the acinus. From Verbanck et al. .

Figure 8. Figure 8.

Normalized slopes of phase III in a multibreath N2 washout study of a hyperresponsive subject, plotted as a function of lung turnover (proportional to breath number). Baseline slopes (solid symbols) vary but little over the maneuver, while there is a significant increase in slope following histamine provocation (open symbols). These are interpreted as reflecting diffusion (Sacin) and convection (Scond) dominated inhomogeneities, respectively. From Verbanck et al. .

Figure 9. Figure 9.

Cartoon of gas exchange units with (A) parallel ventilation and (B) serial ventilation. Both pictures exhibit parallel perfusion. For O2, in (A), gas concentrations are uniformly distributed across the parallel units, whereas in (B), O2 concentrations are diluted distally due to O2 uptake in more proximal acinar regions. From Sapoval et al. .

Figure 10. Figure 10.

Streamlines within the alveolus and alveolar duct in an axisymmetric model of the acinus with expanding boundaries simulating parenchymal expansion during breathing. Panels A, B, and C show streamlines with increasing values of flow into the alveolus relative to flow in the central alveolar duct. From Tsuda .

Figure 11. Figure 11.

Panel A: realization of multiple stretch and fold patterns in the trachea of a rat, after a single breath. Two colors of polymerizable silicone were used to visualize resident and tidal fluid. Scale bar = 500 μ. Panel B: longitudinal striations in a small airway (7th generation) showing the folded patterns originating from deep in the lung. Scale bar = 100 μ. From Tsuda et al. .

Figure 12. Figure 12.

Complex patterns of irreversible kinematics within the acinus. Panel A shows evidence of recirculation within alveoli, and Panel B shows significant folding patterns in a small intra‐acinar airway. Scale bars = 100 μ, From Tsuda et al. .



Figure 1.

Single breath N2 washout experiment, demonstrating the fractionation of the expirate into dead space and alveolar portions. Upper panel shows flow and N2 concentration following an O2 inspiration. Lower panel shows three phases of the washout, and the fractionation of ventilation into dead space and alveolar volumes. From Fowler .



Figure 2.

Cast of the human bronchial and vascular trees. White resin corresponds to airways; red and blue resins correspond to pulmonary arterial and venous vascular trees, respectively. From Weibel ; color photograph courtesy Weibel .



Figure 3.

Sharply increasing net area of the bronchial tree (closed symbols) with generation number in a Weibel model, and the reciprocal fall in gas velocities (open squares at rest, open triangles at moderate exercise) proceeding from proximal to distal airways. Data modified from .



Figure 4.

Casts of the peripheral conducting airways in (A): human and (B): rat. Scale bar = 5 mm. Reprinted from Weibel et al. , with permission from Elsevier.



Figure 5.

A scanning EM picture of terminal bronchioles branching into respiratory bronchioles, ducts, and alveoli. Specimen from Weibel , image courtesy Weibel .



Figure 6.

Peclet number, representing the relative magnitude of convective or bulk transport to diffusive transport, versus generation number. As in Figure , open squares represent conditions at rest, and open triangles moderate exercise. Note the transition point around generation 16 at rest, and generation 19 at exercise. Data taken from Figure .



Figure 7.

N2 concentrations at the first and 20th breath of a multibreath washout test. The slopes of phase III reflect the combined effects of convection‐dependent parallel inhomogeneities in ventilation and diffusion‐dependent inhomogeneities within the acinus. From Verbanck et al. .



Figure 8.

Normalized slopes of phase III in a multibreath N2 washout study of a hyperresponsive subject, plotted as a function of lung turnover (proportional to breath number). Baseline slopes (solid symbols) vary but little over the maneuver, while there is a significant increase in slope following histamine provocation (open symbols). These are interpreted as reflecting diffusion (Sacin) and convection (Scond) dominated inhomogeneities, respectively. From Verbanck et al. .



Figure 9.

Cartoon of gas exchange units with (A) parallel ventilation and (B) serial ventilation. Both pictures exhibit parallel perfusion. For O2, in (A), gas concentrations are uniformly distributed across the parallel units, whereas in (B), O2 concentrations are diluted distally due to O2 uptake in more proximal acinar regions. From Sapoval et al. .



Figure 10.

Streamlines within the alveolus and alveolar duct in an axisymmetric model of the acinus with expanding boundaries simulating parenchymal expansion during breathing. Panels A, B, and C show streamlines with increasing values of flow into the alveolus relative to flow in the central alveolar duct. From Tsuda .



Figure 11.

Panel A: realization of multiple stretch and fold patterns in the trachea of a rat, after a single breath. Two colors of polymerizable silicone were used to visualize resident and tidal fluid. Scale bar = 500 μ. Panel B: longitudinal striations in a small airway (7th generation) showing the folded patterns originating from deep in the lung. Scale bar = 100 μ. From Tsuda et al. .



Figure 12.

Complex patterns of irreversible kinematics within the acinus. Panel A shows evidence of recirculation within alveoli, and Panel B shows significant folding patterns in a small intra‐acinar airway. Scale bars = 100 μ, From Tsuda et al. .

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How to Cite

James P. Butler, Akira Tsuda. Transport of Gases between the Environment and Alveoli—Theoretical Foundations. Compr Physiol 2011, 1: 1301-1316. doi: 10.1002/cphy.c090016