Transport of Gases between the Environment and Alveoli—Theoretical Foundations

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 chapter 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.

Physiology for the pulmonary functional imager

“…Gas moves from the external world to the acinus by two processes: convection and diffusion [29]. Convective or bulk transportation of gas occurs within the conducting airways from the trachea to the terminal bronchioles.…”

Particle Transport and Deposition: Basic Physics of Particle Kinetics

“…For instance, over one breathing cycle, a 0.1 μm particle travels a distance equivalent to only a small fraction of the alveolar depth by its own diffusive intrinsic motion. This highlights a remarkable difference between the behavior of aerosol particles and that of highly diffusive respiratory gas molecules in the pulmonary acinus (see “Transport of gases between the environment and alveoli – theoretical considerations” chapter in this handbook series 39 . More details and comprehensive discussion of acinar geometry can be found elsewhere in the handbook chapter.…”

A General Overview on the Hyperbaric Oxygen Therapy: Applications, Mechanisms and Translational Opportunities

“…In a simple manner, O 2 is introduced in our body by two distinguished process: ventilation, in which gases are transported from the environment to the bronchial tree and diffusion, where an equilibrium in the distribution of O 2 between alveoli space and blood is reached. Given that the partial pressure of O 2 (PO 2 ) here is low, and rich in carbon dioxide (CO 2 ), gas exchange occurs [ 12 ]. Simultaneously, the difference in the pressure and volume in the chest wall and lungs are essential to permit the oxygen flow, as atmospheric pressure does not vary at all [ 13 ].…”