This chapter and the two succeeding it together share responsibility for presenting the physiological basis of normal pulmonary gas exchange. Gas exchange occurs by an integrated series of gas transport steps between the environmental air we breathe and the Hb molecule of the red cells passing through the pulmonary capillaries. These transport steps are of two types—diffusive and convective, and a number of conceptually separate diffusive as well as convective processes interact to accomplish the gas exchange mission. This is true both for gases that are taken up from the environment into the blood (i.e., O2 and occasional toxic gases or volatile anesthetics) and for gases that are eliminated from the body (i.e., CO2 and volatile anesthetic agents).
This chapter deals principally with the convective processes and Chapter 16 with those involving diffusion. However, since the two types of process occur simultaneously they are closely linked.
Basic Outline of the Gas Exchange Pathway
This section dwells on O2, being the gas of principal physiological interest. However, the pathway components are of course identical for all gases and furthermore do not depend on whether the gas is being taken up (O2) or eliminated (CO2). On the other hand, distinct quantitative differences in the uptake or elimination patterns of different gases exist, but those are readily explained by differences in their fundamental physical or chemical properties, and not by transport pathway differences.
To understand the gas transport pathway one must first appreciate the anatomy of the lungs, laid out in detail in Chapter 2. The salient functional features are presented in Figure 14-1.
Principal anatomical features of the lung related to gas exchange. A shows the organization of branching airways, mirrored by a photograph of a lung slice showing terminal and respiratory bronchioles and the alveolar parenchyma (see also B) (A. Modified with permission from Weibel ER: Morphometry of the Human Lung. Heidelberg, Springer-Verlag; 1963) C shows how the capillaries are wrapped around alveoli and D is a scanning electron micrograph indicating the rich capillary networks in the alveolar walls (PA, pulmonary artery). E is a transmission electron micrograph showing the capillaries (C) and the three layers of the blood–gas barrier (EN, endothelium; BM, basement membrane; and EPI, epithelium). (B, D, and E. Reproduced with permission from Weibel ER. The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
The chest wall (rib cage and diaphragm) contains muscles that on contraction expand the volume of the chest cavity and thus reduce the hydrostatic pressure of the pleural space, expanding the lungs with air drawn in via the mouth and nose. Although there is but a single air ...