Walter F. Boron
Although diffusion is at the very heart of the gas exchange, as we discussed in Chapter 30, two other parameters are also extremely important. Ventilation and perfusion—both of which require energy—are critical because they set up the partial-pressure gradients along which O2 and CO2 diffuse. Ventilation is the convective movement of air that exchanges gases between the atmosphere and the alveoli. In Chapter 27 we discussed the mechanics of ventilation. In the first part of the present chapter we consider the importance of ventilation for determining alveolar and , and also see that ventilation varies from one group of alveoli to the next. Perfusion is the convective movement of blood that carries the dissolved gases to and from the lung. In Chapters 17 through 25 we discussed the cardiovascular system. In the second part of the present chapter, we examine the special properties of the pulmonary circulation and see that, like ventilation, perfusion varies in different regions of the lung. Finally, in the third part of this chapter, we see that the ratio of ventilation to perfusion—and the distribution of ventilation-perfusion ratios among alveolar units—is critically important for gas exchange and thus for the composition of the arterial blood gases: , , and pH.