Fig. 1.

Schematic of the multicompartment and 2-compartment models of nitric oxide (NO) exchange in the lungs. A: multicompartment model consists of a central airway compartment that branches into 2 peripheral airway compartments, each emptying into separate alveolar regions. Each airway region is trumpet-shaped, incorporates axial diffusion, and has a constant NO flux per unit volume; alveolar compartments have a steady-state NO concentration. B: 2-compartment model consists of a single trumpet-shaped airway compartment with axial diffusion and constant NO flux per unit volume that empties into a single alveolar compartment that is at constant NO concentration. C: exhaled NO concentration in a representative child with asthma plotted as a function of exhaled airway volumes [exhaled volume (Vex)/airway volume (Vaw), unitless]. Key features include mean NO concentration of phase III (∼50 ppb) and the phase III slope (approx −2 ppb). See Glossary for abbreviations.