Fig. 4.

Differential effects of virtual airway CO₂ load vs. apparent metabolic CO₂ load on ventilatory control in different types of slug CO₂ loading. a: Fenn-Craig diagram showing the metabolic hyperbolas for pulmonary CO₂ exchange at rest and during exercise, CO₂ breathing and (ideal) slug CO₂ loading. See text. b: CO₂ response curves (or more appropriately, CO₂ tolerance curves) are no different under constant-flux slug CO₂ loading than CO₂ breathing in two subjects. Panels a and b are adapted from Fenn and Craig (1963) with permission. c: CO₂ response curve in anesthetized cats is twice steeper in early-inspired slug CO₂ loading than in CO₂ breathing at constant P_{I CO2}. Adapted from van der Grinten et al. (1992) with permission. d: The mechanism underlying the control of exercise hyperpnea remains intact in early-inspired slug CO₂ loading, except that the apparent homeostatic “set point” is shifted to higher Pa_CO2 levels at rest and during exercise compared with corresponding control values (data points at left). By contrast, CO₂ breathing (at 5% level) also induces hypercapnia because of its CO₂-clogging effect (Comroe’s law) but homeostatic regulation at the higher Pa_CO2 level is lost during exercise. Adapted from (Swanson, 1978) with permission.