A breathing model of the respiratory system: The controlled system

Abstract

(1) We have adapted the model of Grodins, Buell & Bart (1967) to include cyclic ventilation, dead space, shunt, and a separate muscle compartment. (2) With a controller equation similar to that of Lloyd, Jukes & Cunningham (1958) , governed by filtered values of arterial P CO 2 and P O 2 the model matched both steady state and dynamic results from man breathing increased CO 2 concentrations. (3) With a technique using step inflow changes in CO 2 , we would predict steady-state ventilation to occur in three-four min, a potential experimental advantage. (4) To maintain stability in simulated exercise at 100 W it was necessary: (i) to decrease shunt to a minimal value of 1% of cardiac output; (ii) to divert blood flow from other tissues to the muscle compartment; (iii) immediately to increase total blood flow to an appropriate physiological level, and to increase ventilation to that level required to maintain iso-capnia in the steady state. (5) Wrong dynamics obtained in the on-transients of exerclse, and in breathing hypoxic mixtures, would be more appropriately corrected by adjusting the controller, rather than the controlled system ( Saunders, 1980 ).