Control of Intermittent Ventilation in Lower Vertebrates

Abstract

In an attempt to understand the quantitative aspects of gas exchange for the control of intermittent ventilatory activity in lover vertebrates we developed a conceptual model of this control system. It was assumed that in most species the onset and duration of ventilatory episodes are governed by an oxygen sensitive hypercapnic drive. As a consequence, the controller governing ventilatory activity in the model was designed as an on-off switch coupled with a sensor monitoring a blood compartment. The internal signal used by the controller to begin and terminate a burst of ventilation is partial pressure of carbon dioxide in blood (PbCO₂). The controller initiates a breathing episode when PbCO₂ rises above a critical value which is influenced by partial pressure of oxygen in blood (PbO₂). Ventilation is terminated when PbCO₂ falls below a constant threshold value. The regulated system consists of the stored quantities of oxygen and carbon dioxide in the lungs and the blood compartments. These are separated by a diffusive barrier. The model treats the blood as a uniform pool. The quantities of both respiratory gases, reflected by their partial pressures, are altered by continuous metabolic activity and intermittent bursts of ventilation.