A mathematical model is presented describing the respiratory chemoreflex relations between the partial pressures of carbon dioxide and oxygen at the central and peripheral chemoreceptors and the resulting pulmonary ventilation. These chemoreflex relations in terms of carbon dioxide partial pressures are transformed to relations in terms of hydrogen ion concentrations in brain tissue and arterial blood using the Stewart approach to acid-base balance. In this way, the chemoreflex relations can be expressed in terms of the actual stimulus to the respiratory chemoreceptors.