A comprehensive simulator of the human respiratory system: Validation with experimental and simulated data

A comprehensive model of oxygen (O₂) and carbon dioxide (CO₂) exchange, transport, and storage in the adult human is presented, and its ability to provide realistic responses under different physiological conditions is evaluated. The model comprises three compartments (i.e., lung, body tissue, and brain tissue) and incorporates a controller that adjusts alveolar ventilation and cardiac output dynamically integrating stimuli coming from peripheral and central chemoreceptors. A new realistic CO₂ dissociation curve based on a two-buffer model of acid-base chemical regulation is included. In addition, the model explicitly considers relevant physiological factors such as buffer base, the nonlinear interaction between the O₂ and CO₂ chemoreceptor responses, pulmonary shunt, dead space, variable time delays, and Bohr and Haldane effects. Model simulations provide results consistent with both dynamic and steady-state responses measured in subjects undergoing inhalation of high CO₂ (hypercapnia) or low O₂ (hypoxia) and subsequent recovery. An analysis of the results indicates that the proposed model fits the experimental data of ventilation and gas partial pressures as some meaningful simulators now available and in a very large range of gas intake fractions. Moreover, it also provides values of blood concentrations of CO₂, HCO⁻ ₃, and hydrogen ions in good agreement with more complex simulators characterized by an implicit formulation of the CO₂ dissociation curve. In the experimental conditions analyzed, the model seems to represent a single theoretical framework able to appropriately describe the different phenomena involved in the control of respiration.