E.S. Ermolaev1, A.I. Dyachenko2, Yu. A. Shulagin3

1–3 Tambov State Technical University (TSTU) (Tambov, Russia)
1,2 Institute of biomedical problems RAS (Mosсow, Russia)

It is known that in response to the onset of physical exercise, lung ventilation and blood flow increase very fast, within a few seconds. Existing theories of breathing control based on the blood gas composition cannot explain the rapid ventilation increase solely due to an increase in the tissues’ carbon dioxide production and oxygen consumption. Cardiodynamic hyperpnea hypothesis and muscle pump with peripheral vasodilation mechanisms suggest that systemic and pulmonary blood flow increases rapidly at the physical exercise onset although particular mechanisms of this increase are still under discussion. A mathematical model of this paper based on the mentioned hypotheses simulates responses of the human cardiorespiratory system on the physical exercise and variations of oxygen and carbon dioxide contents in the inspired air. It is clear that, the details of physiological mechanisms require experimental validation, but mathematical simulation allows us to investigate which certain mechanisms can explain the experimental data on the response of the cardiorespiratory system at the physical exercise onset.

There are two main phases of the cardiorespiratory system response to the transition from rest to steady-state physical activity. The first phase includes more rapid processes, e.g. a rapid increase in ventilation, systemic blood flow, and a subsequent increase in oxygen delivery, and, possibly, an increase in CO2 delivery to the lungs. The following phase is characterized by a relatively slow response of the cardiorespiratory system, with additional more strongly blood flow and ventilation increasing in comparison with the first phase of the reaction, e.g. as a result of increased O2 consumption and CO2 production, and due to chemoreceptor stimuli.

Mathematical simulation confirmed that, compared to the rapid neurogenic response of the cardiorespiratory system, slower blood flow and respiration response to physical activity induced by O2 and CO2 fractional concentration changes in the alveolar space, metabolic needs changes and further blood flow and ventilation adjustments due to chemoreflexes. The model shows a less marketable increase in end-tidal CO2 partial pressure (PETCO2) during physical activity in comparison with experimental data. There is no perfect match between the model results and the experimental data because the coefficients of the equations were selected based on different literature sources with different experimental subjects at different time periods and using different methods applied in these experimental protocols. Nevertheless, the mathematical model also demonstrated the physiological mechanisms of cerebral blood flow regulation in response to the partial pressures of O2 and CO2 deviations in arterial blood that occur during physical exertion. As a result of the increase in CO2 tension in arterial blood, cerebral blood flow increased. The simulation model allows testing various hypotheses about cerebral blood flow regulation and predicting changes that may lead to cerebral hypoxia.

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