The model in this example makes the simplifying assumption that the rate of production of CO2 is constant. It also assumes that the total volume of air exchanged by the lungs per minute is constant (this is referred to by physicians as the “ respiratory minute volume”).
Despite the fact that this model of ventilation predicts unrealistic CO2 concentrations when the ventilation rate is extremely low, it has nevertheless been an invaluable tool for physicians. It is sometimes considered to be one of the four most important equations in clinical practice. Note, however, that the steady state concentration of CO2 is usually expressed in different units in clinical settings. For simplicity we have expressed it in terms of the concentration of CO2 in mg/mL but it is usually expressed as the partial pressure of CO2 in clinical settings (measured in mmHg). One can convert between the two different units using the ideal gas law (which relates the number moles of a substance to pressure) and the molar mass of CO2 (which gives the mass of CO2 per mole).