Physiologically based pharmacokinetic (PBPK) models are systems of ordinary differential equations that estimate internal doses following exposure to toxicants. Most PBPK models use standard equations to describe inhalation and concentrations in blood. This study extends previous work investigating the effect of the structure of air and blood concentration equations on PBPK predictions. The current study uses an existing PBPK model of xylene to investigate if different values for the maximum rate of toxicant metabolism can result in similar compartmental predictions when used with different equations describing inhalation. Simulations are performed using values based on existing literature. Simulated data is also used to determine specific values that result in similar predictions from different ventilation structures. Differences in ventilation equation structure may affect parameter estimates found through inverse problems, although further investigation is needed with more complicated models.