The administration of CO2/O2 gas-challenges during BOLD-fMRI is appealing due to the hemodynamic impact these stimuli have. With the support of computational modeling, arterial gas manipulations can provide a means to separate neuro-vascular signal contributions. Hence, we performed simulations in virtual vascular architectures for different blood volumes and oxygen saturation levels. The effect that these states have on the magnetic field was simulated, separately, for arteries and veins. We present look-up tables to derive the possible vascular contribution responsible for measured BOLD signals in clinical or research fMRI settings including changes in local blood volume, oxygen metabolism or cereberovascular diseases.