To better understand how radiotherapy delivery parameters affect the depletion of circulating lymphocytes in patients treated for intra-cranial tumors, we developed a computational human body blood flow model (BFM), that enables to estimate the dose to the circulating blood during the course of fractionated radiation therapy. A hemodynamic cardiovascular system based on human body reference values was developed to distribute the cardiac output to 24 different organs, described by a discrete Markov Chain. For explicit intracranial blood flow modeling, we extracted major cerebral vasculature from MRI data of a patient and complemented them with an extension network of generic vessels in the frontal and occipital lobes to guarantee even overall blood supply to the entire brain volume. An explicit Monte Carlo simulation was implemented to track the propagation of each individual blood particle (BP) through the brain and time-dependent radiation fields, accumulating dose along their trajectories. The cerebral model includes 1050 path lines and explicitly simulates more than 266 000 BP at any given time that are tracked with a time resolution of 10 ms. The entire BFM for the whole body contains 22 178 000 BP, corresponding to 4200 BP per ml of blood. We have used the model to investigate the difference between proton and photon therapy, and the effect of different dose rates and patient characteristics on the dose to the circulating blood pool. The mean dose to the blood pool is estimated to be 0.06 and 0.13 Gy after 30 fractions of proton and photon therapy, respectively, and the highest dose to 1% of blood was found to be 0.19 Gy and 0.34 Gy. The fraction of blood volume receiving any dose after the first fraction is significantly lower for proton therapy, 10.1% compared to 18.4% for the photon treatment plan. 90% of the blood pool will have received dose after the 11th fraction using photon therapy compared to the 21st fraction with proton therapy. Higher dose rates can effectively reduce the fraction of blood irradiated to low doses but increase the amount of blood receiving high doses. Patient characteristics such as blood pressure, gender and age lead to smaller effects than variations in the dose rate. We developed a 4D human BFM including recirculating to estimate the radiation dose to the circulating blood during intracranial treatment and demonstrate its application to proton- versus photon-based delivery, various dose rates and patient characteristics. The radiation dose estimation to the circulating blood provides us better insight into the origins of radiation-induced lymphopenia.