[en] This article shows some mathematical methods used in the simulation of human metabolism in the presence of radioactive materials with the purpose of evaluating radioactivity in organs and excreta. And application is shown for a particular case. We define the method for calculating the number of disintegrations that the radioactive material produces in the organ that retains it. This amount is proportional to the risk dosage, a quantity calculated in radiobiological protection for the risk of internal contamination