The tracer activity in blood is normally only sampled at a few time points during the acquisition. However, when calculating the operational equations (eg. by numerically integrating a system of differential equations), the blood activities must be available at any arbitrary time point during the acquisition period. This means that the blood curves must be interpolated according to some underlying model, hence called a "blood model". The interpolation models available for the different blood curves are described in the corresponding model reference section.