This thesis aims at building high-fidelity models for the simulation of blood at both the microscopic and the macroscopic scales. Our work focuses on creating a digital replica of human blood through various tools of multi-scale/multi-physics nature, and on contributing novel pieces towards the realisation of a digital lab. We then use these tools for investigating cases that span from fundamental research to problems of clinical relevance. Focal point has been the understanding of the underlying mechanisms of platelet transport. For this, we followed a bottom-up approach, i.e. started from fully resolved blood flow simulations (microscopic scale) and moved towards calibrated stochastic models (macroscopic scale). Our results help clarify platelet transport physics, leading to more accurate modelling and design of PLT function tests (e.g. Impact-R device). Given the limited prognostic capacity of these tests, we believe that our models could lead in next generation tests with higher clinical relevance/readiness.