Study design: Blood will be collected from human volunteers via a venipuncture in the forearm. Each volunteer will donate maximally four times 30 ml of blood over a period of two days. This blood is used in perfusion flow experiments: blood flows over a coverslip covered with collagen in a flow chamber. The investigators will vary several conditions such as the concentration of the proteins and the shear rate. For perfusion flow experiments, the investigators need fresh whole blood because platelets are viable for four hours. After this time, new blood is needed.