The protease-activated receptors (PARs) are a unique family of vascular receptors that confer on cells an ability to sense, and respond to, local changes in the proteolytic environment. They are activated by serine proteases of the blood coagulation cascade, notably thrombin, and are linked to thrombotic and inflammatory effector pathways. In surgery with cardiopulmonary bypass (CPB), thrombin is generated in large quantities in the extracorporeal circuit and can exert systemic effects by way of platelet and endothelial PAR1. Aprotinin (Trasylol), a serine protease inhibitor used in cardiac surgery, preserves platelet function, and attenuates the inflammatory response by protecting the PAR 1 receptor on platelets and endothelium.