Composition of venous thrombosis and the antithrombotic effects of aspirin. Venous thrombosis typically originates in areas of slower blood flow, such as the venous anatomy near valves. Venous clots consist primarily of fibrin, red blood cells, and leukocytes. Platelets are involved, but are less prominent in comparison with the platelet-rich arterial thrombus. Aspirin exerts various antithrombotic effects on the participating cells and proteins of thrombus formation, and fibrinolysis via cyclooxygenase (COX) and COX-independent pathways. Professional illustration by Patrick Lane, ScEYEnce Studios.