inhibits prostaglandin and thromboxane A2 synthesis by acetylating the enzyme cyclooxygenase. Prostaglandins sensitize pain receptors resulting in a lower pain threshold. By inhibiting their production, aspirin raises the pain threshold so fewer pain impulses reach the CNS. Thromboxane A2 induces platelet aggregation and causes vasoconstriction. Aspirin permanently inhibits the thromboxane A2 in platelets for the life of the platelet.