Aspirin in very low doses inhibits the formation of:-

Aspirin inhibits COX enzyme irreversibly and thus results in decreased synthesis of TXA2 as well as PGI2. TXA2 is produced by platelets and as platelets do not contain nuclei, TXA2 is not synthesized till there is production of fresh platelets, whereas vessel wall contains nucleus and thus can resume the synthesis of enzymes required for the formation of prostacyclins. The net effect is inhibition of TXA2 synthesis leading to anti-aggregatory effects.