What part of aspirin thins blood?

Aspirin thins blood by blocking an enzyme called COX (cyclooxygenase) in platelets, which cuts off production of thromboxane A2—a chemical that helps platelets clump together to form clots. By reducing thromboxane A2, aspirin makes platelets less able to aggregate, lowering clotting tendency.

When aspirin is taken, the effect is mainly on platelets rather than on blood itself. Platelets can’t make new thromboxane A2, so the anti-clot effect lasts until new platelets are produced (typically several days).