Prostacyclin is produced in endothelial cells from prostaglandin H₂ (PGH₂) by the action of the enzyme prostacyclin synthase. Although prostacyclin is considered an independent mediator, it is called PGI₂ in eicosanoid nomenclature, and is a member of the prostanoids.The series-3 prostaglandin PGH₃ also follows the prostacyclin synthase pathway, yielding another prostacyclin, PGI₃. The unqualified term 'prostacyclin' usually refers to PGI₂. PGI₂ is derived from the ω-6 arachidonic acid. PGI₃ is derived from the ω-3 EPA. Prostacyclin (PGI₂) chiefly prevents formation of the platelet plug involved in primary hemostasis (a part of blood clot formation). It does this by inhibiting platelet activation. It is also an effective vasodilator. Prostacyclin's interactions in contrast to thromboxane (TXA₂), another eicosanoid, strongly suggest a mechanism of cardiovascular homeostasis between the two hormones in relation to vascular damage.