Catalyzes the biosynthesis and metabolism of eicosanoids. Catalyzes the isomerization of prostaglandin H2 to prostacyclin (= prostaglandin I2), a potent mediator of vasodilation and inhibitor of platelet aggregation (PubMed:12372404, PubMed:15115769, PubMed:18032380, PubMed:25623425). Additionally, displays dehydratase activity, toward hydroperoxyeicosatetraenoates (HPETEs), especially toward (15S)-hydroperoxy-(5Z,8Z,11Z,13E)-eicosatetraenoate (15(S)- HPETE) (PubMed:17459323). {ECO:0000269|PubMed:12372404, ECO:0000269|PubMed:15115769, ECO:0000269|PubMed:17459323, ECO:0000269|PubMed:18032380, ECO:0000269|PubMed:25623425}. This is the function of Ensembl gene identifier ENSG00000124212 (PTGIS, prostaglandin I2 synthase).