Prostaglandin H2 (PGH2) is a type of Prostaglandin which is derived from arachidonic acid and is a precursor for many other biologically significant molecules. Prostaglandins are synthesized from arachidonic acid by the prostaglandin-endoperoxide synthase enzymes, which are also called Cyclooxygenases. These enzymes generate a reactive intermediate PGH2 which has a reasonably long half-life (90-100 s). PGH2 is converted into the biologically active prostaglandins by prostaglandin isomerases, yielding Prostaglandin E2, D2 and F2 (PGE2, PGD2, and PGF2), or by thromboxane synthase to make Thromboxane A2 (TxA2) or by prostacyclin synthase to make Prostaglandin I2 (PGI2). Prostaglandin H2 was first isolated from incubations of arachidonic acid with ovine seminal vesicle microsomes, and was described as a potent vasoconstrictor. Thromboxane receptors can be activated by PGH2 and TxA2, as well as isoprostanes, all of which produce vasoconstriction. Activation of TP receptors by either PGH2 or TxA2 contributes to hypertension of pregnancy and to the elevation of blood pressure in some forms of experimental hypertension, particularly those that involve activation of the renin-angiotensin system.