Converts arachidonate to prostaglandin H2 (PGH2), a committed step in prostanoid synthesis. PTGS1 is involved in the constitutive production of prostanoids in particular in the stomach and platelets. It is a key step in the generation of prostaglandins in gastric epithelial cells and the generation of thromboxane A2 (TXA2) in platelets. PTGS2 is constitutively expressed in some tissues in physiological conditions, such as the endothelium, kidney and brain, and in pathological conditions, such as in cancer. PTGS2 is responsible for production of inflammatory prostaglandins. Up-regulation of PTGS2 is also associated with increased cell adhesion, phenotypic changes, resistance to apoptosis and tumor angiogenesis.