Prostacyclin, molecular model. This is a hormone that acts to widen blood vessels (thus lowering blood pressure) and to prevent aggregation of platelets (preventing blood clotting). These effects are the opposite of those of the related hormone thromboxane, and the balance between the two is important in regulating the functioning of the circulatory system. When taken as a drug it is known as epoprostenol. Prostacyclin is synthesised in the body from prostaglandin H2, itself produced by the cyclooxygenase enzymes COX-1 and COX-2. In this model, atoms are shown as spheres and are colour-coded: carbon (cyan), oxygen (pink) and hydrogen (white).