Prostacyclin

 Prostacyclin (also called PGI₂) is a prostaglandin belonging to the eicosanoidfamily¹, produced by intact vascular endothelial cells². When prostacyclin is produced, it prevents platelet adhesion and aggregation so that clots are not unintentionally formed within blood vessels². When the endothelium lining blood vessels is damaged, prostacyclin production is stopped and platelet aggregation occurs, allowing a clot to form and seal the break². Prostacyclin is also a vasodilator². 

References: 

1) https://en.wikipedia.org/wiki/Prostacyclin

2) Silverthorn U.D., Johnson B.R., Ober W.C., Garrison C.W., Silverthorn A.C., 'Human Physiology: An Integrated Approach', 6th Edition, Pearson Education, 2013