Any of a group of lipid-soluble organic compounds causing a range of physiological effects in animals. Prostaglandins have been detected in most body tissues, in some cases acting locally and in others having effects on more distant target tissues. They are synthesized within cell membranes from arachidonic acid, which is derived from membrane phospholipids by the action of phospholipase A. The arachidonic acid is converted by the enzyme cyclo-oxygenase (prostaglandin endoperoxide synthase) to prostaglandin G₂, the precursor of all the various classes of prostaglandins, prostacyclin, and thromboxane A₂—collectively known as the prostanoids. PGs are short-lived signal molecules that bind to G-protein-coupled cell receptors. Among their array of effects, they cause the contraction of smooth muscle, e.g. in stimulating contractions of the uterus during parturition. Hence, natural and synthetic prostaglandins are used to induce abortion or labour in humans and domestic animals. Prostaglandins are also involved in inflammation, being released from macrophages and mast cells in response to injury or infection. Drugs such as aspirin and ibuprofen exert their pain-relieving effects by blocking prostaglandin synthesis.