Prostaglandins and the structurally associated molecules prostacyclins, leukotrienes and thromboxanes, are called as eicosanoids because they having 20 carbon atoms. These hormones are associativaly short-lived and thus act locally near to their site of synthesis in the body. They are derived from the common precursor arachidonate described in the figure. This polyunsaturated fatty acid is a derivative of linoleate .The Prostaglandins  encourage  inflammation,  modulate  synaptic  transmission   between  nerve  cells,  and  induce  sleep.  Whereas aspirin (acetylsalicylic acid) has been used for centuries to reduce inflammation, fever and pain, it was not until year of 1974 which John Vane discovered how aspirin works. Aspirin hold back the synthesis of prostaglandins by irreversibly inhibiting prostaglandin synthase. This enzyme catalyzes the first phase in the synthesis of prostacyclins and thromboxanes, prostaglandins.

        Figure:  Biosynthetic relationship of the eicosanoids.