Heparin (from Ancient Greek ἧπαρ (hêpar), ""liver""), a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule. It can also be used to form an inner anticoagulant surface on various experimental and medical devices such as test tubes and renal dialysis machines.Although it is used principally in medicine for anticoagulation, its true physiological role in the body remains unclear, because blood anticoagulation is achieved mostly by heparan sulfate proteoglycans derived from endothelial cells. Heparin is usually stored within the secretory granules of mast cells and released only into the vasculature at sites of tissue injury. It has been proposed that, rather than anticoagulation, the main purpose of heparin is defense at such sites against invading bacteria and other foreign materials. In addition, it is observed across a number of widely different species, including some invertebrates that do not have a similar blood coagulation system.In nature, heparin is a polymer of varying chain size. Unfractionated heparin as a pharmaceutical is heparin that has not been fractionated to sequester the fraction of molecules with low molecular weight. In contrast, low-molecular-weight heparin has undergone fractionation for the purpose of making its pharmacodynamics more predictable.Heparin is on the World Health Organization's List of Essential Medicines, a list of the most important medications needed in a basic health system.