EPO (erythropoietin) is a hormone (a type of protein) produced by specialized cells in the kidneys.These cells are sensitive to the oxygen concentration in the blood, and increase the release of EPO when the oxygen concentration is low. Heparin, 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.Pharmaceutical grade heparinis derived from mucosal tissues of slaughtered meat animals such as porcine (pig) intestine or bovine (cow) lung. Although used principally in medicine for anticoagulation, the true physiological role in the body remains unclear, because blood anti-coagulation is achieved mostly by endothelial cell-derived heparan sulfate proteoglycans. 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 in a defensive mechanism at sites of tissue injury against invading bacteria and other foreign materials. In addition, it is preserved across a number of widely different species, including some invertebrates that do not have a similar blood coagulation system.