Menstruation is initiated each month by falling levels of estrogen and progesterone and the release of prostaglandins, which constrict the spiral arteries. This causes them to spasm, contract and break up. The blood supply to the endometrium is cut off and the cells of the top layer of the endometrium (the stratum functionalis) become deprived of oxygen and die. Later the whole layer is lost and only the bottom layer, the stratum basalis, is left in place. An enzyme called plasmin breaks up the blood clots in the menstrual fluid, which eases the flow of blood and broken down lining from the uterus. The flow of blood continues for 2–6 days and around 30–60 milliliters of blood is lost.