Menstrual Phase

a. Menstrual phase : The beginning of each cycle is taken as the first day of menses — the loss of blood (about 45–100 ml) — which lasts on average about 3–7 days (roughly 5 days). Under the effect of prostaglandins released as progesterone and estrogen levels fall, the endometrium of the uterus breaks down, and blood, tissue fluid, mucus, the endometrial lining and the unfertilized oocyte are discharged through the vagina; the endometrial lining thins to about 1 mm. This menstrual blood does not clot, because of the presence of an enzyme called fibrinolysin. The menstrual phase occurs specifically because an ovulated egg from the previous cycle was not fertilized and is therefore shed along with the menstruum — which is why it is sometimes described as the 'funeral of the unfertilized egg.' …