What is the reason for the brown blood on the fourth day of menstruation

At the beginning of menstruation, often due to the sudden decline of estrogen and progesterone, resulting in endometrial exfoliation, spiral arteriole bleeding, menstrual blood is often bright red, the amount is more, why is this? Because menstrual blood in the vagina retention time is relatively short, can be quickly discharged from the body, and this situation out of the current menstruation 1-3 days. With the complete exfoliation of the endometrium, the spiral arterioles are thrombosed to prevent bleeding, vaginal bleeding will be significantly reduced, and the reduced bleeding flow rate will be slower. Under the action of the acidic environment of the vagina, the blood reacts with the bacteria in the vagina, showing brown blood, which is a phenomenon of blood reduction, which is a normal phenomenon.