| Many people think of the discharges that leave the body during a woman's monthly period as "blood." This is not a particularly accurate description; there is blood in the the discharge, but its color masks the fact that those discharges contain a lot of elements not found in the ordinary blood circulating throughout the body. The endometrium, the uterine lining, is made of three layers of mucous membrane which thicken during ovulation to support any embryo that implants in the uterus. (Birth control pills and some other medications containing progesterone or related hormones cause lighter periods by thinning the build-up of the lining.) The outer two endometrial layers are shed during menstruation. So the menstrual discharge contains mucus, made up of white blood cells, the cast-off tissue cells, water, and a sticky protein called "mucin." Blood enters the discharge from the three spiral arteries that feed into the endometrium; about a day before the period is due to start, they constrict sharply and stop delivering blood to the outer layers. The tissue in those layers dies when it is not fed with oxygen and nutrients. After the tissue is dead, the arteries briefly start up again and the burst of new blood pushes the dead cells away from the living layer underneath; this is when the discharges start leaving the body.
Menstrual blood does not contain much of the elements that make normal blood clot, or it would not reach the outside in a liquid state. (Anything in the discharges that looks like a blood clot is probably a clump of tissue or mucus.) Leaving the uterus and passing through the cervix and vagina, the flow picks up cervical mucus (which blocks sperm as well as outside infectious agents from entering the uterus when a woman is not ovulating) and vaginal secretions as well. The total discharge is usually about three and a half to six tablespoons, or two to three fluid ounces. The normally acid pH of the vagina changes to become a little more alkaline during menstruation. However, the discharges are sterile and not "unclean" as ancient beliefs often stated.
Sources:
Angier, Natalie. Woman: An Intimate Geography. New York: Anchor Books, 1999.
Baron-Faust, Rita. Being Female: What Every Woman Should Know About Gynecological Health. New York: William Morrow and Company, 1998.
The Boston Women's Health Collective. Our Bodies, Ourselves. New York: Simon & Schuster, 1998. | 
