Information about the endometrial lining and menstruation.

There is a substance produced in the uterus which is supposed to help prevent clots from forming, it is called fibrinolysin. When the blood flows too quickly (profusely) the anti-clotting fibrinolysins can't keep up and clots form. Heavy bleeding is called menorrhagia.

"Blood clots or flooding are indications of heavy loss. Normally the blood lost from the vessels in the lining of the womb forms small clots and this tends to reduce the flow. The small blood clots within the uterus are broken down by chemicals called fibrinolysins and the normal menstrual loss should be a fluid. When the bleeding is heavy, the blood is extruded too quickly for it to clot within the uterus. In this situation, the blood clots in the vagina and the menstrual flow includes blood clots. Whilst menstruation is inevitably an inconvenience, it should not result in limitation of social activity."
http://askwaltstollmd.com/archives/menstruati/95779.html#95952
(This doctor had his licensed revoked but the information seems valid so I included it, you will see mention of it if you scroll down through the previous link so I thought I would link to this page FYI: (http://askwaltstollmd.com/faq.html)

Check out these two interesting pages with detailed descriptions and diagrams of how the endometrium forms each month, for your reference:

NORMAL MENSTRUATION

THE ENDOMETRIUM

For more information search with these key words in different
combinations (such as "endometrium anti-clotting"):

menorrhagia, endometrium, fibrinolysin, anti-clotting