Question: why is it that the menstruation blood does not mix with the common blood

Cheryl Andisi answered on 23 Jan 2017:

@ Cubby,
We, human beings, have a closed circulatory system which means that all our blood is within vessels and these vessels supply all our tissues and organs. The uterus, and its lining is also supplied using these vessels throughout- any tissues that do not have this blood supplied will die, ie it becomes ischaemic.
The thickening of the uterine lining occurs due to the hormones released in preparation for pregnancy. This is due to the continuous supply of blood in response to the hormones and if a pregnancy occurs, there will be continued maintenance of the uterine lining until the baby is delivered. When fertilization of the released ova does not occur, there is no more progesterone released and this leads to constriction of the vessels supplying blood to the uterine lining, and eventually dying off and shedding of the uterine lining and menstruation.
So I guess in response to your answer, the “common blood”, will supply the tissue when it is still alive, but once it starts dying, the connection between the lining and the rest of the circulation has been cut off.

Just another thing to think of. Some of the cases of excessive bleeding after delivery cases occur because the body’s feedback mechanisms and hormonal messages to stop vessels supplying the uterus after a delivery fail. This results in continuous bleeding and sometimes loss of life.