Can we say that the Aliyah is the eurethra, the prozdor is the external opening, and the cheder is the uterus, so that the woman checks the external opening for dam. Dam on the bottom would be lesslikely to be from the wound above so would be more likely to be tamei.

Then we can learn lul as a wound that has dam seeping down to the prozdor, and a good sefarah for why there is blood and why it is tahor.

Perhaps we can even put the lul somewhere towards the exterior? But this is difficult.

The Aruch says that the Aliyah is the bladder. Accordingly, the Lul is most likely the urethra, and blood on the floor of the Prozdor is from the Cheder and Tamei. Blood on the roof of the Prozdor could have come from the bladder (Aliyah) through the Lul over the vulva (which is also part of the Prozdor) and be Safek Tamei.

Moshe Kaplan, MD