Answer:
The ROSH, quoting the Sar Mikutzi, explains that we rule that if
blood went directly from the Makor out of the woman's body through a tube
then she is Tahor. Why is that so? Is it because the blood never made
contact with the woman's flesh outside of the Makor, or is it because it
left her body in an unusual manner?
Rebbi Eliezer understood that the woman is Tahor if blood left her Makor
through a tube because the blood did not make contact with the flesh of the
woman out of the Makor. Based on this understanding, he rules that if the
blood was contained in pieces which had cracks, allowing the blood to make
contact with her flesh on its way out, she is Tamei. However, the Rabbanan
understood that the reason she is Tahor in the case of a tube is because
the blood left her body in an unusual way. If so, even if blood came out of
her body in pieces which had cracks it would not render her Tamei, since
this is also an unusual manner to shed blood. (This appears to be the
intention of Rashi DH Ki as well.)
The DARKEI MOSHE and BACH (YD 188) add that even if pure blood came out in
some unusual way, not related to pieces with cracks, the woman is also
Tahor.
HALACHAH: The Mechaber (YD 188:3) rules that if blood left the Makor
through a tube, the woman is Tahor. If pieces fell from the woman with
cracks from which blood could make contact with her body, she is still
Tehorah.
The SHACH (8) quotes the opinion of the BACH that it is correct to be
consider her Tamei in a case where pieces with blood fell from her body,
but if bloodless pieces fell from her body, she is Tehorah.
(Permission is granted to print and redistribute this material as long as
this header and the footer at the end of the mailing are included.)