The gemara questions whether blood that would exit the uterus through a tube would render her tamei. The gemara concludes that the pasuk of בבשרה is to the exclusion of a tube. Meaning, blood exiting a tube wouldn't render her tamei. The gemara makes a distinction between blood in a tube and blood in a piece of flesh, in that blood in a piece of flesh is normal, whereas through a tube is not normal.
It isn't clear from the gemara whether the issue of the blood exiting through a tube is a problem of chatzitzah, meaning that בבשרה would demand that the blood be in contact with her flesh when it exits her body, or whether it the problem is that the blood is being drawn out using the tube rather than flowing out naturally. According to the second approach, the fact that the blood isn't in contact with her body is irrelevant, since it flows out naturally it would render her tamei.
Rashi clearly writes that the issue is that of a chatzitza because rashi says that the advantage to a the piece of flesh is that מין במינו לא חייץ, implying that by a tube the issue is that it is a chatzitza. However, the Shulchan Aruch HaGraz (188:8,9) cites two opinions. He understands that the opinion of the Shulchan Aruch who says that blood in a piece of flesh that is cracked and therefore in contact with her body on the way out, would still not render her tamei, clearly implies that the issue is not one of chatzitza but rather any blood being forced out by a tube or by a piece of flesh, wouldn't render her tamei. According to this approach, a woman could not avoid being a nida by simply lining her cervix with some synthetic material. However, he quotes from Tosafos and the Rambam that it is an issue of chatzitzah. Based on that approach a woman can avoid becoming a Nida by lining her cervix so that the blood exits the uterus without contact to her body. Even if the blood flows out in its natural course and is not forced out, it would still not make her tamei.