Niddah, Chapter 9, Mishnah 2. This Mishnah takes up another case in which blood appears, but where there is reason to say that it did not come from the uterus. A woman sees blood at the moment she is relieving herself, and the Tannaim debate whether that blood must be treated as uterine blood.
The Mishnah opens: "Ha'isha shehi osa tzeracheha, vera'asa dam" - a woman who is urinating and saw blood.
"Rebbi Meir omer: im omedes, temei'ah" - Rebbi Meir says that if she was standing while urinating, she is temei'ah. Standing produces a stronger, more forceful stream. Rebbi Meir is concerned that the force of the stream caused some of the urine to be driven back up inside her body, where it could have mixed with blood originating in the uterus. Urine itself does not come from the uterus at all; its source is the bladder. But because of that pressure, uterine blood may have become commingled with the urine that then emerged, and she must therefore reckon with the possibility that what she saw is uterine blood. She is temei'ah.
"Ve'im yosheves, tehorah" - but if she relieved herself while sitting down, she remains tehorah. A seated position generates a gentler flow with far weaker pressure behind it, so there is no reason to suspect that anything was forced backward into her body and gathered blood from the uterus along the way. She is entitled to assume that the blood which appeared together with the urine had its origin in the bladder or in the passage through which the urine leaves. Such is Rebbi Meir's position.
"Rebbi Yosi omer: bein kach uvein kach, tehorah" - Rebbi Yosi rules that she is tehorah either way. Standing or sitting makes no practical difference to him, since he does not accept that any mingling of the sort Rebbi Meir worried about actually takes place. Blood that appeared during urination may be traced to whatever produced the urine, which is to say that it is not uterine blood at all, and it is to be ascribed to a wound she has somewhere in that area. She stays tehorah.
The halachah follows Rebbi Yosi: whether she was standing or seated, she is tehorah.