ציטירנדיקער פּירוש אויף ויקרא 15:19

The Laws of Niddah
"When a woman has a discharge, her discharge being blood from her body, she shall remain in her impurity seven days" (Leviticus 15:19). Our sages learnt from explication of the verses that not all blood flows render the discharger impure, but specifically those of uterine blood (a braita from Torat Kohanim). The Ri"f writes in the second chapter of Tractate Shevuot, and the Rosh at the end of Niddah, and this is his language: "'When a woman has a discharge' (ibid.) -- I could have thought any flow of blood would be impure. The Torah teaches therefore 'he has uncovered the source [uterus] of her blood' (Leviticus 20:18) -- teaching regarding the blood that it speaks only of uterine blood."

Bei allen Frauen. Welche menstruieren und demgemäß den Reinheits-Vorschriften in Lev. 15, 19 ff. unterworfen sind.

All women are subject to uncleanness [as soon as the blood appears] in the outer chamber, as it says, "her discharge being blood in her body" (Leviticus 15:19). But a zav and one who emitted semen convey no uncleanness unless the discharge came out of the body. A woman (niddah or zavah) is impure as soon as the blood reaches the outer chamber, which I assume means the vagina. The blood need not flow out for her to be impure. This is midrashically read into the word "in her body." In contrast, men are impure only when the semen or other discharge has left the penis.