Niddah, Chapter 5, Mishnah 2. This Mishnah draws a sharp contrast between the tum'ah of a woman and the tum'ah of a man: at what point does the flow render the person temei'ah, while it is still within the body, or only once it has emerged?
A Woman: The Outer Chamber
The Mishnah states: "kol hanashim mitam'os beveis hachitzon", all women become temei'os in the outer chamber. As was explained back in the first perek of our masechta, a woman becomes temei'ah the moment blood leaves the uterus, even though it has not yet left her body at all. Once it has entered the vaginal canal, she is already a niddah, even though nothing has appeared outside.
The Mishnah anchors this in a pasuk: "dam yeheyeh zovah bivsarah", her flow of blood shall be in her flesh. The Torah speaks of blood that is still within her flesh, and that is precisely the point: the flow generates tum'ah while it is yet inside her.
A Man: Only Once It Has Left
With a man the halachah runs in the opposite direction. The Mishnah states: "aval hazav uva'al keri einan mitam'in ad sheteitzei tum'asan lachutz", and the translation is that a zav and one who has had an emission of keri are not rendered temei'in until the discharge has passed to the outside. Two forms of male tum'ah are being addressed here: the zivah discharge and the emission of semen. In neither case does the tum'ah take hold while the fluid is still contained within him; even if it has moved and travelled but has not yet fully passed beyond his body, his status is unchanged and he remains tahor. What activates the tum'ah is the exit itself, the point at which the discharge reaches the outside.
The Mishnah thus sets two different standards side by side: for a woman the Torah's own words place the onset of tum'ah while the blood is still inside her flesh, whereas for a zav and a ba'al keri nothing takes effect until the tum'ah has actually left the body.