As we learned on yesterday’s daf the only blood whose consumption would make the transgressor liable to receive the punishment of karet is that blood which comes out at the time of death. The Gemara on today’s daf asks for clarification of this point and we learn that there is a disagreement on its definition. Rabbi Yoḥanan says “That blood which spurts forcefully” while Reish Lakish says “From the last black drop onward.”
Rashi explains that when the arteries are cut the escaping blood is at first dark and then red. In its second stage it begins after a while to gush forth with force and when the pressure ceases the stream weakens and the blood oozes gently. There is thus at the beginning as well as the end a period when the blood escapes in a gentle flow. According to Rabbi Yoḥanan, only the blood that escapes with force is considered the life blood; according to Reish Lakish it is all blood that escapes after the last black drop, even when flowing gently.
Although the Mishna is clear that blood in the animal’s inner organs is not the blood whose consumption would make someone liable to receive karet, nevertheless, Rabbi Zeira rules that blood found in the animal’s heart does fall into that category. He explains that the heart absorbs the forbidden blood.
At the moment of slaughter – as in every situation where there is sudden blood loss – the animal enters a state of hypovolemic shock. At that time there is a physiological response where the blood vessels contract in order to direct as much blood as possible to the heart. This phenomenon is supported by the reaction of the heart’s right ventricle that creates a vacuum drawing in the blood in the veins, as well. In addition, a well-known response to the severing of blood vessels is their contraction in order to limit blood loss. This process explains Rabbi Zeira’s contention that the blood found in the heart is the blood for which a person would be liable to receive karet.