As we have learned on the previous dapim (=pages) of the Gemara, according to the Torah (see Vayikra 11:38) ritual defilement of food will only take place when someone or something that is tameh (=ritually defiled) comes into contact with food that has been made wet. Although the passage in Vayikra mentions specifically that the wetness comes from placing water on the food, the Gemara explains that the “wetness” necessary to “prepare” the food for defilement can only be by means of one of seven liquids – wine, blood, oil, milk, dew, honey or water.
On today’s daf we learn a baraita that was taught in the School of Rabbi Yishma’el that not all blood will be able to serve this purpose. Based on the passage in Sefer Bamidbar (23:24) that compares the Jewish people to a lion that drinks the blood of its victims, i.e. like water, the baraita concludes that only blood from a dead creature – dam halalim – will serve this purpose; if the blood comes from a live animal, then it is dam kilu’ah – “flowing blood” – that cannot “prepare” food for potential defilement.
Rashi explains that dam halalim refers specifically to blood that flows from the animal at the moment of death; Tosafotargue that that blood would still be considered dam kilu’ah – blood from a living animal and that it is only blood that comes from the animal at a later stage that is considered to be dam halalim. Both would agree that any blood that is collected from the animal when it is alive, e.g. from an injury or from bloodletting, would not meet the criteria for dam halalim and such blood would not “prepare” food for potential defilement.
The Gemara does consider the possibility that any blood from ritual slaughter should be considered like water for this law, based on the passage in Sefer Devarim (12:16) that forbids eating blood and requires that it be “poured upon the earth as water.” Nevertheless, the Gemara rejects that suggestion, arguing that that passage is used as a source for other laws.