מוח הרי הוא כבשר. ודם הנבילה אינו מטמא כנבילה אלא הרי הוא כמשקין טמאים שאינו מטמא לא אדם ולא כלים מן התורה: [רמב"ם הלכות שאר אבות הטומאה א-ה]
The marrow of a bone is considered like flesh. The blood of an animal carcass does not impart impurity like an animal carcass. Instead, it is considered as an impure liquid which does not impart impurity to a person or to utensils according to the Torah.
1] The Chazon Ish [Machshirin 9] asked that the Rambam made a rule [Tumas Ochlin 1-4] that any liquid that does not not make food susceptible to becoming tamei [מכשיר לקבל טומאה] can also not be tamei itself and only the 7 liquids that are מכשיר can become tamei. And from the Rambam in Tumas Ochlim [10-3] it appears that דם נבילה is not מכשיר, so how can it be מטמא??
2] The Rambam implies that the shiur of blood is a כל שהוא but in his Peirush Hamishnayos he says that the shiur is a רביעית.
3] The Ritva [Shabbos 77a] and other Rishonim argue and say that דם נבילה is טהור.
