The mishna gives a list of items such as the kometz of the mincha and th blood of the korban which re used to be matir other things, therefore the eating of these things would not be considered a violation of pigul to be chayev kareis. Tosafos asks, why by the kometz of the minchah do we need to exempt it from pigul based on the rationale of pigul only applying to "something which has a matir, not something which IS a matir". Even without that rationale one couldn't be in violation for eating the kometz since another condition of pigul is that the matir must be brought on the mizbei'ach according to it's mitzvah (and by eating it, he isn't doing it's mitzvah)? Tosafos rejects rashi's approach that we need it for case where he removed it from the mizbeiach after the fire took hold, because then it would no longer be considered pigul at all. Tosafos answers that we need it for a case where part of the kometz is brought on the mizbeiach and he ate the other half. The half on the ground that was never brought up one would be chayev for eating if not for the rule of only being chayev on something that has a matir, not on something that IS a matir.
However, this approach doesn't resolve the same question regarding the blood that is sprinkled on the mizbeiach. Why do we need to say that you can't be in violation of pigul because it IS a matir, if anyway there can't be pigul unless the mitzvah is done properly? The case can't be when he ate from the remaining blood in the mizrak (vessel) after sprinkling because that blood is not a matir and the rationale to exempt doesn't even apply? Tosafos suggests that the concept of pigul falling off by it being brought on the mizbeiach may only apply to things that are burned on the mizbeiach, not things that are sprinkled on the mizbeiach like blood. Another suggestion Tosafos has is that since all the blood in the mizrak was fit to be sprinkled, it is all called a matir.
The difficulty with Tosafos first answer is that the concept of pigul falling off when it is brought on the mitbeiach isn't learned from a pasuk, it is pure logic. Since the burning of the kometz on the mizbeiach can establish the other parts of the korban mincha to be pigul by it being accepted as a korban, it certainly can cause the status of pigul to be removed from it itself. Why wouldn't that exact same logic apply to the blood - since it can cause the meat to become pigul by the sprinkling being accepted, it should certainly remove the status of pigul from itself? Tosafos hints to the answer by saying - כי אם גבי הקטרה דוקא שנעשה לחמו של מזבח. Meaning, the logic if it removing pigul from itself is by becoming food of the mizbeiach. It is not the act of burning but rather the result of burning which removes the pigul. Therefore, the blood which doesn't become the food of the miz'beiach would not lose it's status of pigul and the only reason one wouldn't be in violation of pigul by eating the already sprinkled blood is that it IS a matir.