Throughout the tractate, from the first Mishnah to the last, we have mentioned the prohibition against eating blood, whose penalty is kares. Now we will focus on the prohibition itself. It is a complex prohibition, as it is mentioned in the verses several times and in various ways. Therefore, we will first introduce the general framework, so that we know where things are heading.
The general framework of the prohibition:
The verse states, "Lo socheilu dam" - you shall not eat blood, and continues to specify birds and animals, the land creatures. These are the species whose blood is forbidden by Torah law, whereas other species, such as fish and grasshoppers, are not included in the prohibition at all, since they are neither birds nor animals.
Even in the world of birds and animals, blood is divided into three categories:
Dam hanefesh: Lifeblood that leaves the animal as a result of a fatal blow. According to Tosafos, this refers specifically to the blood that flows out at the very last moment. According to others, however, and as is the halachah, when an animal is slaughtered, all the blood that spurts from the neck while it is dying, as long as the heart is beating, is dam hanefesh for which one is liable to kares.
Dam hatamtzis: Blood that drips from the animal before or after its death. This blood is forbidden by Torah law, but only carries the penalty of lashes, not kares.
Dam haeivarim: Blood located within the animal's limbs that has not separated from its place. According to halachah, this blood is not forbidden by Torah law at all.
The term "dam hanefesh" is the language of the Sages, and it is based on the verse that forbids it: "For the life of all flesh, its blood is its life... whoever eats it shall be cut off." Another verse states: "Any person who eats any blood, that soul shall be cut off from its people" - here too, the penalty is kares, but it says "any blood," and it is possible that the word "any" comes to include a broader category, as we will see later.
The language of the Mishnah:
"Dam shechitah" - shechitah is a specific method of killing: cutting the esophagus and the trachea, the food pipe and the windpipe. While the animal is dying, the heart is still beating, and the blood bursts and spurts from the neck at the site of the cut. This blood, "bein temei'im bein tehorim" - whether from non-kosher or kosher species, whether it comes from a domesticated animal, a wild animal, or a bird, and even from a non-kosher animal, such as horse blood and the like - one is liable to kares for it.
"Dam nechirah" and "dam ikur" - these are also methods of killing other than shechitah. Nechirah is stabbing the animal with a knife; ikur is tearing the signs, the trachea and esophagus, from the animal's neck. The rule is the same: since the animal is dying from the wound, the blood that flows out during the death throes is what carries the prohibition of kares.
"Dam hakazah shehanefesh yotza'ah bo" - bloodletting for an animal is done for the sake of its health, just as they used to let blood for people in past generations as a form of medicine. If the animal became sick, they would let a little of its blood in the hope that it would recover, but with excessive bloodletting, it will eventually die. The final blood that comes out, when the heart is beating its final beats and the soul departs with it, is the blood for which one is liable to kares. However, blood that comes out of a wound that the animal does not die from, as well as blood that flows after its death, does not carry the prohibition of kares.
"Dam hat'chol vedam halev" - this refers to the blood found in the spleen itself and the blood found in the heart, and not to the blood in the cavity of the heart. We know today that the heart is composed of four chambers, but the Mishnah does not distinguish between them and simply uses the term "cavity." The discussion here is about the blood found in the tissue of the heart, in the flesh of the heart itself, as well as in the flesh of the spleen. This blood, if squeezed out from the flesh of the limbs, is forbidden - but not with the penalty of kares, since it is not dam hanefesh.
However, the blood in the cavity of the heart, which accumulated inside its chambers, blood that did not spurt out but arrived after the cut and was drawn into the heart to be pushed and exit again - one who eats it is liable to kares. In contrast, blood of the limbs is only subject to a regular negative prohibition.
"Dam beitzim" - two explanations are given for this:
Eggs in the literal sense: Blood found in an egg. If it does not contain a developing chick, there is no Torah prohibition at all, and according to halachah, it is merely a rabbinic prohibition. However, if it contains a developing chick, it is prohibited by the Torah, but it does not carry the penalty of kares.
Eggs meaning testicles: According to this interpretation, their halachic status is like that of the spleen and the heart, organs that contain a large amount of blood. If the blood separates from the organ, it involves a negative Torah prohibition.
"Dam dagim vedam chagavim" - the blood of fish and locusts is not prohibited at all and is permitted to eat. There is no obligation to salt fish or locusts, and they may be eaten with their blood. If the blood separates from the fish, there is a halachah of maris ayin (appearance of prohibition), and fish scales must be placed in it so that onlookers will see that it is fish blood and not animal blood.
"Dam hatamtzis" - this is the blood that drips out. The circulatory system, the veins and arteries, is like a closed system of pipes in which blood flows; once it is cut during shechitah and the animal dies, any blood remaining in the pipes is liable to drip and ooze out. This blood is prohibited only by a standard negative prohibition and carries the penalty of malkos (lashes) alone, which is why the Mishnah says "ein chayavin aleihem" - one is not liable to kares for them.
The Measure of the Prohibition:
It turns out that the condition for liability to kares is the intentional eating of blood that emerged specifically from a wild animal, a domesticated animal, or a bird, at the time the animal is dying from that wound. The measure for liability for eating this blood is a kezayis, which is an exception: usually, the prohibited measures for liquids are a revi'is, but since the verse uses the term eating - "lo socheilu" - the prohibition depends on an act of eating. An act of eating, as we have learned in this tractate, requires a measure of a kezayis. Therefore, a person is liable to kares for a kezayis of blood in this scenario.
Other Types of Blood:
Dam hatamtzis: A Torah prohibition, but it only carries the penalty of lashes.
Blood of the limbs that separated: This is also a Torah prohibition.
Blood of the limbs that did not separate: A piece of meat washed in water is permitted to be eaten even without salting, since the blood did not separate or leave its place. However, cooking with heat draws the blood out of its place, so one must salt it before cooking; but raw meat is permitted with just washing.
Blood of eggs: According to halachah, this is only a rabbinic prohibition, as long as there is no developing chick inside it.
"Rabbi Yehudah mechayev bedam hatamtzis" - Rabbi Yehudah holds that one is liable to kares even for dam hatamtzis, which drips from the veins. This is not because he considers dam hatamtzis to be the life-blood, for it certainly is not, but because of the verse, "Any person who eats any blood, that person shall be cut off from his people." From the inclusive phrasing "any blood," he derives that every type of blood is included, not just the life-blood that spurts out during the dying process, but even dam hatamtzis that drips out afterward. However, the halachah does not follow Rabbi Yehudah.
In summary: We learned that the Torah prohibition of blood applies only to birds and animals, and not to fish and locusts. The liability to kares applies to the life-blood that exits while the animal is dying - whether through shechitah, stabbing, tearing, or letting blood through which the life departs - as well as the blood in the chambers of the heart, and its measure is a kezayis. In contrast, the blood of the spleen, the blood of the heart tissue, dam hatamtzis, and the blood of the limbs that separated are prohibited only by a negative prohibition and lashes, and the blood of eggs is only rabbinically prohibited. As for Rabbi Yehudah, who disagrees and holds one liable to kares for dam hatamtzis based on the derivation of "any blood" - the halachah does not follow him.