Bikkurim, Chapter 2, Mishnah 7. Continuing the theme of the chapter, which deals with things that resemble one another in one respect and differ from one another in another respect, we learn: "Dam mehalchei shtayim shaveh l'dam beheimah l'hachshir es hazera'im" - the blood of those that walk on two legs is like the blood of an animal in that it renders seeds susceptible to tumah. 'Those that walk on two legs' means those who walk on two feet, that is, human beings.
The similarity to the blood of an animal - rendering food susceptible to tumah:
Human blood and animal blood are alike in that both are counted among the seven liquids that render food susceptible to tumah. An item that is capable of becoming tamei does not contract tumah as long as it has not come into contact with one of the seven liquids; once it has come into contact with one of them, it has been rendered susceptible and can become tamei. Blood is one of those seven liquids - and not only the blood of an animal, but the blood of a human being as well.
The similarity to the blood of a sheretz - exemption from the prohibition of eating blood:
The Mishnah goes on to teach that the blood of those that walk on two legs also resembles the blood of a sheretz - one of the eight creeping creatures listed in the Torah in Parashas Shemini. Regarding the blood of those eight creatures, the halachah is that it is not considered blood for which one is liable for eating blood: one who eats it is liable for the prohibition of eating a sheretz, and not for the prohibition of eating blood.
So too with a human being: one who eats human blood is not liable by Torah law for eating blood, and the prohibition of blood that carries kareis does not apply to it. By rabbinic law one may not eat blood that has separated from the body, but the Sages permitted someone whose gums are bleeding to suck the blood as long as it has not left his body - since there is no concern of maris ayin here, that he might appear to be eating animal blood, because the blood is inside his mouth.
It emerges that human blood is not forbidden by Torah law, and in this it resembles the blood of a sheretz, which does not carry the prohibition of blood. The difference is that with a sheretz itself there is a prohibition of eating a sheretz, whereas for eating its blood there is no liability on account of blood.
In summary: our Mishnah presents human blood from two sides of comparison: on the one hand it is like animal blood with regard to rendering seeds susceptible to tumah, since blood is one of the seven liquids that render food susceptible; and on the other hand it is like the blood of a sheretz, for which one is not liable on account of the prohibition of eating blood.