Among the most important parts of the sacrificial service is kabbalat ha-dam – collecting the animal’s blood – and zerikat ha-dam – sprinkling the blood on the altar. The Mishnah on today’s daf (=page) teaches that if the blood spilled on the floor it cannot be gathered up and used for zerikah. The ruling in our Mishnah only applies if the blood from the animal spilled immediately after the animal was slaughtered, for if it was first collected properly and only then did it spill on the floor, it can be collected and used (see the Mishnah at the beginning of the third perek, or chapter, daf 32a).
Thus, our Mishnah is focused on the initial kabbalat ha-dam, and the Gemara quotes a baraita that teaches that the blood that is collected and used for sprinkling can only be dam ha-nefesh – the life-blood – and not blood of the skin or the draining blood. This is derived from the repeated use of the term mi-dam ha-par – the blood of the bull – (see, for example, Vayikra 4:5) which is understood to mean that the requirement is the blood that comes directly from the bull at the moment of slaughter.
When an animal is slaughtered, the very first blood is dam ha-or – blood of the skin – meaning the blood that is part of the small blood vessels that carry nutrients to the skin. Once the slaughterer’s knife reaches the main arteries, the dam ha-nefesh – the life blood that flows as long as the pumping action of the heart continues – will be spilled. This blood is calleddam ha-nefesh because it is the blood that keeps the animal alive and with its loss will bring about cessation of the activity of the heart and ultimately, death. Even after the animal has lost this blood, there is dam ha-tamtzit – draining blood – that was in the arteries and will continue to flow out of the animal due to capillary action after the animal is already dead.
The only blood that is appropriate for use in the sacrificial service is the dam ha-nefesh, not dam ha-or or dam ha-tamtzit.