Based on article from Talmudology.com
וְטָהֲרָה, מִמְקֹר דָמֶיהָ “and cleansed from the source of blood” VaYikra 12,7
The Gemura (Nidah 19) derives that there are 4 main types of red blood. This pasuk uses plural for blood and the pasuk (20,18) also uses plural (2+2=4)
נדה יט, א
תניא נמי הכי שחור כחרת עמוק מכן טמא דיהה אפי' ככחול טהור ושחור זה לא מתחלתו הוא משחיר אלא כשנעקר הוא משחיר משל לדם מכה לכשנעקר הוא משחיר
It was taught in a baraita: The black color of blood that is impure is blood as black as cheret. If the black is deeper than that, the blood is ritually impure; if the black is lighter than that, even if it is still as dark as blue, the blood is ritually pure. And this black blood does not blacken from its outset, when it is inside the body; rather, it blackens only when it is removed from the body. This is comparable to the blood of a wound, which is initially red, but when it is removed from the body it blackens.
In the discussion over the various kinds of uterine bleeding that render a woman ritually impure, the Talmud pauses to consider the color of the blood itself. Perhaps blood is only considered to be, well, blood, when it is red. Rabbi Abahu thought so, and brought a proof from the Second Book of Kings (3:22):
Rabbi Abbahu said that the verse states: “And the sun shone upon the water, and the Moabites saw the water some way off as red as blood” [which indicates that blood is red. The Talmud asks:] Is this to say that blood is red? If so, one can say that only blood that is red like the blood of a wound [is ritually impure], and no more colors of blood are impure.
The Many Colors of Blood
This is not the first time we have encountered Talmudic hematology and a discussion of the color of blood. When we studied Chullin (87b) we read the following:
Rav Yehuda says in the name of Shmuel: All mixtures of blood and water that maintain a reddish hue are considered blood and render food susceptible to contracting ritual impurity, and are included in the obligation of covering the blood and effect atonement by being presented on the altar...Rabbi Asi of Neharbil says: The statement of Rav Yehuda is referring to the clear part of the blood [i.e., plasma. If the plasma has a reddish hue due to the blood, it has the status of blood and can render food susceptible to contracting ritual impurity].
That is one of the earliest references to a component of the blood known today as plasma. Here is how Rashi explains what Rav Yehuda was describing:
בצללתא דדמא - באותן מים שהם מן הדם עצמן כשהוא נקרש יש סביבותיו צלול כמים ואם יש
The clear part of the blood: The liquid that is part of the blood itself. When blood clots, it is surrounded by a clear liquid...
It took another couple of thousand years for us to understand the nature of this “clear part of the blood.” Sure, blood looks uniformly red, but if you let it stand (and not clot) or better yet spin a sample in a centrifuge you will notice that to the naked eye it is made up of several components. At the bottom is a layer of dark red stuff, made up of red blood cells - you know, the ones that carry oxygen from the lungs around the body. On top of that is a thinner, lighter layer, made up of white blood cells that fight infection and platelets that are vital in forming blood clots. This layer is known as the buffy coat, from the word buff meaning yellowish (sort of like a manilla envelope). At the very top is a third layer with a yellowish tinge. We call that plasma, and it is what Rav Yehuda called “the clear part of the blood” - צללתא דדמא . So that is the plasma.
The Talmud does not address the separate color of the plasma, but the combined color of blood that has been left to clot outside of the body.
The Components of Blood
As anyone who has cut their knee will know, the blood that first oozes out is not bright red but a darker hew. This is blood from the venous side of the circulation; there is little oxygen in this blood since it has all been extracted by the muscles and organs that need it. The hemoglobin molecules in the red blood cells shine a different, darker color when they are not carrying a molecule of oxygen. Hence the darker blood.
Hopefully you’ve never done this, but cutting an artery is a whole different story. The blood does not trickle out. It gushes out like a little fountain. And it is bright, bright red. It comes out under force because the arteries are connected directly to the heart and carry the blood under pressure. It is bright red because the hemoglobin has just taken a trip through the lungs where they have happily bound to a molecule of oxygen. And oxyhemoglobin is bright red.
As the Talmud notes, once blood leaves the body it darkens (as oxyhemoglobin and other proteins break down) and turns black. The rabbis ruled that this denatured black blood is as ritually impure as its brighter red former self.
