For the life of the flesh is in the blood.” (Leviticus 17:11)
The word translated here as “life” in the Hebrew original is nefesh, i.e., “soul.” Torah appears to be telling us that soul of every live creature is in its blood. Indeed, Rabbi Schneur Zalman of Liadi says so explicitly in the Tanya:
The abode of the animal soul derived from kelipat nogah in every Jew is in the heart; in the left ventricle, as it is filled with blood, and it is written, ‘For the blood is the soul’ (nefesh)…. just as the blood has its source in the heart, and from the heart it circulates into every organ…” (Likkutei Amarim, 9)
The animal soul is primarily vested in the blood (from where it spreads to the whole body and beyond). What does this mean exactly?
The soul, of course, is a spiritual entity, whereas blood is material. The Kabbalah explains that the soul illuminates the material body as the light illuminates an object it shines on. But there is more to that. The body is a “garment” of the soul. A well-tailored garment must follow the shape of the body for a perfect fit. A jacket has to have two sleeves for the arms to fit in. Accordingly, we would expect to find some structural parallels between the notion of a soul and the blood, in which it is clothed (or in which it vests).
Kabbala tradition recognizes four levels of the animal soul: nefesh, ru’ach, neshamah, and chayah (in Zohar terminology, neshamah d’neshamah). (The godly soul, nefesh elokit has the fifth level, yechidah, but it is not found in the animal soul, nefesh habahamit, which is the subject of our inquiry.)
The word nefesh means “soul.” This is the lowest level of the soul that vivifies the body. Nefesh is responsible for the instincts and motor functions of the body. It is the first level of the soul that vests itself in a child upon its birth. That is why a newborn baby can move its limbs before anything else.
The word ru’ah means “spirit.” The is a higher level of the soul that nefesh. Ru’ach is responsible for the emotive faculties. Ru’ach vests itself in the newborn baby at a later point, when the baby begins to smile or cry upon seeing the mother leaving the room, showing emotions.
Neshama (“breath”) is the highest level of the soul that is vested in the body. Neshamah is responsible for the intellectual faculties. It is the last level of the soul to enter the body of the newborn when the newborn begins to exhibit thoughts and mental activities.
Chayah, meaning “life,” or “living essence,” is the highest level of the animal soul, nefesh habahamit. Chayah does not enter the body but hovers over it surrounding it like an aura. If the first three levels of the soul—nefesh, ru’ach, and neshamah—represent ohr pnimi (the “penetrating light,” or the immanent aspect of the Divine emanation), chayah, represent ohr makif or ohr sovev (the “surrounding light,” or the transcendent aspect of the Divine emanation).
With this background in mind, we can now turn to blood. The function of blood is to deliver fluid, oxygen, and nutrients to all cells of the organism, and to remove carbon dioxide and other byproducts of cell metabolism from the organism. The cardiovascular system responsible for blood circulation is complex and comprises several organs: bone marrow, where blood cells originate, the heart that pumps blood, and vasculature—vessels through which blood flows (arteries that deliver fresh oxygenated blood with glucose to the cells, veins that remove carbon dioxide and other end-products of cellular metabolism from cells, and capillaries). Although lungs are not part of the cardiovascular system per se, lungs oxygenate blood and remove carbon dioxide, playing an integral role in the process. How do these major components of the cardiovascular system correspond to the four levels of the soul?
Nefesh (soul) clearly corresponds to blood itself. Torah testifies to that by stating, “the nefesh of the flesh is in the blood.” Moreover, nefesh is the most basic level of the soul that vivifies the body. Often, the word nefesh is translated simply as “life.” So too blood vivifies the body by providing oxygen and other nutrients to all cells of the organism, and by removing carbon dioxide and other waste products from the blood. Any blockage of the blood flow immediately leads to tissue necrosis. This is why Rashi comments on this verse that life depends on the blood.
The next level is the ru’ach. The word ru’ach means wind. From this point of view, ru’ach may be associated with lung that exhale producing “wind.” On the other hand, four levels of the soul correspond to the four letters of the Tetragrammaton, the proper name of G‑d, Y-H-W-H, or yud-heh-waw-heh. Yud corresponds to the chayah, heh corresponds to the neshamah, waw corresponds to ru’ah, and the second heh corresponds to nefesh.
As we see, ru’ach corresponds to vav, which is just a vertical line resembling a vessel. Thus, from this perspective, ru’ach could be said to correspond to blood vessels.
Neshamah, as mentioned earlier, is responsible for sentience and intellectual faculties. Therefore, it corresponds to the bone marrow, the primary site of blood cell production, or hematopoiesis. In some languages, including Hebrew, bone marrow is called the “bone brain” and for a good reason. White blood cells produced by bone marrow form the basis for the adaptive immune system, which in its complexity is only second to the brain. Its “intelligence” —being able to recognize the invaders and eliminate them, while avoiding damaging healthy body tissue—is mind-boggling. Furthermore, in the correspondence of the levels of the soul to the letters of the Tetragrammaton, neshamah corresponds to the first letter heh. The first heh in the Tetragrammaton is the level of Binah, the “understanding,” alluding to the intellectual faculties. Binah, is the source of Malchut, which is the second letter heh. Similarly, bone marrow is the source of blood. This is yet another reason for the correspondence of neshamah with the bone barrow. On the other hand, the word neshamah literally means “breath.” Breathing in done by the lungs, which are a part of the system because they oxygenate blood on the inhale and expel carbon dioxide on the exhale. Thus, alternatively, we may also associate neshamah with the lungs.
The highest level of the nefesh habahamit, the animal soul, is chayah, the “living essence.” In the analogy with Tetragrammaton, chayah corresponds to the letter yud, which is essentially a dot. The shape of the heart, which is approximately round, could be roughly pictured as a large dot as well. Thus, chayah may be said to correspond to the heart. Another reason for this analogy is that, as mentioned above, chayah corresponds to the level of ohr makif (or, ohr sovev—the “surrounding light”). The heart pumps blood to the whole body, not discriminating between different limbs and organs. As discussed in many works of the Chasidic philosophy of Chabad, the heart represents the level of ohr sovev, similar to chayah.
Let us now summarize our analogy in the table:
|Heh||Neshamah||Bone barrow or lungs|
|Waw||Ru’ach||Vessels or lungs|
The above mapping is on a systems level, where we draw a parallel between various levels of the soul and corresponding organs involved in the production and circulating of blood.
Let us now look at the blood itself. After all, the verse equates the soul with blood!
Blood is made of blood plasma, which contains proteins (albumin is the main protein in plasma that regulates the osmotic pressure of blood), glucose, mineral ions, hormones, and carbon dioxide; erythrocytes or red blood cells that contain hemoglobin; leukocytes, or white blood cells that form the base for the immune system; and thrombocytes, or platelets, that are responsible for the clotting of blood.
Red blood cells are the predominant group of cells in the blood that give blood its characteristic red color. These cells are often identified with blood, and we are going to associate them with the level of nefesh, for the reasons explained above. The main function of red blood cells (erythrocytes) is to deliver oxygen to all cells in the body. Oxygen is the most important element of the air. Kabbalah identifies the element of air (one of four proto-elements of antiquity: air, water, fire, earth) with Keter. The sefirah of Keter is directly connected with the sefirah of Malchut, because the Malchut of the higher world, becomes the Keter of the lower world. Sine the second letter heh is the level of Malchut, this is another connection between nefesh and red blood cells. Lastly, nefesh is often translated as “life,” because nefesh is the level of the soul that gives life to the body. Similarly, oxygen carried by red blood cells also gives life as cells cannot function without oxygen.
One of the meanings of the word ru’ach is “wind,” i.e., the movement of the air. That is what plasma does—it moves through blood vessels carrying blood cells and nutrients. Therefore, we are going to draw a parallel between ru’ach and blood plasma. Plasma also carries hormones that are, in no small degree, responsible for our emotions. This also lends support for this association, because ru’ach, as mentioned before, is responsible for the emotive faculties.
Neshamah, as mentioned above, is responsible for the intellectual faculties of the organism. Neshamah also corresponds to the first heh of the Tetragrammaton, the letter that signifies the sefirah of Binah (understanding). White blood cells (leukocytes) form the core of the immune system, whose intelligence and complexity are still beyond our understanding. Thus, we are going to draw the parallel between neshamah and white blood cells (leukocytes).
The biggest challenge is to find the constituent elements of the blood that correspond to the level of Chayah. Let us recall that that the level of chayah is the level of ohr sovev (the surrounding light). It seems that platelets and albumin—both present in plasma—fit the bill. The difference between the surrounding (ohr sovev) and penetrating (ohr pnimi) light is that the former exerts only general influence, whereas the later exerts specific influence. Other elements of blood, such as erythrocytes (red blood cells) or leukocytes (white blood cells), have specific purposes. Erythrocytes, for example, deliver oxygen to cells, leukocytes are responsible for the immune system. Thrombocytes (platelets) and albumin, on the other hand, have no specific function besides affecting the properties of the blood as a whole. Albumin is responsible for maintaining the osmotic pressure of the blood, and platelets are responsible for forming clots to prevent the organism from bleeding out in case of a trauma. These general functions place these two cells into the category of the surrounding light, which is why we are going to associate them with the level of chayah.
Let us now summarize these parallels in the following table:
|Heh||Neshamah||White blood cells (leukocytes)|
|Heh||Nefesh||Red blood cells (erythrocytes)|
By way of disclaimer, none of the above is purported to be true and authoritative. At best, these are educated guesses that need to be taken with the grain of salt. Perhaps, hematologists, who understand blood better than I, will be able to improve these structural parallels by associating levels of the soul with other constituent elements of blood. I only hope to have demonstrated that, at least in principle, it may be possible to draw some parallels between the levels of the nefesh habahamit, the animal soul, with the structure of the cardiovascular systems or the blood itself, as seems to be implied by the verse in the Torah portion of Acharei Mot.
 English translation is from Weinberg, Yosef. Lessons in Tanya. : The Tanya of R. Shneur Zalman of Liadi.
 Nefesh (sometimes written as nephesh) literally means rest or repose. Thus, some translate nefesh as the “resting soul.”