I know as well as anyone that, psychologically, blood isn’t merely a fluid among many, especially in reference to one’s own. Brace yourself for icky anecdotes … OK, for example, once I did a header over the arms of my bicycle and didn’t tuck my head enough through my roll-out, thus gashing my scalp on the way. Scalp wounds bleed like all get-out. I rode my bike home and shocked my roommates, as I was literally painted with the stuff to the waist.
Tough guy, huh? Well, it’s also the case that I display the classic vasovagal response to a blood draw. It doesn’t happen at the sight of the needle – for example, I dislike getting shots but I don’t faint – but when I see the blood creeping up it, I sure do. Wham – like a sack of grain.
This post is similar to Breath of life, in investigating a phrase-and-concept that’s completely disconnected between words and reality, and it goes way beyond the educated snob’s distaste for ordinary and often metaphorical speech. This is major values stated as raw and categorical reality – when it’s not. Blood is, indeed, merely a fluid among many. But try telling someone so. Or believing it yourself.
Here’s the reality: all life is aqueous. Cells only function in the context of water, no matter where the creature happens to live; so-called “terrestrial” organisms are merely aquaria. A big multicellular one such as you or I is a tank of body fluid, of which quite a bit is interstitial fluid – what the cells sit in. A fair amount of it is subject to bulk transport, meaning, organized movement of a subset of this fluid from here to there ’round the tank, called circulation. (For those allometricians among us, this is why size curves go up exponentially at 3/4, not 2/3 as the cube-square would suggest – above a certain size, organisms display a fourth dimension of structure due to internal infolding and branching.)
Everyone thinks they understand circulation, but stick with me for a moment – the water in blood is constantly exchanging with interstitial fluid. Got that? Blood, or at least, the water that composes most of it, is not sequestered away in those tubes. When I say blood is a subset of the more general category, I really mean it, and I also mean, a dynamic subset, shifting back and forth.
That’s why the vaunted distinction between closed vs. open circulatory systems is actually not that big a deal. The closed one is doing what the open one does, but in the context of a larger organism, for which pressure and diffusion need a bit more structural support.
Regarding our own blood’s specific contents, as with so many things, that’s merely one of many versions that do similar stuff across all sorts of organisms. Among other vertebrates, the only notable difference is that mammalian red blood cells lose their nuclei; the whole two-chamber, three-chamber, and four-chamber heart distinction is actually not that big a deal. There is no “mixing” of oxygenated vs. deoxygenated blood in the three-chamber heart, or so little that it makes no difference. The full septum between left and right ventricles in mammalian and avian hearts is a minor refinement of already-existing function, not a foundational shift in function.
Moving farther afield, some good examples of convergence show up, as hemoglobin and its tricks with iron are not the only way to carry extra oxygen in blood. There are several other relevant proteins found across a number of animals, most famously the “blue-green blood” effect in crustaceans and molluscs who use hemocyanin instead.
On to the mental constructs. “Life-blood” is the minor example, related to my state of mind as described above. It’s not literally true. Pound for pound blood is less alive than most tissues, as it’s mostly water, plus the red blood cells lack nuclei and metabolism, and therefore are ex-cells. I obviously stress about blood in a specific way, and not too unreasonably; we all know how important it is for vertebrates to keep that stuff enclosed and moving about the body. Given that biological analysis of blood is very recent compared to the hard-and-harsh personal experience of it, obviously, when you lose blood, you die. That is less the indicator of a perfect or wonderful physiological feature than of a breakpoint in structure – given an injury to my circulatory system, I’d rather have an open system than the high-pressure closed kind, which tends to empty itself when breached …
The more interesting, less obvious example lies in its puzzling association with kinship.
- He’s my blood
- The same blood flows in our veins
The logic might be, well, blood is important, and kinship is important, so if I say kinship is like – or rather is – blood, then I am underscoring its importance … but the phrasing and psychology go beyond analogy into pure literalism. My perception from teaching is that people really do think that siblings, and other kin, have the same blood, actually sharing blood in some essentialist way. It’s incredibly hard to shake students of it, to the point that people think you’re questioning the values when you clarify the physiology. It’s one of the best examples of naturalism I’ve encountered.
The more so regarding ethnic distinctions, which are often over-determined conceptually into issues of kinship far more than the actual genetics indicate.
But … isn’t it functionally linked in some way? Nope. Blood doesn’t do anything different from anyone else’s, kin or not. Maybe … and it’s about the only thing I can think of … hemophilia could reinforce the concept, in that a person’s blood is “thinner,” i.e., doesn’t thicken or coagulate upon exposure to air, and that it’s clearly consolidated in family lines. But European royalty aside, hemophilia isn’t all that common. But how that could be extended into thinking different whole families and whole ethnic groups could have distinctive blood, I don’t know. Especially since families and ethnic groups are enthusiastically out-breeding every minute.
What about blood type and compatibility re: transfusion? To some extent, but only insofar as siblings’ blood types are derived from combinations of the parents’ – there’s no guarantee that any two siblings have the same kind. And prior to transfusion (which as a technology is merely a century-plus old), blood type was just about the most inconsequential phenotype in history, so in looking for a historical source for this long-standing association, we have to look somewhere else. Again, I turn to the historians of science with a quizzical eyebrow, only to be met with silence.
It’s such a powerful construct, too. People who vociferously reject notions of genetic similarities and differences as important to behavior, e.g., in recognizing kin and in choosing mates, turn right around and talk of “blood” as a fixed and crucial element of kinship and identity. Check into your own mind, think about how you relate to blood, conceptually. What do you find?
Next: Just look at the label