Crabs have blue blood; why don't we? Plus: Does your blood thicken in a cold climate?

April 25, 2008

Dear Cecil:

The horseshoe crab is one of the oldest species on earth, yet it is one of the few (if not the only) species with copper-based blue blood. The rest of us recently developed beings have red, iron-based blood. Did all animal life on earth at one time have copper-based blood? Did natural selection favor red-blooded beings? Or would all earth life still have blue blood if it hadn't been for an invasion of iron-blooded beings from the red planet, Mars?

Cecil replies:

All right, today’s blood day; cookies and juice at the end of the column.

One of the main functions of blood as it's evolved over the eons has been to transport oxygen around an organism's body. Early oxygen-breathing life forms were limited in their complexity in part because their primitive circulatory fluid just wasn't very good at this task. But things really perked up life-on-earth-wise once creatures started producing blood pigments — metal-containing compounds that are able to grab onto oxygen molecules and release them when and where needed. In red-blooded animals the critical pigment is hemoglobin, the primary constituent of red blood cells. Hemoglobin molecules are structured around atoms of iron, and it's these that give the blood of vertebrates its bright red color. (Well, bright red when it's laden with oxygen, duller red when it's on its way back to the lungs.)

Hemoglobin's good at what it does, but as you note it's not the only game in town. The horseshoe crab indeed relies on hemocyanin, a copper-based pigment that makes blood blue. You're right, too, that horseshoe crabs are some of the more ancient animals still in business — they've been scuttling around for something like 540 million years — but they're far from the only extant species to have gone the copper route. After hemoglobin, hemocyanin is the second most commonly encountered blood pigment, and plenty of other arthropods (including lobsters, crabs of the nonhorseshoe variety, and assorted insects) and mollusks (among them snails and octopuses) have blue, copper-based blood. There are other pigments out there as well, and some animals employ more than one oxygen carrier; scientists aren't sure, but the invertebrates known as sea squirts may do their breathing via a combo of hemocyanin and vanadium-based compounds.

Neither hemoglobin nor hemocyanin seems to have evolved from the other — evidence suggests each originated independently a billion-plus years back. (There's speculation that both pigments may have initially developed in organisms to which oxygen was toxic, and that their original function was to neutralize O2, not ferry it around.) Generally speaking, hemocyanin isn't as efficient at carrying oxygen as hemoglobin, but you can pack more of it into the same space. It seems to work well for aquatic creatures in oxygen-rich habitats, and it may provide some advantage where the water is cold and relatively acidic.

Horseshoe crab blood is extremely useful stuff, it turns out, and not just to the crabs. One danger in the manufacture of pharmaceuticals is a particularly rugged group of bacterial contaminants called endotoxins. So say you're mixing up a batch of meds — how do you determine whether it's endotoxin free? Add some horseshoe crab blood. A substance in it called Limulus amebocyte lysate, or LAL, will quickly bind to any endotoxin present and clot, telling you you've got bad medicine. Harvesting LAL is a $50 million-a-year industry; a quart of crab blood will set you back about 15 grand.

Dear Cecil:

Me and a friend got into a conversation about moving to a different weather climate. She told me that blood thickens or dilutes every seven years, so after seven years you get used to any weather. To me that does not seem exactly right, but I was curious if there is any truth to this.

Cecil replies:

You read a question like this and your first reaction is: I am now looking at one of the dumbest things ever committed to writing. Just as you're set to move on, though, you can't help thinking: OK, but mightn't there be a mangled nub of something in there worth addressing? And after some reflection you realize: Actually, there is.

Blood isn't like motor oil — it doesn't just thicken up when the weather gets brisk. Yes, low temperatures affect blood viscosity, and exposure to severe cold might result in temporary blood thickening in the extremities. But it would take a very serious drop in body temperature to cause any significant thickening of one's blood overall.

No, what most determines the viscosity of blood is the proportion of red blood cells in it, which is called hematocrit. An 11 percent increase in hematocrit will raise blood viscosity about 20 percent. And here we arrive at the mangled nub. At high altitudes, your body adds red blood cells to the mix to pull more oxygen out of the thin air. So if you move from LA to La Paz, your blood will in fact thicken to help you acclimate.

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