Blue Bloods

With the exception of the crocodile icefish (a topic for another day), all the vertebrates and a few of the invertebrates on Earth rely on hemoglobin* to transport oxygen from the lungs to the tissues that will use it.  That means that everything from fish to lizards to birds to mammals all have red blood blood when it's oxygen rich, and a blueish-purpley blood when it's oxygen depleted.  Basically, red going out from the lungs, blue coming back into it.  If you're as Caucasian and untanned as I am, you can prove that just by looking at your wrists.

Alright, the vertebrates make up about 5% of the known species, so what about the rest?  Well, most insects (which make up roughly 90% of all animals) don't really have a blood circulatory system as we'd think of it, so we'll just leave them out of this.  That pretty much leaves us with the non-insect branches of the arthropods and the molluscs.  And what did we learn were molluscs a couple of days ago?  That's right!  The sensational cephalopods!  Alright, so what do they all use for their blood if not hemoglobin?  Hemocyanin.

Hemocyanin, which sounds too much like cyanide to make me feel entirely comfortable thinking about something requiring it to live, basically uses copper atoms in place of the iron atoms in hemoglobin.  It's also a much larger molecule than hemoglobin, which means that it doesn't need to attach itself to a blood cell to keep itself from clogging the body's fine filters like the kidneys.  That means that even though cephalopods (alright, and a whole lot of other critters, but this is an unofficial Cephalopod Week) have an equivalent to hemoglobin, they don't have an equivalent to red blood cells.

Now hemocyanin, since it's a copper compound instead of an iron compound, doesn't go from blue to red when it's attached to an oxygen molecule like hemoglobin does, it goes from colourless to blue.  I couldn't find any pictures of octopus blood (and believe me, I tried), but I managed to find a whole bunch of horseshoe crab blood, which is also coloured by hemocyanin instead of hemoglobin.  Check it out:

Click to embiggen, clicky-poppy

Pretty cool, huh?

Cephalopods: The blue bloods of the seas.

Damnit, I know there are only 800 or so species of cephalopods, and over 85,000 species of other molluscs that also rely on hemocyanin, plus an unknown number of arthropods, which means they aren't really the blue bloods of the seas, but this is an unofficial Cephalopod week, so let them (and me) have that, okay?

*For the record, I prefer the British spelling, haemoglobin, but I hate that it sets off my spell-check, so I'm using the American spelling.

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Sources:
PressOfAtlanticCity.com article - great picture of blue horseshoe crab blood that I couldn't hotlink to
Image taken from PSB.org article on horseshoe crab blood
HowStuffWorks.com article on the octopus body plan
Xenology.info page on non-hemoglobin-based circulatory systems - with regards to alien physiology, but with solid chemistry info nevertheless
Article from DavidDarling.info - "The Internet Encyclopedia Of Science", basically the same article as the xenology one above, but with better presentation and slightly less science
Fascinating video on horseshoe crab blood collection from Journal of Visualized Experiments - only loosely related, but interesting nonetheless
Wikipedia article on Hemocyanin
Wikipedia article on Octopuses
Wikipedia article on Red blood cells
Wikipedia article on Hemoglobin