Blood Chemistry

Our blood is red. But red-bloodedness isn't universal in the animal kingdom. In some creatures, the vital fluid is purple; in others it's yellow, green or even white.

What is responsible for these differences? Blood Chemistry!

The rich red of vertebrate blood comes from haemoglobin, a protein in red blood cells that contains iron atoms. Haemoglobin's job is to deliver oxygen, and its iron atoms, which has affinity to oxygen, make our blood look red. When iron atoms are attached to oxygen, blood is a bright cherry red; when less oxygen is bound, it's a muddier brown-red.

But many creatures don't use haemoglobin.
Octopuses and squid, for example, and crustaceans such as shrimp and crabs, rely on a protein called hemocyanin to ferry oxygen around. Hemocyanin has copper atoms—which look blue when bound to oxygen—instead of haemoglobin's iron.

Hemocyanin doesn't bind to oxygen as efficiently as haemoglobin does. But it functions especially well in cold temperatures, such as those found in the deep ocean. And it can have more jobs than carrying oxygen in the creatures that use it—research suggests that haemo-cyanins can play an immunity role, aiding in healing wounds, for example.

Other marine creatures, such as clam-like brachiopods and the seafloor-dwelling peanut worm, bleed violet—but not because of hemocyanin. Instead, these creatures use an iron-rich, purple-coloured protein pigment called hemerythrin to shuttle oxygen throughout their blood.
Some sea squirts—squishy, potato-shaped ocean-living animals—bleed greenish yellow. They do not produce haemoglobin, and there's some debate about what imparts the yellowish hue. One candidate is the element vanadium, which many sea squirts concentrate in their blood.

Another theory points to diet. When sea squirts metabolize their staple meal, marine phytoplankton, they create carotenoids, pigments that are yellow, red or orange.

Different mechanisms can make animals bleed full-on green. Several lizard species in New Guinea use haemoglobin to transport oxygen around, like vertebrates typically do. But then they break down the haemoglobin into a naturally green pigment called biliverdin (the same molecule, when present in high concentrations, causes the yellow-green appearance of jaundice in people). And unlike most vertebrates that metabolize biliverdin into other, duller compounds, these New Guinea lizards accumulate the pigment in their blood to the point that its green hue overwhelms the red of haemoglobin in the bloodstream and tissues.

Antarctic blackfin icefish of the family Channichthyidae are the only known vertebrates that don't make haemoglobin and do not have red blood cells. Their blood is a milky, translucent white.

Instead of haemoglobin or any equivalent oxygen-binding protein, the icefish rely on oxygen dissolving directly into their bloodstream. They also have larger hearts and capillaries compared with other Antarctic fish, and these can pump and circulate greater quantities of blood. Icefish are patient ambush predators—they lie in wait rather than darting around to catch their meals, expending less energy and requiring less oxygen use.

A recent study of icefish investigated the genomes of several fish species to understand why genes for making haemoglobin have been deleted from the creatures' DNA. Whether icefish first lost their red blood cells and then their haemoglobin, or vice versa, is still a mystery. What drove the loss is also a puzzle. One notion the researchers suggest is that the cold waters of Antarctica's Southern Ocean were oxygen-rich enough to render the middleman job of haemoglobin obsolete.

In high-altitude ecosystems, many species must cope with low oxygen. This has pushed the haemoglobin of some mountain dwellers into overdrive.

The deer mouse, for example, can live up to 14,000 feet (4,300 meters) above sea level—thousands of feet higher than the point that many humans start to develop altitude sickness. The mouse has evolved haemoglobin that makes extra-tight bonds with oxygen. Andean hummingbirds and high-altitude black-spotted frogs have similar alterations to their haemoglobin, too.

Yet there are always trade-offs with adaptations.
That high-elevation deer mouse, for example, has some trouble getting oxygen off the haemoglobin and into its tissues. It makes up for missed oxygen deliveries by having higher quantities of oxygen-releasing mitochondria in its muscle fibres.

Different colours of blood based on their chemistry:

Red Blood (Iron-Based)

Red (Haemoglobin): Found inside red blood cells of vertebrates. When iron binds to oxygen, it reflects a bright crimson or cherry-red colour. 

Animals: Humans, mammals, birds, reptiles, amphibians, fish, and some earthworms and leeches.
Pigment: Haemoglobin (contains iron).
How it works: Turns bright red when bound to oxygen and dark red when deoxygenated.

Blue Blood (Copper-Based)

Animals: Octopuses, squids, horseshoe crabs, lobsters, crabs, and most spiders.
Pigment: Hemocyanin (contains copper).
How it works: Reflects blue light when carrying oxygen.

Green Blood

Green (Chlorocruorin or Biliverdin): In annelid worms, chlorocruorin gives a greenish tint. In the New Guinea green-blooded skink, the green colour comes from a massive build-up of the toxic bile pigment biliverdin that would normally be filtered out by a liver.

Animals:
New Guinea skink lizards (green bones, muscles, and blood).
Earthworms and certain leeches.
Pigment: Biliverdin (in skinks) or chlorocruorin (an iron-containing protein in some marine worms/leeches).
How it works: In skinks, a massive build up of the green waste pigment biliverdin overwhelms the red haemoglobin.

Violet / Purple Blood

Purple (Hemerythrin): Contains iron, unlike haemoglobin or hemocyanin, and turns a violet-pink shade when bound to oxygen. 

Animals: Peanut worms (sipunculids), brachiopods (lamp shells), and some marine worms.
Pigment: Hemerythrin (an iron-based protein that isn’t a true heme).
How it works: Appears clear or pinkish when deoxygenated and turns a violet-purple hue when oxygenated.

Yellow Blood

Yellow (Vanabin): Sea squirts accumulate the heavy metal vanadium in their blood cells, though it is used for other metabolic functions rather than primary oxygen transport.

Animals: Sea cucumbers, sea squirts (tunicates), and certain beetles.
Pigment: Vanabin (contains vanadium).
How it works: Uses the vanadium-based protein in its body fluid, though it is not primarily an oxygen-carrying respiratory pigment.

Blue (Hemocyanin): Instead of being packed into cells, copper-based hemocyanin floats freely in the blood. When it oxygenates in cold or low-oxygen environments, it turns blue. 

Clear / Colourless Blood

Clear / Colourless (No Pigment): The Antarctic icefish lacks red blood cells and haemoglobin entirely, surviving because freezing polar waters hold high amounts of dissolved oxygen that diffuse straight through their skin.

Animals: Antarctic icefish, cockroaches, grasshoppers, and praying mantises.
Pigment: None.
How it works: Antarctic icefish completely lack haemoglobin and transport oxygen dissolved directly into their blood plasma; insects use an open haemolymph system that does not use blood to carry oxygen.

Most insects also have colourless or pale haemolymph because they use a tracheal tube system to deliver air directly to their tissues rather than carrying it in blood.

Image credit: Compoundchem