It used to be a common misconception that oxygenated human blood is red and deoxygenated human blood blue. This probably arises from diagrams like this:
In these diagrams and in models it’s a convention to colour most veins and smaller vessels through which blood flows toward the heart as blue, and most arteries and smaller vessels through which it flows from it as red. The pulmonary circulation may be different as in that subsystem it’s the artery which carries deoxygenated blood and the vein in which it’s oxygenated. The colour of oxygenated blood is more or less accurate, but deoxygenated blood is not blue but close to Tyrian purple:
Veins do look blue. This is because of Rayleigh scattering in their surfaces and blood is not visible through the walls of larger veins, which are however proportionately thinner than arteries and therefore more likely to have visible blood. Clearly blood is visible in smaller vessels since parts of the human body are bright red, such as the palpebral conjunctivæ and the buccal mucosa, but the cause of the colour of the clear sunlit sky and most examples of blue colouring in animals is the same as that of veins: smaller opaque particles scatter shorter wavelengths of visible light more than longer ones and human colour vision is less sensitive to violet than blue. In fact I don’t really see the sunlit sky as blue for some reason, but I’m aware that people generally report it as blue and I certainly see veins and some human irises as blue.
Arterial blood is vivid red, almost unnaturally so if that made any sense, and this is of course due to the fact that it contains oxygenated hæm, an iron-rich porphyrin. Porphyrins are unusual ring-shaped molecules with metal ions at their centres. Another very common porphyrin is found at the centre of chlorophyll and contains magnesium. Likewise, cyanocobalamine, also known as B12, has a cobalt atom at its centre. Cyanocobalamine is in fact the raw material for hæm itself, and is, I think, turquoise.
The porphyrins have a particular property which is very useful. They can flip between two different states, and in conjunction with three dye molecules they respond to photons as logic gates. Therefore, in theory it’s possible to build a highly compact computer with them, since they are not only logic gates in themselves as opposed to logic gates built out of transistors or their equivalents such as valves or relays, but also somewhat smaller than even the smallest miniaturisation is capable of producing today. A porphyrin ring is 840 picometres in diameter, although the dye molecules are also required, compared to the smallest transistors at one nanometre, several of which are needed to build a logic gate. It probably works out at about a third the size in two dimensions, and of course porphyrin rings are only one atom thick. The difficulty would be in assembling that kind of structure at that size.
Hæm only forms the centre of a much larger molecule of hæmoglobin, which is unsurprisingly made of several protein molecules all joined to the central porphyrin. Since it contains iron, a transition metal, it has a colour. Alkali and alkali earth metals usually form white compounds, so I don’t understand why chlorophyll is green, but blood is red because of the iron and the states it’s in. The fatal combination of carbon monoxide with hæm to form carboxyhæmoglobin is famous for being cherry red, which means that carbon monoxide poisoning often makes people look really healthy, but of course the problem is that the combination is not reversible and the days of one’s blood carrying oxygen are now gone, except to the extent that it dissolves in plasma.
The so-called bloodless fish of the Southern Ocean are a whole family of fish, the Channichthyidæ, who completely lack hæmoglobin as adults. Also known as icefish, they have a slow metabolism and live in very cold water, which enables them to survive by using their plasma alone to carry oxygen and carbon dioxide. In order to do this, they have unusually wide capillaries, four times the blood volume of other fish their size and a greater cardiac output. Their blood is transparent and colourless. They also lack myoglobin, the muscle pigment which stores oxygen for use later. It used to be thought that the lack of blood corpuscles in icefish conferred some kind of advantage, possibly based on lower blood viscosity and was a necessary adaptation to living near Antarctica, but this turns out not to be the case and they have simply evolved like that. This probably happened in the past thirty million years.
Didemnum molle
Date
8 October 2009, 05:20
Source
Sea Squirt (Didemnum molle)
Author
Bernard DUPONT from FRANCE
Hæmoglobin is as far as I know universal among vertebrates apart from icefish. It is not, however, the only blood pigment in our phylum. Sea squirt blood contains a vanadium-based pigment referred to as hæmovanadin, which as can be seen from the above photo is green. It does not tranport oxygen and its function is unclear. Nobody knows why sea squirts and some other animals use this pigment, but one idea is that it makes them toxic to potential predators. All species using it are marine and it’s the only incidence of vanadium in biochemistry so far as I know.
A more common green blood pigment, which does carry oxygen, is chlorocruorin. This also uses iron and is found in four families of polychætes. These are very common segmented worms characterised by having more bristles than the oligochætes represented by earthworms (oligochætes are actually not a clade). It differs from hæmoglobin in having nearly two hundred iron atoms in the molecule and is a much larger molecule. It’s also distinctive in being green in dilute solution but red when concentrated, and consequently some worms are green where oxygen conditions are good and red when they aren’t. There are also two species of starfish who contain the porphyrin but not the pigment, and it seems it can arise very straightforwardly from a mutation of the machinery which produces hæm. Consequently the question might arise of whether there have been any green-blooded vertebrate lineages which became extinct, such as the Acanthodii, a group of fish who left no descendants when they became extinct in the Permian. It also means the presence of chlorocruorin doesn’t imply species are closely related. Hæmoglobin also appears to have arisen separately in different groups of animals, notably the annelids, which probably explains why leeches suck vertebrate blood. It’s present in one species of snail, in the protist Paramœcium and in many crustaceans. This can be seen, for example, in water fleas living in stagnant water, who often turn red. Importantly for the production of veggie burgers, it’s also found in the roots of bean plants, where its rôle is to remove the oxygen and concentrate nitrogen, which is fixed by bacteria living in their root nodules. This hæmoglobin is used to make more convincing meat substitutes, although as a vegan I’m not usually keen on eating something which resembles a bit of dead vertebrate. Also, the more recent plant-based burgers are sometimes tested on animals and are therefore not necessarily vegan anyway, which leaves me wondering what the point is.
There is a third iron-based blood pigment called hæmerythrin, which is reddish-violet when oxygenated and colourless when not. This is found in priapulids (also known as “penis worms” because of their appearance and mentioned many times on here), and peanut worms or sipunculids, who are worm-like longitudinally striped animals with a thin tentacle at one end, one of whom glories in the name Golfingia because it was discovered during a game of golf when the ball landed in a seashore pool. Brachiopods also contain hæmerythrin. These three groups are not at all closely related to each other and therefore hæmerythrin is an example of a probable pigment to some extent, although it’s actually rare and the only phylum in which it is universal is the sipunculids. Hæemerythrin is always in cœlomic fluid, i.e. the fluid filling the body cavities, but within this fluid can sometimes be found as blood corpuscles. Therefore it seems that there is some point to it apart from arbitrary mutation. In spite of it being superficially similar to hæmoglobin including its colour and the fact that it contains iron, it is not chemically similar to it and contains no porphyrins.
In science fiction and its allied trades, green blood is a very popular idea. Star Trek’s Vulcans, Romulans, Rigelians and Remans all had green blood, which I presume was based on copper. There is a copper-based respiratory pigment in nature called hæmocyanin, and this is fairly well-known. This is blue, and uses two copper atoms as prosthetic histidine groups to combine with oxygen instead of a single iron atom. It’s found exclusively in arthropods and molluscs, although insects don’t have it because they don’t use their blood for respiration except trivially because it will have atmospheric gases and oxygen dissolved in it. Notoriously, horseshoe crab blood, which is blue and contains this pigment, is used to develop vaccines. It has been claimed that the Covid-19 vaccines used in this country are vegan, so I don’t understand this apparent contradiction. They’re also endangered species. Leaving that moral quagmire aside, the fact that horseshoe crabs use hæmocyanin very probably also means trilobites had blue blood. Like hæmerythrin, it has no porphyrins and I think it’s the only respiratory pigment other than hæmoglobin which occurs in terrestrial animals such as snails, spiders and scorpions. Many charismatic species have it, such as octopodes, the aforementioned horseshoe crabs and scorpions. I’m guessing tarantulas also use this pigment. Hæmocyanin is more likely to occur in cold, low oxygen habitats.
The question at the back of my mind while I’ve been writing all this is whether there are sentient species elsewhere in the Universe who have different coloured blood. This is a long way down the road from the question of any life at all off this planet. It makes the big assumptions that there is, that it’s biochemically based, respires with the help of gases and has something analogous to blood. It’s also a chain of assumptions, appended to many more, made in ‘Star Trek’. Vulcans and the other similar humanoid species I’ve mentioned have green blood, although this doesn’t really show in their complexions. Spock is shown as having a slight green tint, but without other skin pigments he would presumably be expected to be greener than you think, so to speak, but has no plans to take over the world, even accidentally. It’s quite likely that the viewer will find out what colour a particular intelligent lifeform’s blood will be in Trek due to the considerable violence in that franchise. Notably, Klingon blood has been shown as lavender, suggesting it contains manganese, but this is rumoured to be a directorial decision made to keep ‘Star Trek VI: The Undiscovered Country”s PG certificate. In other situations it’s been shown as red. For a more complete breakdown see Memory Alpha.
I can certainly imagine a colder planet than this one, with a thinner atmosphere which nonetheless contains oxygen, having dominant multicellular life forms on its land surface whose blood contains hæmocyanin and is therefore blue. I can even roughly sketch this planet. It orbits a K-type dwarf (orange and slightly cooler than the Sun, and more long-lived) with a mass 90% that of the Sun and a surface temperature of 5200 K. Luminosity is fifty percent of the Sun and the planet orbits 90 million kilometres from its primary, having a year lasting 169 terrestrial days. It has three landlocked oceans, two of which are circumpolar and largely frozen, covering 35% of the planet and a surface gravity of 65% of ours. Mean surface temperature is just above freezing at 45° and partial pressure of oxygen in the atmosphere is 120 millibars, making it impossible for humans to survive there for more than a few minutes without supplemental oxygen. There are no seasons because of the lack of axial tilt. It has relatively large cold deserts and a diameter of nine thousand kilometres. I could go on with this but it’s easy to conjecture. The sky is turquoise and darker than ours, as are the plants, since they use slightly different wavelengths for photosynthesis. Of course it doesn’t actually follow that blood pigments even carry oxygen, or another respiratory gas, and if there were compounds which carry oxygen they may not change colour.
I don’t want to turn this into yet another SF world-building exercise. To be honest, the main thing writing this has raised in my mind is whether the UK government uses the same definition of vegan as I do, as I seem to have just discovered that vaccines are tested on horseshoe crab blood extracted by killing the animal in question, and although I still plan to get the second vaccine I am really quite angry, and also curious, about this, and want to know what alternatives are available. But anyway, pretty blood.
A Box Of Chocolates 