Would it be possible for an alien species to have liquid nitrogen for its blood? What would be required for that to work? What kind of environment would it need to exist in?

  • $\begingroup$ All I know for a fact is that liquid nitrogen only happens in extreme cold. Titan is a good example. $\endgroup$ Jul 2, 2018 at 3:59
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    $\begingroup$ @CelestialDragonEmperor Titan isn't cold enough. You might be thinking of the similarly-named Triton, which has liquid nitrogen geysers. $\endgroup$ Jul 2, 2018 at 4:04
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    $\begingroup$ @Logan R. Kearsly that's an oof, I got them mixed up for a bit. $\endgroup$ Jul 2, 2018 at 4:07
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    $\begingroup$ That would be quite a cool alien. $\endgroup$ Jul 2, 2018 at 11:06

10 Answers 10


Short answer is no. Longer version goes as follows...

Blood exists for a number of reasons in an organism, but the primary one is the distribution of oxygen to all the other cells in the body. Nitrogen isn't inert per se, but the fact that it exists in such large quantities in the Earth's atmosphere but doesn't react with the 22% or so O2 should tell you that it's not going to 'store' oxygen to carry it around the body.

The reason why our blood contains a lot of iron (hemoglobin) is because iron reacts very well with oxygen. You've no doubt seen how quickly iron left exposed rusts; well rust is actually just Iron Oxide, or iron that's reacted with oxygen. That reaction allows the blood in a normal body to capture oxygen and then release it where it's needed so that all the cells of our body can get oxygen for their normal chemical processes.

If on the other hand what you're really asking is can a being exist that captures an oxidiser in a form that allows it to be in a really cold environment, you might want to look at using fluorine as an oxidising agent; my understanding is that you can work at much lower temperatures using halogens to transport the fluorine in much the same way as hemoglobins transport the oxygen in life on earth.

But for the sake of argument

The temperature at which oxygen becomes liquid is only about 10 degrees higher than the temperature at which nitrogen becomes liquid. Which means there's another possibility...

If you have a VERY cold planet, AND instead of an 'atmosphere' you have a 'sea' of liquid N2 and O2 in similar proportions to that on Earth, AND you have some mechanism to replenish O2 once it is used by an organic creature, that creature may not need to have blood at all.

Imagine (if you will) a creature that looks somewhere between a fish and a piece of lace; it swims about in this sea, and instead of gills, it allows the sea itself to pass through all parts of its body, which then extracts the liquid O2 for the metabolic needs of all its cells. It might exist in the form of (say) a hollow tube with propulsion mechanisms inside. The idea is that the tube itself would have many hollow tunnels through it, maximising internal surface area and allowing the organism to breathe.

The precedent may be insect life on earth today, which doesn't have lungs but instead absorbs its O2 needs through its carapace via osmosis. This is sufficient for a very small creature and it's the primary limitation on insect growth; if they grow too large, their internal organs literally suffocate.

But, our lacy or tubular fish should have access to sufficient amounts of O2 that it can feed its own cells out of the sea.

This wouldn't be blood per se, but if the sea ran through many different arterial entry points and through the body before being expelled, it would function while inside those tunnels through the body in a manner similar to blood. It, like insects, would be limited in the size it could grow to but would at least experience liquid nitrogen running through it as an oxidising medium.

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    $\begingroup$ This seems like a non-sequitur. The liquid part of our blood is not what carries (most of) the oxygen content, so replacing water with liquid nitrogen in and of itself should be irrelevant. Even if a cryogenic alien lifeform still used oxygen in its metabolism, presumably it would have some sort of carrier molecule suspended in the blood, just like we do, so if you want to eliminate LN2 blood on the grounds that it couldn't carry oxygen, you would have to show that there are no molecules that can reversibly bind oxygen at that temperature. $\endgroup$ Jul 2, 2018 at 4:15
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    $\begingroup$ @LoganR.Kearsley: While it's right that the water in our veins doesn't carry any oxygen (under normal pressure), the answer is still valid insofar as the temperatures at which nitrogen is liquid will inhibit (or slow down to the point of practically inhibiting) pretty much every chemical reaction, i.e. the creature's metabolism. Plus, nitrogen lacks the other important-for-life properties which water possesses, such as e.g. acid-base equilibrium or a high heat capacity. Organisms, no matter what they are based on, need kinda stable conditions. Water provides much of that "automatically". $\endgroup$
    – Damon
    Jul 2, 2018 at 7:33
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    $\begingroup$ Your answer relies entirely on the assumption that all life forms require oxygen to exist. But alternatives have certainly been theorized, if rarely (never?) observed. $\endgroup$ Jul 2, 2018 at 16:17
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    $\begingroup$ @Damon most of all, water is polar, which makes it more useful as a solvent for interesting compounds. Your argument about reaction speeds isn't valid: you could say much the same thing about many reactions that actually happen in organisms on Earth; coming from pure chemistry one would expect these reactions to run far to slowly to be any use. But life solves this problem with enzymes, which are amazing catalysts. $\endgroup$ Jul 2, 2018 at 20:28
  • $\begingroup$ @TheArchitect you may want to re-read the 4th paragraph where I describe the possibility of other oxidising elements... $\endgroup$
    – Tim B II
    Jul 2, 2018 at 23:23

Liquid nitrogen could form the (majority of) blood of a creature

If the ambient temperature/pressure conditions are right for nitrogen to remain liquid, it would be a reasonable kind of blood substrate, the equivalent of water in human blood. The primary function of blood in a living organism is to transport dissolved molecules around the body, such as hormones and cells in humans.

Several papers have investigated the solubility of chemicals in liquid nitrogen, so it is fairly appropriate for this purpose.

However, the blood would not be pure liquid nitrogen, no more than human blood is pure water. The carried cells and hormones are what makes it blood, and the dissolved materials would be crucial for the liquid-nitrogen-based blood to be blood.


Blood serves to transfer heat. Liquid nitrogen is good at this.

I have speculated about a super cold creature based on the phenomenon of superfluidity. Superfluids are sort of animal like in that they crawl around, seeming to defy the laws that govern hotter things. A boojum is a weird pattern that can form on superfluids, a superconducting current giving way to a geometric shape.


Pair of point defects, called "Boojums," in a thin, hybrid-aligned nematic film.


It is spooky and otherworldly. A creature based on these phenomena would be good stuff for concept-drives SF.

And the liquid nitrogen - it would be for heat transfer. That is what we use liquid nitrogen for and it is also an important role of our blood - moving heat from hot places to places where it can be shed in sweat and breath. So too your Boojum - its liquid nitrogen blood would move heat off to the radiant organs to be emitted, keeping its superconducting body chilly.


First, define "blood". There are lots of terrestrial organisms which we might colloquial refer to as having "blood", but which technically speaking do not. E.g., arthropods have hemolymph, a fluid "equivalent to blood", but which is not actually blood. In insects, it doesn't even need to carry oxygen, because the animals tissues are directly oxygenated by tracheal tubes.

So, what functions do you want a solution based on LN2 solute to perform in order to be comfortable with calling it "blood"? That will determine whether or not having liquid nitrogen is possible!

One of the characteristics of blood is that is an extracellular fluid. I.e., LN2 need not necessarily perform as the primary biosolvent, analogous to water, in an organism in order to have other properties which would allow us to call it "blood". Perhaps, for example, you could have an organism that uses oxygen diflouride as its intracellular biosolvent (like the aliens from Robert Forward's Camelot 30K), despite existing in a larger LN2 environment. Such a creature might have cell membranes that are monolayered, to interface between the polar internal solvent and the non-polar external environment, or even non-existent, with cells being bounded solely by the interface between a bubble of OF2 and the surrounding N2 (although it would be difficult to develop multicellularity in that case, which would kind of preclude the development of "blood" entirely--so we'll assume that the creature does have proper cell membranes of some sort, at least!) Given the relative scarcity of fluorine, this might make a lot of sense for an organism that needs to manufacture its own internal biosolvent, and thus needs to use it as sparingly as possible, while taking advantage of some other more abundant fluid in its environment for other purposes.

Such a creature could "inhale" and "exhale" LN2 for heat regulation or propulsion, but it seems to me that one of the critical defining characteristics of "blood" should be that it is contained inside the organism, and separated from the environment. So, what could such a creature using internally-circulating LN2 for. so as not to waste its primary biosolvent? Well, there's heat regulation, and while LN2 isn't a fantastic solvent, it can dissolve some things, so it could transport simple hormones & metabolites (like dissolved methane or hydrogen)... and really, blood doesn't need to be a good solvent, even to do all the same stuff our blood does, because it can carry non-dissolved, suspended solid particles (e.g., blood cells). So, an LN2 circulatory system could serve as a critical means of transporting immune cells, metabolite transport vesicles, and all sorts of other suspended stuff around a cryogenic creature's body.

Lots of answers focus on the problems of transporting oxygen in LN2-based "blood", but I think that's a red herring. For one thing, the ability of our blood to transport O2 is not dependent on its solubility in water; we use specialized proteins in solid, suspended oxygen-transport cells for that purpose, and so could a cryogenic creature. Furthermore, I don't think oxygen, or even oxidizer, transport needs to be a criterion for calling something "blood", at least in a colloquial sense, anyway. Maybe the creature breathes hydrogen, instead, which may or may not be primarily transported by it's blood.

So, yeah, if we accept that it is plausible for life to exist at the relevant temperatures at all, I'd say liquid nitrogen blood is entirely possible.

And if we accept that much, it's not that much of a further stretch to accept LN2 actually acting as a primary biosolvent after all. LN2 isn't a fantastic solvent, largely because it's just so freakin' cold, but there's actually a surprising amount of stuff that does dissolve in it, including several classes of organic compounds and silanols. Silanols (the silicon-based equivalent of organic alcohols) are highly reactive at our temperatures, but would be far more stable and useful for building metabolic processes out of at LN2 temperatures. It may therefore not be exceptionally likely, but certainly plausible enough for the purposes of a hard science fiction story, for there to exist cryogenic life using a mixture of organic and silicon-based chemistry in an LN2 solvent--and thus naturally having LN2-based "blood" as well.


Nitrogen is liquid at very low temperatures for pressures around 100 kPa, and is not so reactive with other gases to be useful in the same way blood is.

You might object that also water alone is not a so good at carrying gases (no living creature uses pure water as blood), but blood is a water based solution, therefore there could be a liquid nitogen based solution suitable for carrying gases. True, but at those low temperatures solubility and reactivity of other substances into nitrogen is going to be very low, therefore you cannot simply rely on physical or chemical transport. And, again, at those temperatures also other oxyding gases (oxygen, fluorine, chlorine, etc.) are liquid, so it would be hard to have some breathing as we know it, without even mentioning the hardship of having life suited chemical reactions in those conditions.

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    $\begingroup$ Solubility of other gasses doesn't really seem relevant. Oxygen transport in human blood doesn't rely on solubility of other gasses, after all; it relies on oxygen being bound to solid particles suspended in the blood stream. $\endgroup$ Jul 2, 2018 at 4:46
  • $\begingroup$ @LoganR.Kearsley, fixed $\endgroup$
    – L.Dutch
    Jul 2, 2018 at 4:54
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    $\begingroup$ Also, if it did rely on solubility (which it doesn't for life on earth), it seems relevant to mention that the solubility of a gas inside a liquid actually improves at lower temperatures. That's why carbonated beverages go flat if you don't keep them cold. $\endgroup$ Jul 2, 2018 at 5:04
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    $\begingroup$ @someone-or-other, I made clear that at those temperatures there are no other gases, but liquids. $\endgroup$
    – L.Dutch
    Jul 2, 2018 at 5:08
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    $\begingroup$ It could just be very slow life. Makes sense, as everything is slow at that temperature. $\endgroup$ Jul 2, 2018 at 12:26

This is pretty tricky and I miss few points in the answers provided by others.

First lets understand the purpose of blood itself. It exists mainly to transport oxygen and other substances needed for keeping the chemical processes in the organism. Oxygen is necessary for burning - to produce the energy. Other materials are used either as a fuel for burning (sugar), as a building particles of the body (proteins, to some level also fat) or to be used to control the chemical balance of the organism (pretty much everything else). Of course the role assignment is somewhat simplified.

As pointed out in other answers, blood also transports heat.

Now before we move on, one should argue what should we actually call life. In a traditional approach it is a form based on carbon, that uses oxygen to produce energy etc. But a more general approach is that it can be any form that is self sustainable, capable of reproduction, consuming whatever nutrition it needs and producing various kinds of metabolism products. Again it is a vast simplification and Wikipedia has a good article to start with this discussion.

The carbon based protein cannot serve their role in life in the temperature necessary to keep the nitrogen liquid. They need a water based solvent and water freezes below 0 Centigrade. Some organisms manage to keep their temperature higher than that but still it is nowhere close to the temperature where nitrogen is no longer a gas.

Now let's drop the carbon part of life as we know it on Earth and think of some very cold environment (already suggested by others in their answers and comments - like Triton). Your life form will have to be build of something totally different, like silicone, metals, maybe some naturally created alloys and other chemicals that are fluid in the range of temperatures where the nitrogen remains fluid. If those minerals can be solved in nitrogen then your creature could use it as main body solvent and consequently as a base for your blood.

I am no expert in this area but the basic Google search results show there are minerals for which liquid nitrogen can be a solvent.

So imagine a creature that in terms of its building particles resembles more our electronic devices, with silicone based "neural" cells for example and you might end up with a creature whose blood is mostly liquid nitrogen.

Note - I lack knowledge to imagine other necessary particles to make such life form, but you have a point to start.

  • $\begingroup$ by coal do you mean carbon? $\endgroup$
    – user371366
    Jul 3, 2018 at 15:25
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    $\begingroup$ yeah, sorry. I'm not a native English speaker. Fixed. $\endgroup$
    – Ister
    Jul 3, 2018 at 17:43

One thing to consider is that it would be quite difficult for life to evolve in temperatures around the temperature of liquid nitrogen. Evolution and biology need chemical reactions, and those are rather few and far between at -196 C.

I am not saying it's impossible, but your aliens would be very special indeed ;)

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    $\begingroup$ Welcome to the site @jtwilson! Your answer is nice and concise, well done :) You might want to consider adding some links or citation(s) to go further into the details with extreme negative temperatures and chemical/biological reactions. Keep it up! $\endgroup$
    – dot_Sp0T
    Jul 3, 2018 at 8:05

Unlikely; consider a nitrogen atmosphere with ammonia oceans/blood instead

First, large reservoirs of molecular nitrogen (N2) are rarely found in the universe. Instead, nitrogen is more commonly found in ammonia (NH3). We happen to have N2 in Earth's atmosphere because it was a byproduct of early life; it came from the NH3 in the primordial atmosphere. We have yet to find another planet with naturally occurring N2.

Using N2 as a biological solvent thus creates a chicken-and-egg problem. How does life which uses N2 arise from a planet which does not naturally have N2?

Second, the boiling point of N2 is -196°C = 77 K. At those temperatures, chemical reactions will occur so slowly that life will arise and evolve at an incredibly slow rate.

Finally, at the temperature and pressure needed for liquid N2, there would be few materials left in a gaseous phase to comprise the atmosphere of your planet. You need at least some atmosphere to create pressure to keep the N2 from boiling! And don't expect N2 to stably co-exist as both a liquid and a gas -- the rotation of a planet is one of many causes that would shift the temperature and pressure off of a phase boundary!

Instead, consider ammonia (NH3) itself as the solvent for life. Advantages include:

  1. It is the most common form of nitrogen in the universe, and ubiquitous among known planets.

  2. Liquid form between -78°C and -33°C. This would be a planet much colder than Earth, but still common enough in the universe and warm enough to permit a decent rate of chemical reactions.

  3. Like water, it is polar and dissolves nearly the same materials. Alkali metals such as sodium and potassium easily ionize -- producing blue solutions and thus a blue planet -- as well as other electropositive metals such as calcium and magnesium. Salts ionize easily in ammonia. Many organic compounds undergo acidic or alkali reactions when dissolved in ammonia, forming polar compounds and thus becoming soluble.

  4. Has its own acid-base chemistry. Just as water dissociates into hydronium (H3O+) and hydroxyl (OH-) ions, ammonia dissociates into ammonium (NH4+) and amide (NH2-) ions, with a dissociation constant of about $K = 10^{-30}$.

  5. Water dissolves easily in ammonia, as both are polar molecules, forming a solution that is more acidic than neutral ammonia. Indeed, water could a poisonous waste product of life, just as ammonia is to our life!

  6. Like water, dissolved substances diffuse readily. This another important property that contributes to life, as living things often do not need to actively transport molecules to bring them together for chemical reactions. Diffusion happens more rapidly in ammonia that water due to the lower viscosity.

  7. Also like water, nonpolar molecules don't mix with ammonia. Lipids (oils, fats, and waxes) form into membrane sheets, which can further wrap into 3-dimensional compartments (organelles and cells) with solutions of ammonia on the inside and outside. The membrane keeps the inside substances inside, and the outside substances outside, preventing diffusion. Because living things need structure, they need to keep things where they belong, and membranes provide this separation.

  8. Ammonia is a good material to transport and dissipate heat. It has a decent heat capacity and heat of vaporization. Many organisms could circulate sap or blood made primarliy of ammonia, to help the organism avoid overheating. Some organisms could also evaporate ammonia for "sweating".

  9. As shown in the Miller-Urey experiment, it is part of an abiotic mixture of compounds that can form simple organic molecules like lipids, sugars, and amino acids.

  10. Nitrogen gas N2 could evolve as the equivalent of our O2. It would be a waste product of early photosynthetic bacteria, become abundant in the atmosphere, and then become the respiratory gas for most living things. It is very soluble in ammonia (0.1124 vol/vol) -- no need for a hemoglobin-like carrier!


You can't use ONLY liquid nitrogen. Blood is supposed to transfer O2 to various parts of the body, keep your warm, etc. It could be the plasma (the liquid in which the cells go around in), and have many other cells in it. But, those cells would have to be adapted to surviving in an EXTREMELY cold environment (eg: Uranus, Pluto).


Would it be possible for an alien species to have liquid nitrogen for its blood?

Since I am not familiar with all the biological processes of this alien I am unable to determine whether or not liquid nitrogen would be a viable blood analog. That said, it seems reasonable enough to me and verisimilitude is always context dependent.

What would be required for [liquid nitrogen as a blood analog] to work?

Most answers have referenced a low temperature as being a necessity for liquid nitrogen; however, it is theoretically possible for nitrogen to be in the liquid phase at normal ambient earth temperature given enough pressure. From what I can tell, this hasn't been done for practical reasons but it may be within the realm of possibility. The pressures required are likely a few orders of magnitude higher than earth atmosphere. Related discussion may be found here https://physics.stackexchange.com/questions/29523/pressure-of-sealed-in-liquid-nitrogen

What kind of environment would [the alien] need to exist in?

In order for internal pressures of this magnitude to exist in this alien, I can think of two possible scenarios.

  1. The alien has an incredibly robust vascular system to resist the strain caused by the difference between the internal and external pressure
  2. The alien lives in an extremely high-pressure environment to counteract the internal pressure.

This opens up the possibility of an alien with a highly developed internal hydraulic system. Perhaps it would be similar to what can be seen in jumping spiders. They are able to jump great distances without the necessity of highly developed muscles which other creatures such as a grasshopper rely on. https://en.wikipedia.org/wiki/Jumping_spider#Behavior


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