After reevaluating the creature that cools its blood with lye and sal ammoniac, I removed the middleman - the cooling chamber and use of blood as coolant - and replaced the potassium hydroxide/ammonium nitrate reaction with urea (that "some of the stuff in pee" the title mentioned) dissolving in water. After all, why transport the heat being removed to a central location when it can be removed on-site instead? Each individual cell in the creature's body now has its own, miniature cooling system, connected to its own system of thermoreceptors. This has several advantages:

  • Each individual cell can "decide" when to cool itself (whenever its thermoreceptors sense a high temperature and tell their cell's cooling organelles to mix urea and water) as opposed to the hypothalamus deciding when to activate the centralized, body-wide cooling system.

  • No heat seeps back into the body en-route to the cooling chamber; it gets removed on-site, ensuring as much as possible of it is taken from the cells.

  • Although the dissolution of urea in water is a weaker cooling mechanism, removing 15.39 kilojoules of energy (see the first page's bottom right) per 1 mole of each substance involved instead of the 30.56 kilojoules removed by potassium hydroxide and ammonium chloride reacting, such a reaction produces ~63.5 milliliters of aqueous solution as its product, rather than ~45 milliliters of salt water...and more than 22.414 liters of toxic, reactive, caustic ammonia gas (even at STP, it takes up 22.414 liters, and the higher temperatures in a living thing will increase that). In other words, while urea dissolving in water might be ~1.986 times weaker as a cooling mechanism than potassium hydroxide reacting with ammonium nitrate, its byproducts take up more than 353 times less volume.

  • Urea is already produced by Earthly biology on a regular basis, and is therefore much more readily available than lye or sal ammoniac.

The body has about ~37.2 trillion cells. Taking a fairly average 70-liter person and dividing their volume by 37.2 trillion finds the average volume of each of their cells: ~1881 cubic micrometers. If a mere 1% of that volume is reserved for reactants and 4% of that volume is dedicated to extra thermoreceptors and the organelles for containing and reacting the reactants, that's 18.81 cubic micrometers for reactant storage per cell, or 699.732 milliliters/cubic centimeters across all 37.2 trillion cells.

Urea has a density and molecular mass of 1.335 grams per cubic centimeter and 60.056 daltons/atomic mass units per molecule, respectively; since the value of the molecular mass of a molecule in daltons is the same as the molar mass of said molecule in grams, that gives urea a molar mass of 60.056 grams. 60.056 grams/1.335 grams gets us a molar volume of urea of ~44.986 cubic centimeters per mole. Water has a density and molecular mass of 0.9970 grams per cubic centimeter and 18.015 daltons/AMUs per molecule, respectively, giving it a molar volume of ~18.069 cubic centimeters per mole.

44.986 + 18.069 = 63.055. Since 699.732 cubic centimeters are on hand to store urea and water, that means we can store ~11.097 moles of each. Dissolving 11.097 moles of urea in 11.097 moles of water uses up ~170.783 kilojoules of energy, enough to negate the heat from ~19 to ~57 minutes of exercise by a fit adult. I'd say that's a fairly good amount of cooling.

The question, however, is how the urea gets there in the first place. Normally, the body wants to pee urea out; indeed, after each cell in this organism mixes its urea with water, that's exactly what happens: it gets pumped out of the cell as waste. As far as I know, there are no pathways - metabolic, chemical, biological, or otherwise - for a living organism with Earthly biology to deliberately supply what is ordinarily a waste product to its own cells.

As such, the question: via what chemical or biological pathway can a creature with Earth-like biology get a small quantity of urea to each and every one of its cells so that they can have their own cooling system? While this might seem rather in-depth for any piece of writing (or it might not), I'd like to know whether there are any interesting side-effects to this that I could spin out into further wacky biology, so please be as specific as possible, and cite actual chemistry if you can. It'd be fascinating if the chemical transport mechanism to get urea to every single cell could also somehow be used to transport other substances as well.

  • 2
    $\begingroup$ If you mix urea with water and then extract it out from water I'm not sure why you think that you will get any net cooling effect... I would expect that on the contrary the inherent inefficiencies will produce even more waste heat. (And we land animals excrete urea because we cannot excrete ammonia directly. Converting toxic ammonia into non-toxic urea is energetically expensive, but as land animals we don't have oceans of water around to let ammonia diffuse at low enough concentration to avoid toxicity.) $\endgroup$
    – AlexP
    Jun 6, 2022 at 11:24
  • $\begingroup$ @AlexP It isn't extracted back out of the water, it's just peed out. Yes, it's inefficient in the net scheme of things, but it's designed to provide a sudden cooling burst for a brief period of extreme exertion, not a long, efficient cooling mechanism. $\endgroup$
    Jun 6, 2022 at 11:34
  • $\begingroup$ To pee it out you need to filter it out of the cytosol. You cannot really empty the cells and hope that life will continue. (And remember that the "natural" end product of protein metabolism is ammonia, not urea. Urea is synthesized from ammonia, consuming energy in the process, and of course producing waste heat. Aquatic animals have an entire ocean of water around them and usually just let the ammonia diffuse in the water.) $\endgroup$
    – AlexP
    Jun 6, 2022 at 11:35
  • $\begingroup$ @AlexP Let's say, for the purposes of this question, that it's contained within a special organelle, not distributed into the cytosol. I even set aside some extra volume for that. Yes, I know synthesizing urea produces waste heat, but the point is that the organism in question can synthesize urea when it's not exerting itself and dissolve it when it is. $\endgroup$
    Jun 6, 2022 at 11:43

1 Answer 1


A system such as this is going to require metabolizing ammonia to urea within a special cellular organelle. The organelle would require membrane bound water pumps that would (almost) constantly be evacuating water from within the organelle. Remove enough water, and the urea will crystallize. This will release heat, which would have to be removed from the creature's body using other thermoregulatory mechanisms.

The crystalline urea stored in this organ can then be used to control the temperature of the cell. If the water pumps were particularly temperature sensitive, and ceased functioning if their temperature rose as little as a degree above the usual operating temperature of the cell, the cessation of their function would allow water to enter the organelle through water (but not urea) permeable pores that evolved to open if the cell's temperature rose.

As water enters the organelle, the reaction will provide a cooling effect. If the cell's temperature drops sufficiently, the pores will close, the pumps will start, and the urea will begin to recrystallize. If the recrystallization takes place too fast and the temperature rises too much, the water pumps will stop and the pores will open, slowing the process and cooling the cell. This would become a negative feedback mechanism that automatically regulates the temperature of the cell.

Of course, if a high-temperature condition persists, the urea will all dissolve and cease to provide a cooling effect, at which point, other thermoregulatory mechanisms must take over, and body temperature may rise, limiting the heat-producing activity through heat stress.

This isn't a magical cooling superpower. It will give a creature the ability to engage in maximal exertion for several minutes without needing to deal with excess heat. However, this comes at the cost of a higher baseline metabolic rate, and doesn't address the necessity of providing energy for this extended period of maximal exertion. Once this period of maximal exertion is over, it will be necessary for the creature to rest for longer than might otherwise have been the case in order to recrystallize its urea and shed the associated excess heat.

Such a mechanism would best suit an apex predator, which could use it to prolong pursuit of prey. To use such a mechanism as a prey species would risk a sudden, debilitating and prolonged collapse in the event of being pursued by a predator.

Evolution of other thermoregulatory mechanisms would still be an important factor in the evolution of species with this ability.

Such a mechanism is unlikely to evolve in a single species. It is more likely to be found in a group of species equivalent to a class or phylum.

Of course, some ammonia would be metabolized to urea outside these thermoregulatory organelles, or would escape, and would be excreted in the usual manner.

As for how the ammonia gets into the thermoregulatory organelles it would be carried in the blood to the cells, and being a small molecule, could pass through ammonia pores impermeable to larger urea molecules.

  • 2
    $\begingroup$ This is all perfect - this system is supposed to be in the cells of an artifically-designed, giant apex predator with a high metabolism that needs to cool itself off to an extreme degree but for short periods. $\endgroup$
    Jun 6, 2022 at 21:05

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