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How could a large ectotherm (say, the size and shape of a horse) have a truly unidirectional system of respiration? Not a looping system as found in birds and other archosaurs, but a system where air flows continuously in a straight line through the lungs and out of a completely separate outlet, allowing a constant inflow that isn't interrupted by the need for deoxygenated air to go out the same way it came in.

One way might be to use a chambered heart-like organ, pumping air instead of blood. Alternatively, you could use peristalsis to draw the air in. However, I doubt these mechanisms would be fast or efficient enough to sustain a large organism.

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    $\begingroup$ You mean a truely unidirectional respiratory system like all the fish have had since the beginning of time? Including those which are larger than horses. $\endgroup$
    – AlexP
    Commented Dec 6, 2022 at 22:42
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    $\begingroup$ Hello Daniel. Please note that our help center prohibits providing answers to your own question in the question itself. From Stack Exchange's point of view, you already have an answer, so why are you asking the question? If you wish to answer your own question, please do so by posting it as an actual answer. That has value as the community can vote on your answer along with any others. $\endgroup$
    – JBH
    Commented Dec 6, 2022 at 22:43
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    $\begingroup$ @JBH The second paragraph isn't answering the question. It's clarifying the question by stating that a chambered organ and peristasis are not answers to the question because they probably wouldn't work for a large organism. $\endgroup$ Commented Dec 7, 2022 at 11:28
  • $\begingroup$ @Tanner-reinstateLGBTpeople I might agree with you if the second paragraph didn't start with the phrase "one way might be..." and continue with "Alternatively...." An expression of doubt isn't a substantiated reason to exclude those solutions that helps focus the question (we allow lists of potential answers when the OP specifically explains why those answers are not suitable). Note that I neither down voted nor voted to close the question, I was instructing a new user about the protocol of this Stack for future reference. Training new users is important here. $\endgroup$
    – JBH
    Commented Dec 7, 2022 at 15:59

6 Answers 6

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Maybe not too creative. But indisputably effective.

The gill.

gill

https://en.wikipedia.org/wiki/Fish_gill

Water moves one way through the respiratory system in fish. In the mouth and out the gills. Gills work good. 500 million years old and going strong.

If you want air gills you could have them be external, salamander style. Air blows by them. You could do like fish and take air into some antechamber (like the mouth) then close the mouth and expel air through the gill. You could have some valved piston arrangement (I am imagining a piston like tongue) which fills an air chamber on the back stroke then expels air past the gills on the forward stroke.

Gills. Gills good.


A broad-scale comparison of aerobic activity levels in vertebrates: endotherms versus ectotherms

The body mass scaling of maximum oxygen consumption rates (i.e. VO2 max) in ectotherms was statistically indistinguishable from that of endotherms... based on the 95% CI of the scaling exponents from PGLS regression analyses ... Moreover, within both endotherms and ectotherms, the body mass scaling of VO2 max was statistically indistinguishable from that of resting rates based on the 95% CI

o2 demand

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  • $\begingroup$ Gills not so good for air though. They dry out too fast. $\endgroup$
    – ShadoCat
    Commented Dec 6, 2022 at 23:01
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    $\begingroup$ @DanielMazdakHonar, please don't misunderstand what Will did. The fact that fish are ectotherms is irrelevant. What he provided was a Real Life example of something that's known to work that you can use to creatively design your creature. If what you're looking for is something that will work in Real Life, you're kinda in the wrong place. Evolution didn't do what you're looking for with land animals for a number of reasons (like doubling the chance of flooding the lungs of an air-breathing creature is bad). Remember to use our site to help you build worlds. $\endgroup$
    – JBH
    Commented Dec 7, 2022 at 0:22
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    $\begingroup$ @DanielMazdakHonar: ??? The question explicitly asks about an ectotherm animal. "How could a large ectotherm"... $\endgroup$
    – AlexP
    Commented Dec 7, 2022 at 0:33
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    $\begingroup$ I added some material on oxygen consumption: ecto vs endo. At max activity both groups are the same. Gill good. GILL GOOD! $\endgroup$
    – Willk
    Commented Dec 7, 2022 at 3:49
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    $\begingroup$ Not sure if it was mentioned, but some species of fish (notably sharks) cannot stop moving because their gills require constant flow of oxygenated water over their gills. Orcas have been known to roll juvenile Great White sharks, which stuns them long enough to die from lack of water flow causes the shark to die, allowing the Orca to eat them at leisure. $\endgroup$
    – hszmv
    Commented Dec 7, 2022 at 14:59
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As AlexP mentioned in a comment fish already do this. That is good for water breathers but not so good for air breathers.

The main reasons the lungs are shaped as they are surface area and moisture retention.

What you seem to be looking for is a modified tube.

You need the surface area to transfer enough gas fast enough to be useful. So, any solution would have numerous pockets or breaking up into multiple thin tubes and then recombining. Either way, the higher surface area will create more "friction" resistance to air flow. Therefore, unless the creatures never go below a certain speed (like a get engine), they would need muscles along the tube that would contract in a wave to push air through.

The real killer here though is moisture retention. The moisture on the surface of the lungs helps the gasses to transfer through the lung lining. Our nose actually adds moisture when we inhale and recaptures a bunch of our moisture on exhale. So, the tube has to have a structure to moisten the air at the beginning and to try to recapture the moisture at the end.

I think that this is a plausible evolutionary adaption if you start with a water born creature that has a tubular breathing apparatus instead of gills. It is just that gills are more efficient for water breathing since the water doesn't have to travel as far through the body (water has more "friction" resistance than air). However, once you posit that gills didn't develop, the switch to air breathing should be smoother. It just has to add a bit of complexity to what is already there instead of creating a completely new structure.

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  • $\begingroup$ Would peristalsis be sufficient? It seems like a very inefficient system compared to pressure-differential respiration. $\endgroup$ Commented Dec 6, 2022 at 23:57
  • $\begingroup$ @DanielMazdakHonar, maybe modified with compressible chambers with alveoli. Then it would look more like a string of pearls than a tube. $\endgroup$
    – ShadoCat
    Commented Dec 7, 2022 at 0:40
  • $\begingroup$ Maybe get around the surface area issue with a rigid bone-like blood/air interface structured like an open cell foam. You could get around the moisture issue by tweaking the environment. Maybe this thing lives in areas with ~90% humidity. $\endgroup$
    – 9072997
    Commented Dec 15, 2022 at 6:14
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The Mighty Sphincter

enter image description here

There are two tubes. One tube sucks good air in through the mouth. The other tube pushes bad air out through the bumhole. Each tube has a sphincter where it joins the lungs. The Diaphragm is same as a human. It is a band of muscle around the lungs that makes the balloons expand and contract. When it expands, the bottom sphincter is closed and the top is open, so air is sucked into the lungs. When the diaphragm contracts the bottom sphincter is open and the top is closed, so bad air is pushed out through the bumhole.

With proper training, these people can learn to breath in and out through their bumhole. This is not optimal but makes them very good at card games and staring contests. It also lets them release the gas at inopportune moments for a practical joke.

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    $\begingroup$ If each lung had independent sphincters at each end, and the diaphragm/skeletal structure was reworked to be able to compress and expand each lung independently, one could even be breathing in while the other is breathing out, making the overall airflow more continuous. $\endgroup$ Commented Dec 7, 2022 at 15:28
  • $\begingroup$ Yes but then they would be independant instead of unidirectional, basically just 2 sets of lungs $\endgroup$
    – Tofandel
    Commented Dec 8, 2022 at 10:53
  • $\begingroup$ Except you'd need a separate orifice - and of course you wouldn't want it immediately adjacent to your actual bumhole. Which means we have just found Mr Arthur Frampton. $\endgroup$
    – Graham
    Commented Dec 8, 2022 at 12:04
  • $\begingroup$ The idea is actually quite workable. The Palatine uvula is actually "such" sphincter (but with different role). The only problem would be the reversal of the air flow - assuming that the other gases are still released at the bottom :) $\endgroup$
    – virolino
    Commented Dec 8, 2022 at 12:56
  • $\begingroup$ Oh, look, an internal combustion engine! $\endgroup$ Commented Dec 8, 2022 at 19:06
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A straight line path isn't much needed for air. It's more important for a denser liquid offering greater resistance. Hence the more or less straight-line design of fish gills.

Birds pass air through their lungs in a single direction, using various bladders in their bodies to create the flow. It's a simple change to have the exhaust vented down an exhaust-only pipe.

We can speculate on why evolution did not take that path. My guess is that it's because of the common body plan that birds share with mammals, reptiles, amphibians ... no way to get there from here by small incremental changes, all viable? However, it's also possible that the common intake/exhaust conserves energy and water by heat-exchange between intake and exhaust. (I expect somebody will have already written a scientific paper on this; if not, it would be interesting research! ) Birds have phenomenally good thermal insulation (feathers) and evolution will certainly be strongly selecting for energy conservation in a half-ounce bird living in North America (black-capped Chicadee, 0.32 to 0.49 oz according to Wikipedia).

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Imagine a structure like a heater / radiator with one input, one output, and many "parallel" tubes between them. Inside the tubes, there are countless cilia which push the whatever (air, water, alien substance) from one end to the other. The walls of the tubes and the cillia contain cells which can perform the substance exchanges (oxygen or whatever exists on that planet). The tubes are "optimum thin" - they allow at the same time big air flow and big surface for exchanges, while till having a relatively low volume. It can grow proportionally with the size of the animal. There are sphincters which protect the organism from dangerous substances. If there is natural substance flow (e.g., air), then the cillia do not need to provide any movement, just substance exchanges.

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With valves, see https://en.wikipedia.org/wiki/List_of_valves

For example, with a Tesla valve. It doesn't require moving parts apart in your case from a diaphragm in the middle (not shown) to make the air move. Many Tesla valves can be used in parallel like in the following image from this link:

enter image description here

An alternative to not having additoinal moving parts is to use vibration to affect its properties as described here.

Energy seems to be unnecessarily lost due to turbulences though, so see the list of valves linked above for more efficient valves.

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