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So I’m sitting here populating my alien world with critters to run around, eat each other and do critter stuff (as is the custom). At some point a thought came over me that their organs should be weird crazy shapes to mix things up a little.

What are spiral lungs good for?

The organs are sort of shaped like conches that sit comfortably inside the rib cage. Like a very long lung that’s coiled up like a python, except it’s one structure and can’t be straightened out. The trachea connects to both ends of the lungs for optimal airflow (because birds are unmatched in the breathing department). Aside from confusing the air molecules inside I’m not entirely certain what the advantage would be. My hypothesis is that air stays in the lungs longer this way in order to maximize gas exchange.

Tips for answerers: The method of respiration can be by compression or with air sacs circulating air through stiff lungs. Both versions should be explored in the answers to give a good idea of where they stand.

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  • $\begingroup$ What is the mechanism for creating the air flow? Lungs work because muscles contract and expand, alternately creating a void to suck air in, and squeezing to expel it. Not sure how that action occurs in your lungs. $\endgroup$ Oct 13 at 14:04
  • $\begingroup$ @JustinThymetheSecond It's written in the tips for answerers. Like we do or like birds do. Since I'm not designing one particular creature the question is broad. $\endgroup$ Oct 13 at 14:59
  • $\begingroup$ What was left after your search engines told you about the differences among lungs in different species, let alone among lungs, gills and whatever we call the insect respiration system? $\endgroup$ Oct 14 at 21:32
  • $\begingroup$ There is a limit to the ratio of 'lung capacity' to 'delivery tube volume' in two-way respiration. If the lung capacity is less than the delivery tube volume, no new air will ever reach the lungs. The lungs will keep 'breathing' just the air in the 'delivery tube'. That is why it is impossible to breathe underwater through a long tube. The volume of air in the lungs is not great enough to exhale stale air all the way back up through the tube. The lungs are 'pushing' and 'pulling' the same air, over and over again. Thus, there is a limit to how long this spiral tube can be. $\endgroup$ Oct 15 at 15:16

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Countercurrent heat exchange

Not quite sure the exact kind of spiral you mean but if an organism needs to maintain a very different body temperature from the gas around it in an extreme environment and is otherwise well insulated then having incoming and outgoing gas spiral around each other through counterflowing passages might allow a stable temperature to be maintained.

For example penguins feet use something similar for blood to reduce heat loss through their feet in extremely cold environments.

Blood going one direction exchanges heat with blood going the other in a spiral.

You might try something similar with gases being breathed in and out.

Penguins feet

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A one-way air passage

When we (humans) inhale and exhale, the air enters the lungs and exits them through the same passage. So minimizing the distance to the trachea from any part of the lung is an energy-saving optimization in our lungs.

But if your organism has two different entries into the lung, this allows us to use valves to have a directed, one-way flow of air through the lung (even if both those entries connect to the same outside opening). Then their lung might be more similar to a human gut - a long tube, with complications along the inside to increase surface area. But our gut is way too long to fit in our body, so it's all coiled up - a similar adaptation with your one-way lung would seem likely.

Circulation via compression: If we want to contain the tube nicely inside an expanding/contracting muscle membrane, a spiral could be a convenient shape to pack it.

Circulation via "air sacs in stiff lungs": Not really much different--it's still advantageous to pack the lung tissue together; and the air sac(s) can simply be placed at one end or both.

An advantage of one-way airflow: countercurrent exchange

Countercurrent exchange is a well-known feature (already used in the lungs of birds, which you mentioned, and in fish's gills--with thanks to @Turksarama for pointing that out to me) that optimizes the transfer of gas (or heat, or chemicals dissolved in a liquid) across a membrane. For your one-way lung, this would mean you direct the flow of blood the around the lung in the opposite direction to the airflow.

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    $\begingroup$ " The trachea connects to both ends of the lungs for optimal airflow". I might be wrong, but doesn't that allows bird to "inspire/expire" at the same time? So, as described by OP, it's technically a tube, comparable to what you describe (but with only one orifice, potato potato), no? Genuinely curious to know if that would work without adding a second orifice. $\endgroup$
    – Nyakouai
    Oct 13 at 14:17
  • $\begingroup$ That's a good clarification I should make! It should still work if a set of valves somewhere ensure that the air goes in one side of the lung when inhaling, and exits the other end of the lung when exhaling. $\endgroup$
    – Qami
    Oct 13 at 14:20
  • $\begingroup$ Upvote for circulation by compression. A spiral would be great for this and it would work well. $\endgroup$
    – Willk
    Oct 13 at 14:21
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    $\begingroup$ Cross current exchange is also used in the real world with an organ with very similar function to lungs: gills. $\endgroup$
    – Turksarama
    Oct 14 at 5:44
  • $\begingroup$ @Turksarama Thanks! I did not know that (but it makes perfect sense). I'll include that in the paragraph about cross-current exchange. $\endgroup$
    – Qami
    Oct 14 at 12:25
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What Qami said about one way passages, plus what Nyakouai said about increased gas exchange surface, but let me add one more thing: voice organs.

Your aliens might not have vocal cords like we do. Instead they vibrate different sections of the conch in order to speak. The larger the piece the deeper the voice. So an alien's vocal range depends on the sizes of their conch lungs, from the biggest cavity providing the lowest frequency to the smallest cavity giving the highest pitch.

This is very similar to how we humans perceive sound, by the way. We have an organ inside our ear called the Cochlea which is also a spiral (the emphasis below is mine):

The hair cells in the Organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea, due to the degree of stiffness in the basilar membrane. This stiffness is due to, among other things, the thickness and width of the basilar membrane, which along the length of the cochlea is stiffest nearest its beginning at the oval window, where the stapes introduces the vibrations coming from the eardrum. Since its stiffness is high there, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells. The farther a wave travels towards the cochlea's apex (the helicotrema), the less stiff the basilar membrane is; thus lower frequencies travel down the tube, and the less-stiff membrane is moved most easily by them where the reduced stiffness allows: that is, as the basilar membrane gets less and less stiff, waves slow down and it responds better to lower frequencies. In addition, in mammals, the cochlea is coiled, which has been shown to enhance low-frequency vibrations as they travel through the fluid-filled coil. This spatial arrangement of sound reception is referred to as tonotopy.

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    $\begingroup$ So they won't speak by flapping their meat? $\endgroup$
    – dotancohen
    Oct 14 at 8:47
  • $\begingroup$ @dotancohen not necessarily. $\endgroup$ Oct 14 at 12:36
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    $\begingroup$ @TheSquare-CubeLaw This voice box lung idea is very tempting. I'll have to look more into it. Thank you. $\endgroup$ Oct 16 at 10:10
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Extraction

First off, it is a nice idea, but not practical. The air is more difficult to push through the long winded pipes, costing more energy.

That out of the way, you can have super extraction. Lung lining is designed much like the intestines. It's made so there's a lot of surface area. The difference is that lungs are made to easily replace their gas with fresh oxigen holding particles that can swirl around a bit.

Your organisms have much more surface area for the lungs. That means more efficient oxygen extraction. This can change on what is needed. Either more quickly extraction, preventing the organism of using bad anaerobic processes in extended effort situations, or used to hold their breath longer. This last one won't be too much longer mind you.

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  • $\begingroup$ This assumes pushing it, which isnt needed. If pressure is increased and decreased generally, and one or other end blocked during in/outflow, air pressure will naturally do the trick. Like in our lungs - we dont "push" air. We expand the chest cavity volume,and natural gas pressure does the rest. $\endgroup$
    – Stilez
    Oct 15 at 2:40
  • $\begingroup$ @Stilez "Expanding the chest cavity" is functionally the same as pushing air -- there's still resistance from whatever passage the air passes through to get into the lungs, and that resistance is felt by our chest muscles. $\endgroup$
    – RLH
    Oct 15 at 6:59
  • $\begingroup$ With this slight difference. When you expand or contract the chest cavity, air * will * flow. At worst, at a slower rate, but it will, no matter how weak your chest expansion capability may be, there will always be a size increase/decrease that is within capability, and that will drive airflow no matter if slower. Further size change and further airflow is then possible, up to full physical capacity of chest and muscles. So its slowed buut never prevented. When "pushing", thats not so, or not usually so, for what we intuitively think of as pushing. $\endgroup$
    – Stilez
    Oct 15 at 7:08
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What an alien idea. I like it.

I suppose your weird shape could somehow increase the total contact surface of your lungs, making them more efficient compared to boring old regular shaped lungs. There could also be more major blood vessels running in the coils of it, allowing for a quicker transfer of oxygen into vascular system.1

Or (weird idea), it developped this way because your creature is somehow exposed on a regular basis to void. To avoid air being pulled out of the lungs, your coils are lined with sphincters that contracts and isolate sections of your lungs to create pockets of air.

1: Keep in mind, those are ideas from someone that hasn't had a biology class in the past 10 years.

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An internal cooling coil.

Building on Qami's idea of a one-way airway, a coil-shaped "lung" could also serve the purpose of cooling the body from the inside, in the same way a cooling coil would, by running a colder fluid through and around a warmer body or tube.

That might come in handy e.g. for large animals with active metabolisms that generate a lot of heat and have a low (skin) surface area to volume ratio. If a biological process like digestion or other metabolic functions produces a really strong exogenic thermal reaction, maybe the lung could loop around that area to cool it off. Having a one-way system with an 'in' and an 'out' would allow you to cool off continuously with no need to inhale.

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Reality Check = FAIL!

The first important consideration for this kind of breathing system, the conchoid lung, I believe, will be dead space.

The other important consideration is that your system isn't set up and doesn't function like a bird's respiratory system.

So, what will happen? --- Dead space is the term for air that sits in a tube that can not be properly inhaled or exhaled. In other words, your chest cavity is only able to expand so much, so the amount of air inhaled is limited by the available chest volume. The more tubing you add to the system, the harder it will be to bring fresh air in and clear stale air. The air that gets into the very end of the lung will become stale and will be unlikely to clear.

In a human lung, the entire volume is taken up by increasingly smaller branches and then even tinier ones. So, the large trachea divides into two primary bronchi each of which divides further into a couple more levels of smaller secondary & tertiary bronchi. These in turn divide into many bronchioles which terminate in alveolar sacs where gas exchange happens. This works well because the distance from any one alveolus to the outside is relatively short and the respiratory cycle can fill and empty the lungs without (much) air remaining inside (there will always be some).

Your conchoidal lung basically presents us with a very long tube. The distance from the most proximal alveoli to the outside will be comparable to the distance of the alveoli in the upper lobes of the human lungs. However, the distance from the most distal alveoli to the outside might be a metre or more! That part of the lungs will be filled with dead air that will hardly ever be cleared.

You'll have to either scrap the conchoid lung or else scrap the relatively humanoid thoracic cavity you described. See below under reinventing the wheel.

Bird breathing is complex! --- Birds have two lungs, nine air sacs, and a four stroke respiratory cycle. Inspiration brings 1. the current breath's fresh air from outside into the posterior sacs and also 3. the previous breath's stale air from the lungs into the anterior air sacs. Exhalation pushes 2. the current breath's fresh air from the posterior sacs and into the lungs and also 4. pushes stale air from the previous breath back to the outside. The result is a continuous flow of air through the lungs.

The extra tubing coming off the distal ends of the conchoid lungs will not effect continuous breathing. In stead, they will just create more dead space.

Conclusion. --- As it stands, I think your system will both fail to produce a superior volume of air flow & gas exchange and also will defeat itself by taking up too much valuable space within the chest with extra tissue and dead space.

Lastly, if you just connect those extra tubes to air sacs, well, then all you've really done is reinvent the wheel that avian dinosaurs invented millions of years ago. The argument for increased surface area for gas exchange fails again. Whereas before, only the most proximal portions of the lungs would really receive fresh air; now, only the most distal parts of the lungs will receive fresh air. Much of the lung's coiled up length will just have stale air passing through it.

I think diminished returns on investment will eventually lead to smaller, more straight forward lungs.

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  • $\begingroup$ but where is the power stroke in the bird's respiratory cycle? /s $\endgroup$
    – user253751
    Oct 15 at 8:32
  • $\begingroup$ @user253751 -- ;) Glad you caught my meaning! I'd argue that the power stroke is #2. This is where fresh air in the reservoir pushed out and through the lungs. Concur? $\endgroup$
    – elemtilas
    Oct 15 at 17:42
  • $\begingroup$ Exercise for the reader: design an internal combustion bird $\endgroup$
    – user253751
    yesterday
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A spiral-shaped lung could be good for separating out different-sized particles from the air that your creature inhales. The shallower curve at the outside of the spiral will allow smaller particles to flow around it, but the centrifugal force will push larger particles into the lining; and so on, as the curve gets sharper towards the centre of the spiral.

If your creatures live on a planet with lots of different kinds of dust, spores, and other particles which the creatures are adapted to inhale but which need to be filtered (e.g. maybe they eat the spores from the air but need to cough out the dust), then a spiral-shaped lung could function as a filter.

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Two crazy ideas

  1. For symbiosis: posit another creature which normally lives in spiral shapes (like a conch/shell) but sometimes found its way into the lungs where it supplied a symbiotic function like producing oxygen and helping the critter stay underwater longer, or filtering blood and converting waste product into useful product or less toxic waste product. Over time, critters develop spiral shapes in order to better acquire and accommodate the natural shape of their symbiotes, and over time critters have gotten used to having stuff live in their lungs.

  2. For reproduction: if the lung cavity doubles as a reproductive cavity, then perhaps it has undergone an effect similar to the Earth-borne duck.

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You said "air" but didn't really specify the composition of the atmosphere. If it was some sort of electrically-charged fluid, running it through spiral lungs would be useful for inducting electrical current. This could be a sort of "electrical respiration" for this species, rather than chemical respiration like we have.

This could also have the nice side effect of, when they go for a run or a bike ride, they could keep whatever they have that's analogous to cell phones in their pockets. Then by the time the exercise was over, they devices would be fully charged.

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Gas chromatography

The air of your planet is far more complex than the atmosphere of Earth. A rich mix of hydrocarbons, silanes, germanes, amines and fluorinated compounds, it provides many useful chemical precursors. Unfortunately, your species' membranes, like ours, are lipid bilayers with little capacity to control which compounds cross, whether beneficial or baneful.

With the spiral lung, your organism breathes in a brief pulse of external air, followed by a carrier gas recycled internally from the absorbed inert compounds. This adheres differentially to the polysaccharide- and water-coated solid support phase of the spiral lung. After a moment, inhalation stops. This leaves each type of chemical having travelled a different distance depending on its hydrophilicity, and cells specialized to deal with each type of chemical constituent do so - whether to absorb, detoxify, or route through toward small channels for rapid "exhalation" from small ducted glands.

The system is not tremendously efficient as a means of respiration - I'm going to assume the primary means of absorbing oxidizing agents is by drinking compounds that rain down in the wind from the sunlit side of the planet, and that supplemental hydrocarbon intake is done by ordinary eating. But the breathing apparatus could deliver a wide range of useful trace elements from an otherwise hostile atmosphere.

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