Humans (and mammals) have 2 lungs, one right and the another left. The right lung has 3 lobes and the left have 2 lobes because in the "third" there is the heart, so the right lung is bigger than the left.
I want to make a race with two hearts so I have to remove one lobe of the right lung to locate the heart and also I have to move a little the left heart.
If I remove a lobe this race would have a less pulmonary capacity and won't be able to run much. How can I overcome this problem?

My ideas are:

  • Increase the size of the lungs: The problem is that I don't know if we have "free space" in our body to do that.
  • Hollow bones: Some time ago I read that birds have hollow bones and they store air inside to breath later, then I found that is false but it is still an interesting idea. The problem with that is I would make bones less strong and I don't want that.
  • Increase efficiency: I couldn't find any page with this information but 2 years ago I read that we inspire air with 21% O2 and exhale with ~15% O2, so we breathe the ~6% of O2 in air. Maybe it's possible to increase this efficiency and exhale air with 10% O2 or less to compensate for the lost lobe. The problem again is that I don't know if that it's possible or how to do that.
    • Myoglobin: searching more about the ZioBite's answer I found the myoglobin, a substance similar to hemoglobin but instead of transport oxygen it's used to store oxygen in the muscles. This could explain how they can do physical effort but not how they live normally (because this store slowly the excess of oxygen not increase the oxygen input).

I had some ideas but I don't know if they are possible or how to do them.

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    $\begingroup$ Just make the lungs bigger, the chest cage enlarges with lungs so there won't be a problem at all, it will also make your creatures look bigger if paired with some large dorsali and pectoral muscles. $\endgroup$ – Evi Jul 9 '17 at 19:11
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    $\begingroup$ You don't need hollow bones to have air sacs, many of a birds air sacs are in the body cavity. ucmp.berkeley.edu/images/science/profiles/birdxsection.gif $\endgroup$ – John Jul 9 '17 at 19:43
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    $\begingroup$ Uh, oh. Hollow bones are not air storage. Hollow bones simply weight less. Weight is important for flying animals. $\endgroup$ – M i ech Jul 9 '17 at 21:56
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    $\begingroup$ How exactly would a cardiovascular system with two hearts work? How would it be beneficial? As for the efficiency of human lungs, their effect is to saturate the blood with oxygen - and they are quite capable of doing that even in "thin" air with a bit of adaptation. The pressure gradient gets bigger under load (cell/blood oxygen partial pressure gets lower), so we take more oxygen from air when necessary. It should also be noted that humans are very high above the endurance average to start with - do you really need aliens that have bigger endurance than humans when few animals on Earth do? $\endgroup$ – Luaan Jul 10 '17 at 8:42
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    $\begingroup$ The actual need is to increase efficiency of oxygen cycle: more red blood cells, more blood vessels, faster breath, faster blood circulation... Lung efficiency is only one part of the whole picture. $\endgroup$ – mouviciel Jul 10 '17 at 9:59

The way birds do it.

The air passes one-way through the lung and can extract nearly all the oxygen in one pass.


A diagrammatic representation of the cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs unidirectionally (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram). Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.

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    $\begingroup$ to emphasize the point bird breathing is so efficient that if you put them in a pressure chamber and slowly pull out the air a bird will be breathing fine long after a mammal of the same size has suffocated. This is also why if a victorian biologist asks to borrow your pets you say no. $\endgroup$ – John Jul 9 '17 at 19:48
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    $\begingroup$ The bird lung is more efficient for the same volume of space. Instead of adding air sacs, replace the lungs completely with avian style breathing apparatus. $\endgroup$ – pojo-guy Jul 10 '17 at 2:04
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    $\begingroup$ Bonus, they can breathe in and out at the same time. Weird. $\endgroup$ – talrnu Jul 10 '17 at 15:18
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    $\begingroup$ Is this also the reason they pass out more quickly due to carbon monoxide poisoning in mines? $\endgroup$ – Deruijter Jul 10 '17 at 15:49
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    $\begingroup$ @Deruijter CO bonds with hemoglobin in the blood at many times the strength of an Oxygen bond. The continual breathing (as opposed to humans who breath in pulses) make them succumb to CO poisoning much faster. Additionally, they are much smaller than humans, and require less CO breathed in to kill them. However, relative lung capacity mitigates that some, as a human will breath in more CO per breath than a bird. $\endgroup$ – Marshall Tigerus Jul 10 '17 at 17:18

Exit route

Imagine a car parking lot with one single lane entrance. You can either admit a car or allow a car to exit. If you have high volume you can admit a bunch of cars at once, then allow a bunch of cars to exit.

Now you open another lane. Cars can exit all the time, and enter all the time. You can have a steady stream of each. It is more efficient because you never need to stop and reverse traffic.

Do the same with the lung. Open an exit route. This will be a hole somewhere in the bottom of the lung (which might be better configured more like an intestine with this scheme - a air-filled tube rich with alveoli). I envision this right around the xiphoid. Oxygen rich air enters continuously at the entrance and exits continuously at the exit. The mechanics of breathing will be different and might be more like our digestive tract, another system with an entry and exit. The digestive tract operates using peristaltic waves and I can imagine a respiratory tract with an entrance and exit doing the same - perhaps with many small valved chambers along the way which open and close in sequence.

I here assert you will double efficiency by not having to regularly reverse flow.

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    $\begingroup$ Our ribs are not very rigid and the muscles in chest and abdomen are not at all. They are capable of more that what they routinely do. Watch this dude and imagine. youtube.com/watch?v=v8XSmhdAveI The cartilaginous attachments of ribs to sternum are flexible and not a huge deal. If they were not attached one could send waves top to bottom with the muscles we have. $\endgroup$ – Willk Jul 9 '17 at 21:59
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    $\begingroup$ oh, great! I didn't know that that was possible. $\endgroup$ – Ender Look Jul 9 '17 at 22:21
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    $\begingroup$ work your way up to that. He is a professional. $\endgroup$ – Willk Jul 9 '17 at 22:34
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    $\begingroup$ So you're approximately describing a gill but without water? Nice idea - you beat me to it. As for "waves" that's unnecessary - maintain the current system but put a one-way valve at the entrance and the exit. Diaphragm expands, air comes in, then shrinks and air is pushed out. The lung's air passages would be more like the lung's blood passages, a mesh that splits+thins then combines again. Advantages, noone dies from "water on the lungs" because it drains out the bottom. However people die from valve failure/obstruction. $\endgroup$ – Criggie Jul 10 '17 at 7:22
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    $\begingroup$ I wouldn't say it's "double efficiency". We don't spend all of the time breathing in or out, and breathing out is significantly shorter than breathing in. Even under heavy load, we keep air in the lungs for a while. Not to mention that "reversing the flow" doesn't make the lungs less efficient - they still extract oxygen while breathing out as long as the partial pressure is high enough. The main benefit of your gills is that the partial pressure of oxygen is always the same as atmospheric, which means the pressure gradient is more stable and higher on average. $\endgroup$ – Luaan Jul 10 '17 at 8:48

One of the quickest ways to increase the efficiency of the cardio-vascular system is to take EPO/use blood doping. So, it seems the lungs and heart are already big enough to utilise more oxygen carrying capacity than our blood already has. You ought to be looking at increasing the efficiency of the blood. The more oxygen your blood can carry, the more of it will be extracted from the air in the lungs. And the more oxygen in the blood, the more oxygen your heart will pump to your muscles without needing to change the amount of blood being pumped.

See here, from the 90s onward: Tour de France

In short, make the blood better - not the organs.


A few points:

  • Having hollow bones doesn't make bird bones more fragile, it makes them lighter (and you need some other place to to put the "factory" of erythrocyte, but that is another matter). A pipe is much stronger than a bar of same weight and material.
  • If You need a more efficient respiratory system copy cetacean system. They can store oxygen in muscle tissue and exchange very efficiently with a single breathing act (see, for example).
  • In general the rib-cage size is not a real constraint, lung status is: a "normal" human being has a "vital capacity" (volume difference between full and empty lungs) averaging around 5 liters, but normal respiration uses about 3 liters (or less) and athletes can move more than 7 liters in a single respiratory act. Severely handicapped lungs (such as those of long time heavy smokers) can have "vital capacity" lower than one liter and people still survive with only "marginal drawbacks" (i.e.: use a lift instead of stairs ;) )
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    $\begingroup$ Sorry, I thought that ribs doesn't have haematopoiesis function. Anyway there are very little compared to the rest of bones. The liver in the firsts months of life do the haematopoiesis function, maybe in this race the liver do that function for the rest of they life in a smaller rate. Also I didn't take note that hollow bones can be strong also. $\endgroup$ – Ender Look Jul 9 '17 at 21:23
  • $\begingroup$ I found in wikipedia that they exchange the 90% of air per breathing (mammals 15%) but I don't found any information about how they do that. Do you know about that? I don't know if it's possible make that in humans. $\endgroup$ – Ender Look Jul 9 '17 at 21:29
  • $\begingroup$ @EnderLook: Hollow bones are not limited to ribs; most good fliers (e.g.: albatross) have almost all long bones hollow, some of them are "pneumaticized" (connected with lung system) and some aren't. $\endgroup$ – ZioByte Jul 9 '17 at 22:35
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    $\begingroup$ @EnderLook: cetacean efficiency is mostly due to levels of hemoglobin in the blood, while underwater resistance is linked to several aspects (mioglobin, blood system partitioning, etc) that are not pertinent here. ANYWAY, if your only problem is to make room for a spare heart you need to do no special allowance: a bit of healthy training is more than enough to compensate for the reduced lung space (my third point) without need for other special things. $\endgroup$ – ZioByte Jul 9 '17 at 22:39
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    $\begingroup$ @EnderLook Note that human bones aren't solid either - they're less "hollow" than bird bones, but are actually significantly stronger than the same mass of solid bones would be. Solid doesn't mean strong, just more dense - which means the same mass has a lower total load. That's why we build skyscrapers with girders (all that empty space!) and not blocks of steel :) $\endgroup$ – Luaan Jul 10 '17 at 8:53

It seems like you don't want to change human physiology much so the easiest things to do would be to increase the efficiency of the bodies. As you said we only absorb 6% of the oxygen we breathe in, so let's find the bottlenecks.

Potential bottlenecks are:

  • Airways - The amount of air we can breathe in.
  • Blood - The amount of air our blood can absorb.
  • Lungs - The amount of air our lungs can put into our blood.
  • Body - The amount of air our bodies need to function.

Improve Airways

While the first is the least likely, it's obvious that we breathe faster or deeper when we're in a state where we need more air. A solution would be faster or bigger airways (we also tend to use our mouth for breathing instead of our tiny nostrils). Another issue we face is that we can not breathe in and out at the same time reducing the potential efficiency by 50%. Others here have identified this and claimed that it would work if you had an entry and exit hole, but keep in mind that air needs to flow in and out continuously as well. That would only work with a 2 chambered valve system similar to a heart.

Improve Blood

All of this is meaningless if our blood can not absorb enough oxygen. Since extreme divers hyperventilate before a dive to fill their blood with more oxygen, it seems like this is not the case - on the other hand, athletes try this angle when doping. Some of the things they do that increase the amount of oxygen that can be carried by our blood are to increase the number of red blood cells. I assume some other elements might be more efficient in carrying oxygen. This can also change the color of blood, Crustaceans, for example, have copper based blue blood rather than iron based red blood.

Improve Lungs

As I said, divers hyperventilate to increase the oxygen in their blood. This implies that the bottleneck is the lung. While you suggest that the size of the lung has to increase, this is actually not the case. What's more important is to increase the surface area of the lung. Their lungs could be built like window blinds, like a CPU cooler/heat sink with fins, or similar to fishes gills, which are made for breathing in low oxygen environments. The last two options would work best with flowing air, similar to first suggestions. Since your humans are double hearted and their blood would flow faster, chances are this will become an even higher bottleneck than it already is.

Improve Bodies

Our bodies can become more efficient and require less air to perform the same way. If you've ever tried to get in shape, you might have noticed that at first, you'll be out of breath after a short run. However, once you've trained for a while, you can run for a while until you finally run out of breath. Your double hearted humans could have a better physique anyway and thus not be out of breath as fast.

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    $\begingroup$ Our airways aren't anywhere near to being a bottleneck. Having to breathe in and out separately is nowhere near 50% reduction of efficiency - even under heavy load, you're spending most of the time holding your breath, not actually breathing (optimally, you exhale just before CO2 partial pressure gets too high and inhale just before O2 gets too low). Birds have a continuous breathing system that works fine without valves or exit holes (that's what air sacs are for). Divers hyperventilate to get rid of CO2, not increase O2 - blood is normally saturated with O2, there's no more you can get. $\endgroup$ – Luaan Jul 10 '17 at 21:13
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    $\begingroup$ Surface area of human lungs is already pretty crazy - are you suggesting even smaller alveoli? Fish gills are far more efficient than lungs, but they lose water quickly and stop working in air. But again, CO2 seems to be the bottleneck, not O2. Why would two hearts lead to higher blood flows? Why would higher blood flow make lungs more of a bottleneck? It would make them more efficient by increasing the partial pressure gradient. A human in good shape doesn't "run out of breath" while running - you need to compare to humans in our ancestral environment, not to modern urban residents. $\endgroup$ – Luaan Jul 10 '17 at 21:20
  • $\begingroup$ «divers hyperventilate to increase the oxygen in their blood» can you say “shallow-water blackout”? A certified diver is taught not to do that. Freedivers (breath holding) will take a specified number of deep breaths, not hyperventilate. $\endgroup$ – JDługosz Jul 10 '17 at 23:37

Consider the biochemistry of this race

Above answers are all very good! However, although this doesn't specifically apply to the lungs, you could also consider the molecules that are involved in oxygen transport. This works if you were looking for an option that involves biochemistry, rather than further changing the anatomy of your new race. To do this, you may wish to consider the protein hemoglobin, which binds oxygen and then transports it to cells. As you have mentioned, myoglobin is another important protein, and is a key player in diving mammals, though I believe the molecules that allosterically bind to hemoglobin are a better option.

In particular, a real life applicable example is 2,3-bisphosphoglycerate (2,3-BPG). This is the molecule that leads to populations that live in higher altitudes being more efficient at transporting oxygen. In short, this molecule decreases the oxygen affinity of hemoglobin, meaning it is released more easily when it reaches the cells. Since oxygen itself is an allosteric regulator of hemoglobin, as more oxygen is released from the one hemoglobin protein, more will want to be released as well! Populations at sea level have around 5mM concentrations in their blood, whereas populations in higher altitudes can have around 8mM concentrations.

You may also wish to do some research on the other molecules that are involved in hemoglobin regulation, such as hydrogen protons and carbon dioxide (the acidity of our blood near our muscles is higher than at the lungs, and this results in oxygen unbinding from hemoglobin so it can be used by our cells).

In conclusion: this is probably something that you might wish to 'add-on' to one of the other options provided, as there is a limit to how much you can play around with the levels of these molecules in our blood. If you were to increase the concentration of BPG tenfold, then oxygen would never even bind at the lung-blood barrier!

Just a few questions Is this really just a different race of humans? If you are trying to ground your creation in reality, have definitely considered the plausibility of having a different race of humans with two hearts. Can they mate with one-hearted humans? What would the result be? Could this in fact be another species, closely related to humans?

EDIT Here is a graph that shows the binding of oxygen to hemoglobin vs the partial pressure of oxygen for blood with and without BPG. Though this may be hard to understand without having being taught it, there are many resources explaining the role of BPG in the blood.

enter image description here

Also note that it would not be too far-fetched to perhaps say that the hemoglobin in your new race has changed in such a manner that it is more efficient at transporting oxygen. In hindsight, I definitely think that that is probably the best option to go with if you don't want to have to change the anatomy of your new race any more than the extra heart!


To add a little to the 'yoga breathing' line of thinking, with a view to the underlying physical mechanics:

If, say, 2 litres of air goes into your lungs, then necessarily 2 litres of the volume of the rest of your body, needs to be displaced, somehow, somewhere. There is a certain degree of flexibility in how to do this, and some ways are better than others, and some ways easier to learn than others. The former tend to turn up in practices which deliberately teach breathing, the latter turn up as people's typical breathing habits.

Now if our 2 litres of body-volume immediately surrounding the lungs has to move out to allow the lungs to expand, then 2 litres of body volume surrounding that has to go somewhere. And so it continues from the lungs themselves to the extremities of the body, or at least until this 'one bit of the body making way for another' ceases.

Now of course the physical work done to get the air in the lungs is, technically, done by the pressure of the air outside your body pushing the air in, due to a decrease in pressure in the lungs, caused by the volume of the lungs expanding. To breathe in, the body must thus work to increase the volume of the lungs. Part of doing this physical action efficiently, from an engineering perspective, is distributing the workload around the body. Another part is minimising mechanical losses due to one part of the body unnecessarily working against the efforts of another. The other is doing this so that you get an optimal amount of air in and out for the amount of oxygen your body needs.

When you hear yoga (and perhaps tai chi) teachers giving the seemingly crazy instruction to 'breath into your feet' or something like that, what is being got at is, in some sense, this 'mechanically efficient' breathing described above. But to actually do this, it is necessary to understand how to do it in terms of body sensations, and instructions the brain can deliberately give to the muscles, hence the very subjective viewpoint. If you actually do this, (to my experience,) it can feel as if you are breathing into your entire body, and to actually deliberately do this, you breathe as if you feel you are breathing into your entire body. (With years of practice, the deliberate stuff gradually becomes instinctive and automatic, of course, which is why the word 'deliberate' is important here.)

Much of yoga and tai chi is described in terms of how things seem when you are doing them correctly, rather than in terms that an engineer or biomechanics specialist would think of when understanding and explaining things (since they are generally talking about a body other than their own, whereas the yoga and tai chi practices are largely about teaching you about what you do with your body and your mind).

But to conclude, if you think about the mechanics involved with breathing, given a pre-existing piece of breathing apparatus (so you can't swap e.g. human lungs for bird lungs), thinking like the above can arise quite readily, and from there the connections to things like yoga and tai chi start to become apparent.

  • $\begingroup$ Welcome to WorldBuilding.SE John! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jul 10 '17 at 12:28
  • $\begingroup$ In birds, the flapping also works the air sacks like a bellows, and it’s constant volume shifting between fore and aft sacks, so the thorax doesn’t change size. $\endgroup$ – JDługosz Jul 10 '17 at 23:34
  • $\begingroup$ Various efficiency gains from considering the 'overall mechanics' of a system are likely apparent in every living organism. Also things like momentum transfer and elasticity between the moving wings and lungs are possibilities worth pondering. $\endgroup$ – John Allsup Jul 11 '17 at 14:09

One specific idea - separate breathing from eating.

As humans we have to stop breathing for a moment to drink or swallow. Evolutionarily-speaking, its probably to make the best use of a complex system of the mouth for speaking as well as chewing rather than duplicating things.

If the beings had separate eating orifaces from their breathing ones, then efficiency rises and multitasking becomes more possible.

Downside, the human nose depends on air movement to smell things, and having no air movement there would impact on the sense of taste.

On the positive side, there's zero danger of choking on food, or inhaling vomit into the lungs because of the partitioning of systems.

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    $\begingroup$ Human infants have breathing and eating separate (they can't breathe through the mouth at all; the later joining of the two tubes is necessary for speech). It prevents choking on food, but I wouldn't expect it to significantly impact the breathing efficiency - eating and breathing in adults are in "gulps", not in streams. You chew while breathing, then for a blink you swallow, and then breathe again. It's not really a big deal. Multi-tasking in general tends to be overstated - we already multi-task where it makes sense; we're mostly idling even under load, really. $\endgroup$ – Luaan Jul 10 '17 at 9:01
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    $\begingroup$ @Luaan a crying baby is definitly exhaling through the mouth! I found pictures here and explaination that matched what I knew: the connection is present, just higher. To go down the wrong pipe, food would have to go up the throat to get to the connection. $\endgroup$ – JDługosz Jul 11 '17 at 4:00
  • $\begingroup$ @JDługosz Yes, crying is a good example - that's actually how an infant with nasal congestion opens the oral airways. My point was that human infants (and adult primates) already have breathing and eating separate, and that a newborn that cannot breathe through the nose (e.g. due to nasal congestion) might not be able to breathe at all, though it's relatively rare. But you're definitely right that the connection is there (in a healthy infant), just not in the "right" place - my bad. It takes a lot more effort to breathe in through the mouth, but it's possible (though undesirable). $\endgroup$ – Luaan Jul 11 '17 at 7:55

To increase the efficiency of the lungs try breathing using your diaphragm. Commonly called Yoga breathing, you intake by pushing your stomach muscles out to fill your lungs. This generally increases lung capacity by 20–25% intake.

Hold longer to pass more oxygen through your blood, exhale slowly.

  • $\begingroup$ You're totally right - I've noticed this while riding up climbs on a bicycle. Slower deeper breathing helps, whereas the muscles "want" you to breath in synch with your pedalling. $\endgroup$ – Criggie Jul 10 '17 at 7:32
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    $\begingroup$ The question isn't about breathing techniques it's about redesigning the body such that the lungs are more efficient. It's important to read the body of the question, not just the title. $\endgroup$ – Separatrix Jul 10 '17 at 8:34
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    $\begingroup$ @Criggie Rapid breathing is great for high-stress loads, while deep breathing is great for endurance. It doesn't take a lot of training to make deep breathing entirely natural when running or riding a bike - endurance is one of the things that we're quite extraordinary at, compared to most animals. Though it's a lot harder to see that when watching humans in their urban habitats :P Which of course further reinforces the huge breadth of human breathing efficiency, even with the same basic hardware. Losing one fifth of lung capacity isn't anywhere close to that. $\endgroup$ – Luaan Jul 10 '17 at 9:06
  • $\begingroup$ Isn’t the diaphram how breathing normally works? $\endgroup$ – JDługosz Jul 10 '17 at 23:29
  • $\begingroup$ Oh, welcome to Worldbuilding if nobody’s done so already. $\endgroup$ – JDługosz Jul 11 '17 at 3:58

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