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This question already has an answer here:

I am aware that human lungs can't process water due to other reasons, but I am working on a setting that has genetically altered humans of various degrees and one of them can breathe water and air. I don't want them to have gills but rather just have lungs that can process water and air.

They would be;

  1. Salt water creatures
  2. Living in relatively shallow waters, maybe in extreme cases going down to 40 meters but almost exclusively residing at about 20 or less
  3. Possessing the same mental capability as humans

I know I would have to hand wave the details, but I just want to know that if the human lung could take oxygen from water if just making them stronger will work to keep the people from ripping open their lungs or passing out from straining to breathe. Would their lungs have to be bigger? Would they need a different cell structure?

Is this remotely possible or will I have to do as Star Wars does and just ignore the problems this would create?

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marked as duplicate by Willk science-based Dec 17 '18 at 2:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Did not get to post. In short don't be a fish be marine animal. Hold your breath and dive. Some small increase in lung capacity, more efficient and even use, awareness of oxygen left in lungs. $\endgroup$ – Artemijs Danilovs Dec 17 '18 at 3:24
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No. There is no means, genetic or technological, to sustain a human on oxygen filtered through water. The problem isn't the capacity of the lungs or filters or anything else: it's due entirely to the amount of oxygen in the water in the first place.

Wikipedia helpfully tells us the scale of the problem here:

In fresh water, the dissolved oxygen content is approximately 8 cm3/L compared to that of air which is 210 cm3/L.

In other words, there's about 4% as much oxygen in a given volume of water compared to the same volume of air. Water is also more viscous and dense, making it harder to move around. Lungs would not be able to process enough water to derive a livable amount of oxygen.

Gills work for two reasons. One is that their design is optimized for the water environment to make them much more efficient. For instance, while the travel of air through the lungs is two-way (you inhale and exhale), the travel of water through gills is one-way. This means the gill is never idle in terms of oxygen transfer, the way lungs are while exhaling. The other major factor is that the animals that use gills simply need less oxygen, chiefly because they're cold-blooded.

The cheap solution for shallow-water creatures is simply make them air breathers. A wide variety of marine mammals exist, all of them air-breathing water-dwellers with adaptations like oxygen reservoirs to maximize the time they can take between breaths. Many marine mammals are smart, and there's no particular reason why they couldn't support human-like intelligence.

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