system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we create underwater breathing technology

by mimicking what happens in fish' gills?
 
Edit: I mean by "extracting dissolved oxygen from water to stay submersed indefinitely." (thanks ragnarokrudolph)
gpt
We are endothermic and have a high metabolic requirement--one that is further stressed when we are submerged.  Animals with gills, on the other hand, are almost exclusively poikilothermic and require much less oxygen.  For us, the challenge is getting enough oxygen from the water to sustain our metabolism.