I have a character in Seaborn, Michael Henderson, who's a minor character with a background in science, and I've sort of left it up to him to try to explain how people can live and breathe under the sea. He has the "curse" himself, all the abilities the Seaborn have. He writes pages of notes, sketches the things he sees in the deep, imagines why things work the way they do with the Seaborn--all with a scientific mind.
I've written and drawn a bunch of stuff in the character of Michael Henderson, which started out as part of the worldbuilding exercises, and just kept going. I wrote the chapter headings in Seaborn from Henderson's perspective, taken from his notes, his journal, his "conversations" with various notable characters.
Here are some samples from my journal:
I have been to the deep ocean, the Very Deep, and I have set my feet down in billion year old sand. I have kicked through the dark with blind animals that change shape with their moods, with fish ten meters long that glide through the deep sea without fear--and only eat microscopic food, with arthropods made of glass, and creatures that defy classification, I have touched the bioluminescent lures of fanged ambush predators in the abyss, and I still have all of my fingers. I have done all of this without equipment, without SCUBA, without feeling the pressure, or need for air. I am no longer a surface human--or as the Seaborn, say--a surfacer, a Thinling. I have become one of them.
I have experienced, l’ivresse des grandes profondeurs, Jacques Cousteau's "rapture of the deep," but not as the nitrogen narcosis that Cousteau described in Silent World. Say, rather, that I have experienced the rapture of the unexpectedly normal in the most unexpected place on earth: the deep sea.
The Seaborn do not suffer from any of the affects of breathing compressed gases, for example the squeeze of barotrauma on descent, because presumably, these do not exist in effective amounts in their bodies.
SCUBA stands for Self-Contained Underwater Breathing Apparatus. This is a device enabling surface-living humans to recreate, as near as possible, and within well-defined limits, everything the human respiratory system needs above the ocean surface, in the air. While in the water, it appears that the Seaborn do not--or even need to--breathe in the same manner, possessing a different, possibly more advanced system for taking in the same gases and nutrients directly from seawater. Out of the water, the lungs of a Seaborn human appear to function the same way as the lungs of any surface human.
Lungs: Alveoli are the small grape-bunch like structures that line the lungs and take up oxygen, CO2, Nitrogen--gases the human body needs to survive, with oxygen fueling so many of the processes. The Alveoli are highly susceptible to damage from heavy substances like seawater, which really shouldn't be in the lungs. Damage then leads to low blood oxygen levels (hypoxemia) , low tissue oxygen levels (hypoxia), and then death. The alveolar-capillary membrane is a delicate, one cell thick membrane through which the gases we breathe are exchanged. It appears to be the case that the Seaborn possess a more rigid surfactact--a sort of stiffening coat for the alveoli to prevent them from collapsing under the weight of heavier substances like water in the lungs.