Note: this was originally posted on my blog on tumblr.com. I have tried to convert some of the formatting over, however there may be some inconsistencies. Also this particular entry was written before I knew how to cite scientific sources, so sources will either be nonexistent or incorrectly cited.
Not only would a flier (someone who can fly) have to have amazing muscle control, they would also have to be adapted for low oxygen levels, low pressures, and other conditions that come with high elevations that humans usually aren’t exposed to and aren’t adapted to.
While a flier would have to have a lot of muscle to control their movements in their air, they would have to have a lighter skeleton than a normal person to make up for the weight of the muscles. In birds some bones are fused together while other bones are filled with holes or are even hollow to decrease the weight of the skeleton. So for a special to fly (at least at the elevation Nathan appears to fly maybe, different specials can fly at different heights) they would actually probably be lighter than expected for a person of their size. The drawback of this is that the person would probably have brittle bones, due to the hollow areas, that would be easier to break than a normal human. However these hollow bones might be strategically placed, such as the ribs or some other area where a broken bone would not be lethal or prevent flight.
A flier would also have to have a more efficient respiratory system. In mammals the respiratory system is made up of two lungs that inflate when the animal inhales and then deflate when the animal exhales. However when the animal exhales not all the air leaves the lung, so the exchange of gases isn’t very efficient because there is so much dead space. This would be bad for a flier because at high elevations there is less air present due to a lower pressure (this is why cabins of air planes are pressurized) so every breath counts.
Birds have managed to adapt to these conditions by having a series of smaller air sacks throughout their bodies that allow the inhaled air to move in one direction through the body. The bird inhales and the air moves around the bird’s body with each breath before it is finally exhaled. This allows for more oxygen to be dissolved into the blood stream because the air spends more time in the bird’s body and is circulated more evenly.
Some birds, like bar-headed goose, also have more capillaries than the average bird so they can absorb even more oxygen into their blood streams. Bar-headed geese can also hyperventilate without fainting which allows them to exchange oxygen even faster. Other birds, like Ruppells Vulture, actually have a different type of hemoglobin than humans called myoglobin that can carry oxygen more efficiently in the blood. This allows the bird to get more oxygen out of each breath.
It seems unlikely that a flier would have air sacks as opposed to lungs, since when Nathan was shot the doctors did not appear to be seriously freaked out by his anatomy. If he had anything other than normally functioning lungs the doctors would have noticed. So it is more likely that he would have increased capillaries, the ability to hyperventilate without passing out, and a special type of hemoglobin that would allow him to breath at high elevations.
Finally comes the problem of energy. Flying takes a lot of energy out of birds. There is a reason why long distance migrations are so dangerous, may die because of fatigue or not being able to eat enough to replace the energy they have lost on the long flight. So how do they get energy? Some birds (like the bar-headed geese, can you tell that I found a good article about them?) actually have double the mitochondria in their muscle cells. Mitochondria break down glucose and turn it into ATP, which the cell uses as energy to power the muscles. By having twice the number of mitochondria the geese can utilize fat stores and nutrients twice as efficiently. Of course this means they also use it up twice as fast, so they have to eat more. If a special were to have this adaptation then it seems like they would either have to eat carbs before a long flight like a marathon runner or they would be extremely hungry after a long flight.