The short answer: Superman and Kryptonians in general, only share one characteristic with human beings; our external physical, bilaterally symmetrical appearance. Kryptonians sufficiently resemble Humans enough for them to be able to pass as Human but only from a cursory physical inspection. Any trained medical personnel should be able to determine a Kryptonian is not Human with only a modest toolkit of scientific equipment.
The Long Answer: DC Comics has never given us a cohesive or guided tour into Superman's physiology over the decades of his existence. Supposedly born on a world with gravity far greater than Earth's, Kryptonians were thought to have incredible physical capabilities including superhuman levels of strength, durability and stamina. Superman as a biological entity is almost implausible given the parameters of his abilities in the comics.
Given his creators, Jerry Siegel and Joe Shuster were not scientists in the 1940s, his powers were very loosely defined as:
- faster than a speeding bullet
- more powerful than a locomotive
- able to leap tall buildings in a single bound
- nothing less than a bursting shell could even pierce his skin
Under these parameters, Kryptonians were later thought to be very advanced humanoids externally very similar to Humans. The internal workings, however, had to be vastly different to allow them to do even these relatively simple feats able to be done by the Golden Age (Earth-2), Pre-Crisis Superman.
The Golden Age Superman (and the only version which has any modicum of a chance to be remotely physically possible) had to be a specimen of incredible physical capability to perform even his relatively limited suite of super feats:
Due to his denser muscle and bone tissues he was able to resist small arm fire and was nearly invulnerable to conventional injuries.
His denser musculature allowed him to be able to move far greater weights than any human possible could and allowed him to run as fast as a moving automobile or jump as high as a fifty story skyscraper.
Born to a world with a stronger gravity, under Earth's gravity, Superman's reflexes are many times faster than ours, able to even catch a bullet in mid air.
Unfortunately, modern science cannot replicate or explain how Superman was able to perform these relatively modest superhuman feats without a significant restructuring of both his chemical processes and his physical structure.
I have in the past tried to rationalize his powers but the underlying energy source to explain how he powers his superfeats defies scientific description. We are forced to rely on handwaving of the laws of physics to explain how he:
absorbs solar energy into his body and stores it for later use. How is the energy stored, chemically, as radioactivity, as a matter-antimatter reaction?
being a man-sized body, he cannot possible absorb enough solar energy to perform even one of his average feats of superhuman ability. Where does the rest of the energy come from?
can carry objects much larger than he is and they don't fall apart when he picks them up? If he is a telekinetic, that might explain some of the superfeats in terms of their physics-defying nature, but we then must explain the telekinetic ability as well.
is able to perform any of his modern feats at all. If his powers are purely a physical representation of how his body processes energy, his organs would bear no resemblance to anything human. He would be suffused with a variety of both organs and cellular structures completely different from anything human. They would need to be since his energy profile would be so far removed from anything human. He is human in appearance only. Viewing him under any kind of scanning technology should show a creature as different from us as we are from viruses.
As far as his eyes and other sensory abilities:
they too would seem to be better explained as psionic abilities rather than physical abilities, since it would imply his eyes are capable of absorbing and processing far more light than our eyes can, even though they are the exact same size as human eyes.
Greater muscular strength and precise muscle control might explain the increase acuity of his microscopic and telescopic visions but his abilities exceed that of our greatest machines, its more like magic than science.
The use of heat vision capable of reaching 6,000 degrees as an energy being released from his eyes would imply his eye structures are as different from our eyes as any organ can be. Again, better explained as a form of psychic ability than a physical one.
I am relatively certain, given Superman's abilities, the longevity of the character, and the currently undisclosed nature of his biology, it is safe to assume we will never be given a full disclosure of his anatomy for one simple reason.
Once you define it, there will be people who will want to go back through history and see if the definition has been exceeded. Since we accept Superman's outrageous history as fact (as defined by DC Comics) it is simply easier for them to leave it undefined than to have to retroactively explain how he did any of those feats in the first place.
I created a profile of Pre-Crisis Earth-1 Superman's physiology and an explanation of his powers back in the late 1990s on my website the Starnet Metahuman Information Database. This was back before the comic industry had begun posting their heroes online. Only the Wayback Machine can get us there now. Enjoy the link...