I guess what I am saying is that "but the rules don't simulate the world!" is a false contradiction. That isn't what gameplay is for, and the rules are for gameplay.
Again, let me use Call of Duty 4 as my worked example. It is set in the real world, using real weapons. The characters are, ostensibly, real people who can bleed to death, have indigestion, pay taxes and drink beer.
Your character can soak up bullets and so long as he isn't killed and has time to recover, eventually returns to full health. People do not do this in the real world. Even worse, in cutscenes, normal human vulnerabilities return!
The simulationist (apparently) says: "Contradiction! Inconsistent! I cannot buy into the story or the characters as a result!"
This is clearly not a problem for a great many people, as it is a very popular game. But why, if there is a intrinsic, crippling contradiction to it?
Because the three elements are served in different ways, and because it would be far, far worse to have bad gameplay as a result of attention to "realism."
The world is simulated by all the Tom Clancy crap- real world weapons and jargon, etc. The story isn't told by gameplay but by characters, dialogue and plot twists. The gameplay is served by the (very popular) regenerating health mechanic. There are no contradictions here, because the three elements have different needs.
That is why D&D has hit points. To serve gameplay.
If you want simulation, then do it the same way all other gamers everywhere have always done- keep your world and characters consistent whilst the story is told.
Searching for a "reason" for hit points or second wind or anything is fruitless. It has dominated discussion on this forum and it all ends up in the same place- there is no explanation. Sure, people come up with all kinds of great rationalizations for rules (there was one a while back on why Rings were so powerful) but they're unnecessary.
Why are they unnecessary? Because it's a false contradiction to begin with. Just like someone playing Call of Duty 4...don't worry about it. Everyone at your table is going to be aware they are playing the game. They are sitting at a table. There is a bag of Cheetoes. They have to edit these elements out for their imagination anyway if they're going to get any degree of immersion, and it's trivial to do the same for gameplay artifacts and their "intrusions" on the story or the world building.
In other words, don't worry about whether or not you can explain what a Healing Surge is. You don't have to worry about it. Why don't more games have detailed, realistic wound systems? Because they're bad for gameplay. I mean, Phoenix Command had a way of determining how a bullet traveled through the body. It could take several minutes to resolve a single gunshot. Is that fun?
It is not the role of the rules system to tell you what the world is like. That's both impossible (no system could ever reflect the complexities of a world) and counterproductive (in the trying you'd need to sacrifice ease of play and other considerations).