In Marathon , you're a cyborg. You have some kind of shield - force field, super-hard liquid shell, anti-matter battery, whatever - which you replenish by plugging yourself into a fuel unit on the wall, which is like charging your car battery by driving it since it doesn't run out. You only physically get hurt when your shield is broken.
How do the medical things on the wall in half-life work, then? You bleed, this is somehow healing you. Is it dispensing band-aids? Is it full of bloodbags from blood donors? What is that stuff?