Surprisingly, the body does not signal a low-oxygen state, and the urge to breathe is stimulated by a rising carbon dioxide level. This works fine for healthy i

Surprisingly, the body does not signal a low-oxygen state, and the urge to breathe is stimulated by a rising carbon dioxide level. This works fine for healthy individuals in the environment we have evolved to live in, but it makes hypoxia an insidious killer in an environment where the oxygen partial pressure is substantially less than it normally is, as the victim does not notice that anything is amiss.