Why do people get unconscious after they loose blood?

Answer 1

Two factors typically come into play: Shock and hypovolemia (lack of blood). Shock is your body (and mind) trying to cope with a difficult situation, and its occurrance is not always a bad thing. Unconsciousness beats enduring extreme pain you can't escape from, for instance. And part of the shock process releases endorphins into your system, which can cut pain levels to something you can live with in order to save your own life. I watched films of a woman who, after a fall resulting in a disasterously damaged knee, who was able to CRAWL on it over 100 yards to safety - and then -- once safe -- she passed out. When interviewed, she said it wasn't even all that painful until she awakened afterwards: shock and trauma. It's also fair to say that shock can be induced in a cognitive sense. I have a good friend who passes out when he sees blood. This (I suppose) is an empathy response where he sees blood, identifies with his own bleeding, and shocks out. This says a lot for his empathy; and not as much for his chances for a career in surgery :} The other problem from bleeding out is hypoxia or lack of oxygen, resultant from blood loss (hypovolemia). No blood means no way to carry sufficient oxygen. Unconscious restults, which reduces the body's oxygen needs for a time. This is why first responders put such emphasis on stopping bleeding at trauma sites.