Complex sentence: A general anesthetic by itself can leave a person unaware of a surgical procedure but the body will still mount a fight-or-flight response which is harmful because of the extremes of pulse and blood pressure.
Split into simpler sentences: A general anesthetic by itself can leave a person unaware of a surgical procedure but the body will still mount a fight-or-flight response. This stress response is harmful because of the extremes of pulse and blood pressure.