Feared throughout the galaxy as nightmarishly violent warriors, the krogan are both aided and hobbled by their legendary "blood rage".
In the grip of that madness, krogan become seemingly invincible, but are merely totally unresponsive to pain. "Blood-enraged" krogan fight regardless of injury level, to the extent that krogan shorn of all four limbs continue gnashing past brain death until total somatic death.
The supremely resilient, hyper-violent blood rage is the synergy of two aspects of krogan neurology. The first aspect is a positive feedback loop in which adrenalin, also activated by fear or rage, suppresses serotonin, the brain chemical that induces serenity. The second is the over-developed krogan limbic system. In krogan, as in humans, fear or rage shifts mental control from the frontal lobes, responsible for reasoning, to the limbic system, responsible for aggression and survival. During that shift, krogan and humans exhibit diminished capacity for logic and self-control.
Prior to the ecological devastation of Tuchanka, blood rage was extremely rare among the krogan. Back then, while all krogan were capable of heightened anger and violence in fight-or-flight scenarios, almost none experienced insensitivity to pain. The one percent who did were those suffering serotonin-suppression. At that time, krogan society regarded the condition as pathological, and medicated or imprisoned sufferers to protect them and society.
Following nuclear ecocide four millennia ago, evolution selected only those krogan afflicted with blood rage for survival. Today there is no living memory among the krogan of a life without mindless, murderous fury.