system
Given the following post, respond as if you're commenting on Reddit:
human
In movies "going into shock" after an injury is always bad, but isn't it one of the body's defense mechanisms? How could it help?
gpt
Shock is just hypotension, meaning the blood pressure is not enough to perfuse the body. Perfusion pressures are determined by mean arterial pressure (MAP) and needs to be above ~65 mmHg to be effective. Shock causes organ dysfunction because the organs aren't getting enough oxygen. Shock can be hypovolemic, cardiogenic, or distributive. Hypovolemic shock is the one you see in movies and is usually due to loss of >15-20% of total blood volume. Going into shock, as in the movies, in and of itself isn't a bad thing, but it's a bad sign because the character has lost enough blood for hemodynamic compromise. 

Cardiogenic shock occurs when there is pump failure of the heart causing an inability to maintain perfusion pressures. Distributive shock is vasodilatory, meaning the peripheral arteries have dilated (due to infection, anaphylaxis, toxins, etc), and the total systemic resistance has dropped to a point that the body cannot maintain an effective MAP.