When blood pressure goes to 0, the lights go out. If a shot to the heart destroys the heart, and thus blood pressure goes to 0, it’s game over. If the heart is damaged but still able to provide some blood pressure, you may have some time. Think of a small calibre round, or something other than a direct hit.
If you watch hunting videos, a heart shot with a rifle round just flips the switch and turns them off.
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