caused by air entering the bloodstream
. It eventually travels to the heart, where it fills the right atrium
with frothy bubbles, which form an air lock
in the heart
The heart can't pump blood around the body, because the usual compressing action that causes this to happen just squeezes the bubbles a bit, rather than forcing blood out of the atrium. If nothing is done, death usually occurs.
Air embolism is quite often deliberately inflicted during syringe attacks by people who are unaware of its serious effects.