How could air embolism in the artery ׂ(by syringe) cause heart attack or stroke?

1) I've been reading very well the article on Wiki air embolism , and it's not clear to me how can a bubble that inserted by syringe to the artery of the arm, reach to the heart or the brain and damage them. the blood flow is just in opposite direction (from the heart to the tissue in case of artery). 2) The article there states: "Gas embolism into an artery is a more serious matter than in a vein, because a gas bubble... How could air embolism in the artery ׂ(by syringe) cause heart attack or stroke?
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