When an air bubble enters a vein, it's called a venous air embolism. When an air bubble enters an artery, it's called an arterial air embolism. These air bubbles can travel to your brain, heart, or lungs and cause a heart attack, stroke, or respiratory failure. Air embolisms are rather rare.
An injection of 2-3 ml of air into the cerebral circulation can be fatal. Just 0.5-1 ml of air in the pulmonary vein can cause a cardiac arrest.