<P> There have been rare cases of air embolism being caused by air entering the bloodstream from the uterus or tears in female genitalia . The risk appears to be greater during pregnancy . Cases have been reported that resulted from attempts to perform an abortion by syringing . These appear to have been due to damage to the placenta allowing air to enter the bloodstream . </P> <P> Patent foramen ovale in underwater divers is considered a risk factor for arterial gas embolism due to shunt of what would otherwise be asymptomatic venous bubbles into the systemic arteries . </P> <P> Air embolism can occur whenever a blood vessel is open and a pressure gradient exists favoring entry of gas . Because the circulatory pressure in most arteries and veins is greater than atmospheric pressure, an air embolus does not often happen when a blood vessel is injured . In the veins above the heart, such as in the head and neck, the venous pressure may be less than atmospheric and an injury may let air in . This is one reason why surgeons must be particularly careful when operating on the brain, and why the head of the bed is tilted down when inserting or removing a central venous catheter from the jugular or subclavian veins . </P> <P> When air enters the veins, it travels to the right side of the heart, and then to the lungs . This can cause the vessels of the lung to constrict, raising the pressure in the right side of the heart . If the pressure rises high enough in a patient who is one of the 20% to 30% of the population with a patent foramen ovale, the gas bubble can then travel to the left side of the heart, and on to the brain or coronary arteries . Such bubbles are responsible for the most serious of gas embolic symptoms . </P>

Why is it dangerous to have air bubbles in the blood stream