An air embolism, also known as a  gas embolism, is a blood vessel blockage caused by one or more bubbles of air or other gas in the circulatory system. Air can be introduced into the circulation during surgical procedures, lung over-expansion injury, decompression, and a few other causes. Air embolisms may also occur in the xylem of vascular plants, especially when suffering from water stress. Divers can develop arterial gas embolisms as a consequence of lung over-expansion injuries. Breathing gas introduced into the venous system of the lungs  due to pulmonary barotrauma will not be trapped in the alveolar capillaries, and will consequently be circulated to the rest of the body through the systemic arteries, with a high risk of embolism. Inert gas bubbles arising from decompression are generally formed in the venous side of the systemic circulation, where inert gas concentrations are highest, these bubbles are generally trapped in the capillaries of the lungs where they will usually be eliminated without causing symptoms. If they are shunted to the systemic circulation through a patent foramen ovale they can travel to and lodge in the brain where they can cause stroke, the coronary capillaries where they can cause myocardial ischaemia or other tissues, where the consequences are usually less critical. The first aid treatment is to administer oxygen at the highest practicable concentration, treat for shock and transport to a hospital where therapeutic recompression and hyperbaric oxygen therapy are the definitive treatment.
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.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. Venous or pulmonary air embolism occurs when air enters the systemic veins and is transported to the right side of the heart and from there into the pulmonary arteries, where it may lodge, blocking or reducing blood flow. Gas in the venous circulation can cause cardiac problems by obstructing the pulmonary circulation or forming an air-lock which raises central venous pressure and reduces pulmonary and systemic arterial pressures. Experiments on animals show that the amount of gas necessary for this to happen is quite variable.