A pulmonary embolism occurs when the artery that carries blood to the lungs becomes blocked.
The blockage is usually a blood clot, but it can also be a fat droplet, an air bubble or amniotic fluid (fluid that surrounds unborn babies).
If it's a blood clot, this will have usually come from one of the deep veins in your legs and is known as deep vein thrombosis (DVT).
The main reasons why blood clots develop are outlined below.
If you're inactive, blood tends to collect in the lower parts of your body, particularly in your lower legs. This isn't usually a problem because when you start to move, your blood flow increases and blood begins to move evenly around your body.
However, if you're immobile for a long time, the flow of blood around your body can slow down considerably. You're likely to be immobile:
- after a serious illness such as a stroke
- after an injury or operation
- when travelling on a long journey by plane, train or car
If your blood flow slows down because of a long period of inactivity, your risk of developing a blood clot increases.
Blood vessel damage
If a blood vessel is damaged, the inside of the blood vessel can become narrowed or blocked. This can result in a blood clot forming.
Blood vessels can be damaged by injuries such as broken bones or severe muscle damage. If a blood vessel is damaged during surgery, a blood clot may develop, particularly in operations carried out on the lower half of the body.
Conditions such as vasculitis (inflammation of the wall of a blood vessel) and some types of medication, such as chemotherapy medication, can also cause blood vessel damage.
Blood that clots too easily
Your risk of developing a pulmonary embolism is increased if you have a condition that causes your blood to clot more easily than normal.
Conditions that increase the likelihood of your blood clotting include:
- cancer – also, cancer treatments such as chemotherapy and radiotherapy can make your blood clot more easily
- heart failure
- thrombophilia (an inherited condition where a person's blood has an increased tendency to form clots)
- antiphospholipid syndrome (an immune system disorder that causes the blood to become abnormally sticky, increasing its tendency to clot)
Other factors that increase your risk of developing a pulmonary embolism include:
Your chances of developing a blood clot are very small if you're taking the contraceptive pill or HRT, and your GP will usually assess your individual risk before prescribing either medication.