Pulmonary embolism is a serious and acute condition that occurs when the pulmonary arteries (lung arteries), which carry blood from the right side of the heart to the lungs, become suddenly blocked by a blood clot, air, or fat. This condition is one of the life-threatening medical emergencies. Although it most commonly results from blood clots, more rarely, air or fat embolisms can also lead to pulmonary embolism.