A pulmonary embolism (PE) can be life-threatening. Usually they occur when a clot occludes the pulmonary artery or one of it's branches. The pulmonary artery is the artery that drains the right ventricle in the heart. The blood travels via the pulmonary artery to the lungs where the previously unoxygenated venous blood receives oxygen and is transported back to the left atrium via the pulmonary veins. From there it travels through the mitral valve, into the left ventricle and out through the aortic valve into the aorta; From there it goes into systemic circulation and supplies your body and brain with oxygenated blood. When a pulmary embolism occludes (blocks) the pulmonary artery or its branches, quite simply not as much blood can go through to receive oxygen. The alveolar (alveoli are the distal part of the lung where oxygen and carbon monoxide are exchanged) dead space in the lungs is increased. Your body tries to compensate by increasing heart rate and blood pressure. Eventually, shock will occur due to the decreased amount of blood in circulation. Or, the blood will back up into the right side of the heart and move backwards in the body causing right-sided heart failure. Even worse, a part of the clot could break loose and occlude an artery going to the brain and cause a stroke. Typically when ever there is a clot, there will be mediators of inflammation ...which could lead to some fluid buildup. Any way you look at it, get to the hospital quick. Hope this helped.