A pulmonary embolism happens when an embolus, which is a type of blockage, suddenly gets lodged inside a pulmonary artery.
Depending on which pulmonary artery or arteries are affected by the blockage, that can seriously decrease the amount of oxygenated blood that gets out to the body.
Normally, blood makes it back to the heart from all of the tissues and organs through a network of veins that merge over and over.
Superficial veins drain blood into deep veins, which rely on the skeletal muscle pump to move blood forward. The way it works is that the surrounding skeletal muscles compress the vein and propel blood forward, and the veins prevent blood from moving backwards by using one-way valves.
Ultimately, all of the blood ends up in the superior or inferior vena cava and dumps into the right atrium.
From there the blood goes into the right ventricle and gets pumped into pulmonary artery and eventually into the lungs.
The pulmonary artery splits at a spot called the pulmonary saddle, which looks like a bit like a horse saddle, and then the right and left pulmonary arteries enter their respective lungs.
Subsequent branches off the pulmonary artery lead to smaller and smaller arteries, then arterioles, and finally capillaries that form nets around the alveoli, which is where gas exchange occurs.