In the normal electrocardiogram, the PR interval represents

A pulmonary embolism (PE) occurs when a blood clot, typically originating from the deep veins of the legs or pelvis, travels through the bloodstream and lodges in one of the pulmonary arteries in the lungs.

Deep venous thrombosis (DVT) is the most common source of pulmonary emboli. DVT occurs when a blood clot forms in the deep veins of the legs, pelvis, or arms, and can develop due to prolonged immobility, surgery, trauma, cancer, pregnancy, hormone therapy, or genetic factors.

Endocarditis, valvular heart disease, and left heart failure are not typical sources of pulmonary emboli. Endocarditis is an infection of the heart valves and lining, which can lead to valve damage and heart failure, but does not usually cause blood clots. Valvular heart disease, which includes conditions such as aortic stenosis and mitral regurgitation, can cause blood flow disturbances and increase the risk of thrombosis, but does not directly cause blood clots. Left heart failure, which occurs when the left ventricle of the heart is unable to pump effectively, can cause fluid buildup in the lungs, but does not typically cause blood clots to form.

Ischemia refers to the temporary deprivation of blood supply to a particular tissue or organ, resulting in reduced oxygen and nutrient delivery to cells. This can lead to cellular injury and, if prolonged, irreversible damage. Infarction, on the other hand, refers to the death of tissue due to prolonged ischemia. Necrosis is a type of cell death that occurs when cells are damaged or injured and cannot survive. Inflammation is the body's response to tissue injury or infection, and can be a result of ischemia or necrosis.