Peripheral arterial disease of the lower extremities is atherosclerosis that occurs in the blood vessels of the legs. The primary arteries that supply blood to the legs are the:

Oxygen-rich blood flows down these arteries to supply the muscles of the buttocks, legs and feet. Atherosclerosis impedes the blood flow, much in the same way it can impede blood flow to the heart, by narrowing and hardening the arteries, much in the same way it can impede blood flow to the heart. 

The body compensates for the narrowing or blockage of these major arteries to the legs by enlarging smaller arteries to provide the blood flow to the leg. These smaller arteries, which do not have formal anatomical names, are referred to as collaterals. They are essentially alternate routes that the blood takes to reach its destination.

According to the American Heart Association, up to 75 percent of people with PAD have no symptoms and, therefore, may go undiagnosed. However, because atherosclerosis tends to be a widespread condition in the body, PAD can be an important indicator of heart disease and so it is important that the condition is diagnosed as soon as possible. People with PAD have a six to seven times greater risk of cardiovascular problems, such as heart attack, stroke or transient ischemic attack (TIA or mini-stroke), according to the National Institutes of Health (NIH).

Other forms of PAD include cerebrovascular disease, renal artery stenosis and mesenteric arterial disease