Deep veins are veins that exist deep within the body, primarily in the arms, shoulder, legs, pelvis, and neck. The deep veins in the legs and pelvis are particularly prone to developing blood clots, especially following surgery or an injury, in bedridden patients, or after prolonged sitting. This results in a common and serious condition known as deep venous thrombosis or DVT. Clots that break free from their point of origin and then travel to the lungs are known as pulmonary emboli and can be fatal.