All parts of the body require oxygen to function, which is carried to the cells via blood vessels or the vascular supply. The heart and lungs require a great deal of oxygen, while other parts of the body, like bone, require less. However, bone can also cease to function without an adequate supply of oxygen. In an average long bone, oxygenated blood is supplied by either periosteal vessels, epiphyseal vessels, or a nutrient artery. By having three systems in place, one can take over the function of the other if it becomes blocked or damaged in some way. If bone tissue does not receive enough oxygen, a process known as avascular necrosis or osteonecrosis occurs. This is the death or decay of the bone due to a lack of oxygenated blood. Osteonecrosis can cause the bone to become brittle, breaking into small pieces and eventually crumbling or collapsing if left untreated.