The human circulatory system or cardiovascular system consists of blood vessels, the heart and blood. The piping system in humans is extensive. There are tens of thousands of kilometers of blood vessels in a human body to ensure that every cell of the body is within diffusion distance of a capillary.
Blood leaving the heart enters arteries.
In general, these are large vessels that act as pipe lines to carry blood rapidly to all parts of the body. They have a large diameter and a complex wall structure that contains concentric rings of elastic fibers. As the heart pumps blood out into arteries, the artery walls are pushed outward, increasing the fluid capacity of the vessel. As the walls stretch, the elastic fibers act much as rubber bands and resist the stretching. The major arteries branch into smaller vessels called arterioles.