The veins of the human body are responsible for returning de-oxygenated blood back to the heart. Like a rock rolling down a hill, blood flows from the highest pressure (the blood in the aorta) to the lower venous pressure (the blood in the vena cava as it empties back to the heart.) Unlike arteries, veins are thin walled and distensible, allowing them to accommodate large volumes of blood without significant changes in pressure. In fact, the venous system is so low pressure that veins have valves to keep blood from flowing backward. The motion of the human body helps pump blood through the veins- squeezing leg muscles while walking, for instance, helps push venous blood back up to the heart against the pull of gravity. Unfortunately, without this extra push some blood can sit stagnant in veins, leading to a multitude of clinical problems. The largest vein in the body is the vena cava. The superior vena cava (SVC) drains blood from the top half of the body while the inferior vena cava (IVC) drains blood from below the diaphragm. Elsewhere in the body, veins can be categorized into superficial, primarily associated with the skin and soft tissues, or deep veins, which drain muscles and organs.